How does one calculate an opposite color for use in absorbing and
emitting media ?

  Advanced user question ?

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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(1 - OriginalColor)    ?

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TonyB wrote:
> 
> (1 - OriginalColor)    ?

Won't work...

rgb<1,0,0> - rgb<1,0,0> = black.

The opposite of red is not black at least so I suspect.

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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On Mon, 27 Sep 1999 21:34:51 -0700, Ken <tyl### [at] pacbellnet> wrote:
>
>
>TonyB wrote:
>> 
>> (1 - OriginalColor)    ?
>
>Won't work...
>
>rgb<1,0,0> - rgb<1,0,0> = black.

1 is not rgb <1,0,0>; I'm pretty sure it's rgbft <1,1,1,1,1>.
You want rgb <1,1,1> - OriginalColor.

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Ron Parker wrote:

> You want rgb <1,1,1> - OriginalColor.

so -

rgb<1,1,1> - rgb<1,0,0> = rgb <0,1,1> = opposite color ?

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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On Mon, 27 Sep 1999 21:40:11 -0700, Ken <tyl### [at] pacbellnet> wrote:
>
>
>Ron Parker wrote:
>
>> You want rgb <1,1,1> - OriginalColor.
>
>so -
>
>rgb<1,1,1> - rgb<1,0,0> = rgb <0,1,1> = opposite color ?

Exactly.

Cyan is the opposite of red.

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Ron Parker wrote:

> Cyan is the opposite of red.

Cyan = opposite of red = ok fine by me.

Thank you for your assistance gentlemen.

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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I was trying this out to do it an easy way and stumbled into errors
concerning the color vector, int( ) and #if ( ). It would be fine were
it not for the negative vector created when switching back to original
color.  I didn't know vectors weren't allowed in these (and whatever
else there might be too).

// white pointlight at camera position
#declare LCX = 0;
#declare LCY = 0;
#declare LCZ = -10;

light_source { <LCX,LCY,LCZ> color rgb <1.5,1.5,1.5>
}
camera {
  location  <LCX,LCY,LCZ>
  angle 30
  look_at   <0,0,0>
}

// zero or 1 (more than 1 not acceptable)
#declare EAF=0;
// original color to calc opposite of
#declare OC=.5; // <.1,.3,.9>;  vector does not go into int( )
// opposite color vector (or not) [int(EAF-OC) errors wanting float
only]
#declare RGB=EAF-OC;
 // convert to positive if negative [#if (RGB errors wanting float)]
 #if (RGB<0) #declare RGB=-1*RGB; #end

box {-1,1 pigment {color rgb RGB} rotate <30,45,60>}

Bob

TonyB <ben### [at] panamaphoenixnet> wrote in message
news:37f03117@news.povray.org...
> (1 - OriginalColor)    ?
>
>

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Ken <tyl### [at] pacbellnet> wrote:

:> (1 - OriginalColor)    ?

: rgb<1,0,0> - rgb<1,0,0> = black.

  Since when 1 has been equal to <1,0,0>?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Nieminen Juha wrote:
> 
> Ken <tyl### [at] pacbellnet> wrote:
> 
> :> (1 - OriginalColor)    ?
> 
> : rgb<1,0,0> - rgb<1,0,0> = black.
> 
>   Since when 1 has been equal to <1,0,0>?

Read the rest of the thread and the answer shall be revealed to you...

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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Ken <tyl### [at] pacbellnet> wrote:
:>   Since when 1 has been equal to <1,0,0>?

: Read the rest of the thread and the answer shall be revealed to you...

  Sorry for the sarcasm...

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Nieminen Juha wrote:

> > Read the rest of the thread and the answer shall be revealed to you...
> 
>   Sorry for the sarcasm...

You are forgiven my friend.

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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Ken wrote:
> 
> Ron Parker wrote:
> 
> > Cyan is the opposite of red.
> 
> Cyan = opposite of red = ok fine by me.
> 
> Thank you for your assistance gentlemen.
> 
> --
> Ken Tyler
> 1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
> http://home.pacbell.net/tylereng/index.html

That depends. In rgb-calculations cyan is the opposite of red, if for instance
you were using another hightech multimedia-tool like for instance fingerpaint
you'd have green as the opposite of red.
This is part of a list that seems rather inconsistent in rgb-vectors:
red    <-> green   <1,0,0> <-> <0,1,0>
blue   <-> orange  <0,0,1> <-> <1,.5,0>
yellow <-> purple  <1,1,0> <-> <1,0,1>

And even then this is just an arbitrary definition of these colors (is purple
really <1,0,1> or perhaps <.5,0,1>?).

There's no cyan here, nor fuchsia or whatever.
Also with fingerpaint adding more paint doesn't make the color brighter.

This gives you the impression that this way of working with colours is a bit
retarded, but still, that's how it was done for centuries? 
Of course rgb is based on a more scientific interpretation of the behaviour of
light. Fortunately art (?) needn't be dictated by science.

Just my 2 eurocents...

Remco

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Ron Parker wrote:
> >so -
> >
> >rgb<1,1,1> - rgb<1,0,0> = rgb <0,1,1> = opposite color ?
> 
> Exactly.
> 
> Cyan is the opposite of red.

The only problem being when the original colour is not full intensity, i.e.
something like rgb <.5,0,0>

rgb<1,1,1> - rgb<.5,0,0> = rgb<.5,1,1>

which is of course spectrally correct, but much brighter than the original.

I simply use my HSL macros (I've all but abandoned RGB)
This is what I do:

hsl(OriginalHue-0.5, S, L) = hsl(OppositeHue, S, L)

If the original hue is <0.5 add 0.5 instead of subtracting. The brightness of
the colour remains the same, which is especially important in media.

Margus

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When we talk about lights, the colors are additive, while when we are
talking about paint, the colors are substractive.
  We should know which color system we are using before we can tell the
correct answer.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Yes, the answer depends on whether we want the negative color (for
example, the negative color of white is black and vice-versa) or the
opposite hue (where the opposite of black is still black) where we have
to play with the hue instead of the rgb values.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Ken wrote:
> 
> Nieminen Juha wrote:
> >
> > Ken <tyl### [at] pacbellnet> wrote:
> >
> > :> (1 - OriginalColor)    ?
> >
> > : rgb<1,0,0> - rgb<1,0,0> = black.
> >
> >   Since when 1 has been equal to <1,0,0>?
> 
> Read the rest of the thread and the answer shall be revealed to you...
> 
> --
> Ken Tyler
> 1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
> http://home.pacbell.net/tylereng/index.html

But since when is 1 = <1,0,0> ?

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PoD wrote:

> But since when is 1 = <1,0,0> ?

All right people. I was wrong and I was hasty in my reply to TonyB.
I admit that rgb 1 = <1,1,1>. Is everybody happy now ?

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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On Tue, 28 Sep 1999 08:25:13 -0700, Ken wrote:
>
>
>PoD wrote:
>
>> But since when is 1 = <1,0,0> ?
>
>All right people. I was wrong and I was hasty in my reply to TonyB.
>I admit that rgb 1 = <1,1,1>. Is everybody happy now ?

No, but if you send me money I'll be happier. :)

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On Tue, 28 Sep 1999 14:05:05 +0200, Remco de Korte
<rem### [at] xs4allnl> wrote:


>That depends. In rgb-calculations cyan is the opposite of red, if for instance
>you were using another hightech multimedia-tool like for instance fingerpaint
>you'd have green as the opposite of red.

I'm not sure but I think you are talking about the CMY(K) color model
which is a subtractive one as opposed to RGB which is additive.


Peter Popov
ICQ: 15002700

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>That depends. In rgb-calculations cyan is the opposite of red, if for
instance
>you were using another hightech multimedia-tool like for instance
fingerpaint
>you'd have green as the opposite of red.


This is not entirely true. There is a common misunderstanding, that additive
colors (light) behave different from subtractive (pigments). It is correct
that green is the opposite of red, but the misunderstanding lies in what is
"red" and what is "green".

Here is a show-off from a class I attended once. (This is for information,
and not to make a fool of anyone. It is not easy to come up with all this
one ones own; I know. :) )

Usually, you say that red, blue and yellow are mixed together to make black.
In pigment-theory, yellow is a primary color, which is true, because yellow
pigment is the pigment that reflects everything which is not blue! Red is a
primary color in both systems, however the color is not the same! Red is the
pigment that reflects all colors but green. The red pigment reflects
red+blue light when illuminated with white light, and thus "red" is not
actually red, but red+blue=magenta! "Blue" pigment is really cyan. We have:

"red" (magenta) pigment = <1, 1, 1> - green
yellow pigment = <1, 1, 1> - blue
"blue" (cyan) pigment = <1, 1, 1> - red

The opposite color is the color on the "other side" of the color wheel. Now,
we can see that red is the opposite of green in common language, but it is
actually magenta that is the opposite color of green and vice versa.

If you open the standard color-picking tool in Windows or any graphical
package, you can see the color-wheel, and verify this.

Observe that opposite colors also exists in light-systems, as opposite = <1,
1, 1> - original.

Simen.

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Peter Popov wrote:
> 
> On Tue, 28 Sep 1999 14:05:05 +0200, Remco de Korte
> <rem### [at] xs4allnl> wrote:
> 
> >That depends. In rgb-calculations cyan is the opposite of red, if for instance
> >you were using another hightech multimedia-tool like for instance fingerpaint
> >you'd have green as the opposite of red.
> 
> I'm not sure but I think you are talking about the CMY(K) color model
> which is a subtractive one as opposed to RGB which is additive.
> 
> Peter Popov
> ICQ: 15002700

No, not really, although it's close. CMYK is for print as opposed to RGB for
tubes. Obviously print works more or less the same as paint but there are subtle
differences. I guess that has something to do with the tricks of the trade. One
of the most interesting things: paint has mass. But this is totally off topic
here. 8) 

Regards,

Remco

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Simen Kvaal wrote:
> 
> >That depends. In rgb-calculations cyan is the opposite of red, if for
> instance
> >you were using another hightech multimedia-tool like for instance
> fingerpaint
> >you'd have green as the opposite of red.
> 
> This is not entirely true. There is a common misunderstanding, that additive
> colors (light) behave different from subtractive (pigments). It is correct
> that green is the opposite of red, but the misunderstanding lies in what is
> "red" and what is "green".
> 
> Here is a show-off from a class I attended once. (This is for information,
> and not to make a fool of anyone. It is not easy to come up with all this
> one ones own; I know. :) )
> 
> Usually, you say that red, blue and yellow are mixed together to make black.
> In pigment-theory, yellow is a primary color, which is true, because yellow
> pigment is the pigment that reflects everything which is not blue! Red is a
> primary color in both systems, however the color is not the same! Red is the
> pigment that reflects all colors but green. The red pigment reflects
> red+blue light when illuminated with white light, and thus "red" is not
> actually red, but red+blue=magenta! "Blue" pigment is really cyan. We have:
> 
> "red" (magenta) pigment = <1, 1, 1> - green
> yellow pigment = <1, 1, 1> - blue
> "blue" (cyan) pigment = <1, 1, 1> - red
> 
> The opposite color is the color on the "other side" of the color wheel. Now,
> we can see that red is the opposite of green in common language, but it is
> actually magenta that is the opposite color of green and vice versa.
> 
> If you open the standard color-picking tool in Windows or any graphical
> package, you can see the color-wheel, and verify this.
> 
> Observe that opposite colors also exists in light-systems, as opposite = <1,
> 1, 1> - original.
> 
> Simen.

If you open the standard color-picking tool in Windows you are again relying on
a computerized, scientifically based, interpretation. I'm not debating what
you're saying here. As I already stated: it's not clear what everyone means by
red although there is some common idea about that. If you're looking at the
difference between blue and green it's getting more difficult as a thread in the
binaries.images group illustrates. Apart from the fact that it is sometimes hard
to decide if it's more green or more blue there is also the problem that not
everyone sees it in the same way. A significant part of the male population is
simply colourblind.

It is tempting, if you're working with computers, to pin down colours on their
values, use a scientific approach. I'm not saying there's anything wrong with
that. My point is: colours are a matter of perception. This is not less
scientific as there has been a lot of research on how people react on colours.
The Bauhaus gave a lot of attention to colour-theory, partly based on artistic
principles, but also on psychological research. You could say that that theory
is outdated but I think there's more in those theories than just some
numbercrunching.

Regards,

Remco

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I agree, but there is one fact that has to be made clear, and that is that
many colors are "standarized". I believe (but are not sure) that the colors
in "colors.inc" is taken from the international catalog, which states what
blending of frequencies to use for a specific colour. There are standards to
what one should call "aquamarine", "azure" and so forth. "Blue" and "green"
are more diffuse in the meaning. Many would for example say that the sky is
blue, but that might not be the correct technical term to use. I am not very
bewandered with this; i just happen to know the story behind, if you see
what I mean.

Just another point. A curious fact that I guess you are aware of: Many
colors cannot even be displayed on a computer screen, even with 500 billion
zillion shades of red, green and blue, merely because the human eye can
percieve many more colors. For example, if you have a yellow marking pen and
try to recreate the color on the screen, you'll be dissappointed. If you
scan it you might not even see it!

In the future I think we will get new computer screens wich cover more of
the human eyes capability to see colors. Maybe POV-ray should be able to
extend the usual color-system. One could add infra-red channels, for
example. (We cannot see those frequencies, but they might have scientifical
importance. Who knows.)

Simen.

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>No, not really, although it's close. CMYK is for print as opposed to RGB
for
>tubes. Obviously print works more or less the same as paint but there are
subtle
>differences. I guess that has something to do with the tricks of the trade.
One
>of the most interesting things: paint has mass. But this is totally off
topic
>here. 8)


Light has mass too! According to Einstein, light has no resting mass, but
relativistic mass. It is easy to verfy. The solar winds, for example, can
make a satelite go off couse. And you cannot have force without mass. But,
as you say, this is off topic.

Simen.

Post a reply to this message

Simen Kvaal wrote:
> 
> I agree, but there is one fact that has to be made clear, and that is that
> many colors are "standarized". 

Uhm, this is way off topic again, but standardization seems to be on of the
major diseases of these times. Or better: not the standardization itself but the
false belief in it. 
This is a computer based platform: that is on of the places were you can
experience the consequences of Standardization all the time. 

I believe (but are not sure) that the colors
> in "colors.inc" is taken from the international catalog, which states what
> blending of frequencies to use for a specific colour. There are standards to
> what one should call "aquamarine", "azure" and so forth. 

Yes there are. 
Kodak tried to set some standards for instance. 
Of course Apple had to define some standards.
There are probably as many colorstandards as there are, just to name a thing,
computerplatforms. 
Luckily you see standards converging, but you can never be sure.
I thought Netscape's 256 color palette was more or less a standard. Who
guarantuees that Microsoft in an attempt to beat its opponents on all fields
won't come up with another standard for Explorer? Well, luckily the 256-colro
days are almost behind us 8)

"Blue" and "green"
> are more diffuse in the meaning. Many would for example say that the sky is
> blue, but that might not be the correct technical term to use. I am not very
> bewandered with this; i just happen to know the story behind, if you see
> what I mean.
> 
> Just another point. A curious fact that I guess you are aware of: Many
> colors cannot even be displayed on a computer screen, even with 500 billion
> zillion shades of red, green and blue, merely because the human eye can
> percieve many more colors. For example, if you have a yellow marking pen and
> try to recreate the color on the screen, you'll be dissappointed. If you
> scan it you might not even see it!

Aha! Now this interesting, also in regard to the original question!
What would be the opposite of ultra-violet? How would you express such a colour
in a vector? Is perhaps the way to define colours as RGB vectors not as perfecet
as we (computer-users) would like to believe?
> 
> In the future I think we will get new computer screens wich cover more of
> the human eyes capability to see colors. Maybe POV-ray should be able to
> extend the usual color-system. One could add infra-red channels, for
> example. (We cannot see those frequencies, but they might have scientifical
> importance. Who knows.)

How about describing light by its frequency?
Now that would be cool. Once POV-Ray could work with that you could even make
EM-raytracings ;)
> 
> Simen.

Have fun!

Remco

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Remco de Korte <rem### [at] xs4allnl> schreef in berichtnieuws
37F27701.33A06FB4@xs4all.nl...
>
> How about describing light by its frequency?
> Now that would be cool. Once POV-Ray could work with that you could even
make
> EM-raytracings ;)

The CIE (Commission Internationale de l'Eclairage) standard uses spectrum
measurement to define a colour. IR and UV are not coverd though, they are not
conciderd colour or light. It's the only colour measurement / definition
accepted by ISO. Even in the printing buisiness it's slowly gaining terrain on
pantone etc.

http://www.inforamp.net/~poynton/notes/colour_and_gamma/ColorFAQ.html

http://www.colourware.co.uk/cpfaq.htm

Ingo

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Not that it's likely to happen in POV-Ray (I may wish I didn't say that
later, noticing all the changes to POV and it's cousins over the years) it
would be pretty fantastic to see raytracing cover some of the aspects of
light in the realm of wavelength and the corresponding interactions.
Might be neat to see that ultraviolet (well, not see it exactly unless
shifting the spectrum for a fancy render) interact with something and drop
in frequency to emerge as a deep blue indigo or other fluorescent color;
just like certain mineral rocks.
Fact is, as many people must know, the potential for a vast number of
physical realities being simulated always exists given the will and a way.
That's probably the major stumbling block; I know I can't do 1% of the
things I can think of (actual percentage may vary, greatly....) otherwise
I'd be living a dream.

Bob

Remco de Korte <rem### [at] xs4allnl> wrote in message
news:37F27701.33A06FB4@xs4all.nl...
> Simen Kvaal wrote:
> >
> > I agree, but there is one fact that has to be made clear, and that is
that
> > many colors are "standarized".
>
> Uhm, this is way off topic again, but standardization seems to be on of
the
> major diseases of these times. Or better: not the standardization itself
but the
> false belief in it.
> This is a computer based platform: that is on of the places were you can
> experience the consequences of Standardization all the time.
>
> I believe (but are not sure) that the colors
> > in "colors.inc" is taken from the international catalog, which states
what
> > blending of frequencies to use for a specific colour. There are
standards to
> > what one should call "aquamarine", "azure" and so forth.
>
> Yes there are.
> Kodak tried to set some standards for instance.
> Of course Apple had to define some standards.
> There are probably as many colorstandards as there are, just to name a
thing,
> computerplatforms.
> Luckily you see standards converging, but you can never be sure.
> I thought Netscape's 256 color palette was more or less a standard. Who
> guarantuees that Microsoft in an attempt to beat its opponents on all
fields
> won't come up with another standard for Explorer? Well, luckily the
256-colro
> days are almost behind us 8)
>
> "Blue" and "green"
> > are more diffuse in the meaning. Many would for example say that the sky
is
> > blue, but that might not be the correct technical term to use. I am not
very
> > bewandered with this; i just happen to know the story behind, if you see
> > what I mean.
> >
> > Just another point. A curious fact that I guess you are aware of: Many
> > colors cannot even be displayed on a computer screen, even with 500
billion
> > zillion shades of red, green and blue, merely because the human eye can
> > percieve many more colors. For example, if you have a yellow marking pen
and
> > try to recreate the color on the screen, you'll be dissappointed. If you
> > scan it you might not even see it!
>
> Aha! Now this interesting, also in regard to the original question!
> What would be the opposite of ultra-violet? How would you express such a
colour
> in a vector? Is perhaps the way to define colours as RGB vectors not as
perfecet
> as we (computer-users) would like to believe?
> >
> > In the future I think we will get new computer screens wich cover more
of
> > the human eyes capability to see colors. Maybe POV-ray should be able to
> > extend the usual color-system. One could add infra-red channels, for
> > example. (We cannot see those frequencies, but they might have
scientifical
> > importance. Who knows.)
>
> How about describing light by its frequency?
> Now that would be cool. Once POV-Ray could work with that you could even
make
> EM-raytracings ;)
> >
> > Simen.
>
> Have fun!
>
> Remco

Post a reply to this message

Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
: Light has mass too! According to Einstein, light has no resting mass, but
: relativistic mass.

  I wouldn't say that light actually has mass.
  AFAIK light has no mass. Period.
  This is only my opinion of what I have understood about physics:
  For some calculations it may be easier to consider the light having mass,
but it's only a mathematical trick. It doesn't really have any mass. The
reason why light bends near massive objects is because the space is curved
there and the light tends to move along the geodesic lines of the space
(which are actually the shortest way from one point to another). The
massive object doesn't attract the light, it's just bending the space.

: It is easy to verfy. The solar winds, for example, can
: make a satelite go off couse. And you cannot have force without mass.

  You are confused here. Solar wind is not light, it's gaseous matter that
the Sun is emitting (mainly ions). See:
http://www-spof.gsfc.nasa.gov/Education/wsolwind.html

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Nieminen Juha skrev i meldingen <37f31566@news.povray.org>...
>Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
>: Light has mass too! According to Einstein, light has no resting mass, but
>: relativistic mass.
>
>  I wouldn't say that light actually has mass.
>  AFAIK light has no mass. Period.
>  This is only my opinion of what I have understood about physics:
>  For some calculations it may be easier to consider the light having mass,
>but it's only a mathematical trick. It doesn't really have any mass. The


Light has mass. Period!! It's not just a mathematical trick. Light *has*
relativistic mass, and so have you. When travelling faster and faster, you
gain weight. You cannot travel at the speed of light, because you'd have
infinite mass:

m = m0 / sqrt(1 - (v*v/c*c))

If m0 is your resting mass, v is your velocity and c is the speed of light,
then relativistic mass (aka REAL mass) is m. You can see from the equation
that when your speed approaches the speed of light, your mass diverges into
infinity. These are the equations Einstein found, and they have been
*empirically* verified!

The reason why light has mass, is Einsteins formula:

E = mc^2.

Light has energy, right? Then, light must have mass. It cannot have resting
mass, because light doesn't rest. But as the light travels at the speed of,
say, light, it must have relativistic mass. The energy of a photon is:

E = f * k

where f is the frequency and k is a constant (which i cannot remember; it's
three years since i learnt this). Put this into the "emc" and you get:

m = f*k/c^2

This is the mass of light. Period.




>reason why light bends near massive objects is because the space is curved
>there and the light tends to move along the geodesic lines of the space
>(which are actually the shortest way from one point to another). The
>massive object doesn't attract the light, it's just bending the space.
>

Correct, afaik.


>
>  You are confused here. Solar wind is not light, it's gaseous matter that
>the Sun is emitting (mainly ions). See:
>http://www-spof.gsfc.nasa.gov/Education/wsolwind.html
>

Okay. I was wrong there. But light has mass! The solar winds was just a
(very) bad example. You can verify that photons actually may cause pressure!
This is the empirical prof of the mass of light. You cannot have pressure
without mass.

Post a reply to this message

Nieminen Juha wrote:
> 
> Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
> : Light has mass too! According to Einstein, light has no resting mass, but
> : relativistic mass.
> 
>   I wouldn't say that light actually has mass.
>   AFAIK light has no mass. Period.

  I heard an astronomer say on an educational program just 2 days ago
that light does indeed have mass. The surface of the earth is bombarded
with roughly the equivalent of 2 pounds of light per day. This is of
course heresay and would not hold up in a court of law. The program
that I heard this on a is called "Universe: The Infinite Frontier"
which airs on a local public broadcast station (PBS).

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

Post a reply to this message

Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
: Light has mass. Period!! It's not just a mathematical trick. Light *has*
: relativistic mass, and so have you. When travelling faster and faster, you
: gain weight. You cannot travel at the speed of light, because you'd have
: infinite mass:

: m = m0 / sqrt(1 - (v*v/c*c))

: If m0 is your resting mass, v is your velocity and c is the speed of light,
: then relativistic mass (aka REAL mass) is m. You can see from the equation
: that when your speed approaches the speed of light, your mass diverges into
: infinity. These are the equations Einstein found, and they have been
: *empirically* verified!

  Actually it was Lorentz who first proposed the relativistic transformation
of distances with the factor 1/sqrt(1-v*v/c*c). Einstein always refers to
the Lorentz transformations.

: Light has energy, right? Then, light must have mass. It cannot have resting
: mass, because light doesn't rest.

  Well, the actual reason why it cannot have resting mass is because it
wouldn't travel at speed c if it had.

  Now tell me something:
  When light travels through matter (for example water), it travels at a
speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
gets a non-infinite value, which multiplied with the rest mass of the
light gives us 0. This would mean that the energy of the light travelling
through water would be 0.
  How is this possible?

:>reason why light bends near massive objects is because the space is curved
:>there and the light tends to move along the geodesic lines of the space
:>(which are actually the shortest way from one point to another). The
:>massive object doesn't attract the light, it's just bending the space.
:>

: Correct, afaik.

  Is the bending of the light caused by the curvature of the space,
the mass of the light (which the massive object attracts), or both?
  If both, then it would seem that the light would not stay in the
geodesic line because the massive object is attracting it out of that line.

: Okay. I was wrong there. But light has mass! The solar winds was just a
: (very) bad example. You can verify that photons actually may cause pressure!
: This is the empirical prof of the mass of light. You cannot have pressure
: without mass.

  How can I verify that photons may cause pressure?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

On Thu, 30 Sep 1999 10:39:37 +0200, Simen Kvaal wrote:
>E = f * k
>
>where f is the frequency and k is a constant (which i cannot remember; it's
>three years since i learnt this). Put this into the "emc" and you get:

Planck's Constant, usually denoted with an h.  6.63e-34, if my calculator 
is correct. :)

Post a reply to this message

On 30 Sep 1999 09:44:34 -0400, Nieminen Juha wrote:
>  When light travels through matter (for example water), it travels at a
>speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
>gets a non-infinite value, which multiplied with the rest mass of the
>light gives us 0. This would mean that the energy of the light travelling
>through water would be 0.
>  How is this possible?

The explanation I always got is that the photon is being absorbed and
re-emitted each time it hits a water molecule, but it is traveling at
c in the interstices between molecules.

Post a reply to this message

Ron Parker <par### [at] fwicom> wrote:
: The explanation I always got is that the photon is being absorbed and
: re-emitted each time it hits a water molecule, but it is traveling at
: c in the interstices between molecules.

  The speed of the photon must slow down at least somewhere so that
the average speed could be less than c.
  If the photon is absorbed by a molecule, why does the photon leave the
molecule in the exact same direction as it entered it?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Nieminen Juha skrev i meldingen <37f36942@news.povray.org>...
>Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
>: Light has mass. Period!! It's not just a mathematical trick. Light *has*
>: relativistic mass, and so have you. When travelling faster and faster,
you
>: gain weight. You cannot travel at the speed of light, because you'd have
>: infinite mass:
>
>: m = m0 / sqrt(1 - (v*v/c*c))
>
>: If m0 is your resting mass, v is your velocity and c is the speed of
light,
>: then relativistic mass (aka REAL mass) is m. You can see from the
equation
>: that when your speed approaches the speed of light, your mass diverges
into
>: infinity. These are the equations Einstein found, and they have been
>: *empirically* verified!
>
>  Actually it was Lorentz who first proposed the relativistic
transformation
>of distances with the factor 1/sqrt(1-v*v/c*c). Einstein always refers to
>the Lorentz transformations.
>

Okay, i have no problem with that. But that doesn't mean that light doesn't
have mass? Do you agree when I say that light has mass?

>: Light has energy, right? Then, light must have mass. It cannot have
resting
>: mass, because light doesn't rest.
>
>  Well, the actual reason why it cannot have resting mass is because it
>wouldn't travel at speed c if it had.
>

That's just what I was trying to say, but in other words. (Mine was
unclear.) If light travels at the speed of light, it would have infinite
mass, which it clearly doesn't. (I thing I'd get a headache if I turned on a
lamp in my room if it did...)

>  Now tell me something:
>  When light travels through matter (for example water), it travels at a
>speed which is less than c. Because of this, the factor 1/sqrt(1-v*v/c*c)
>gets a non-infinite value, which multiplied with the rest mass of the
>light gives us 0. This would mean that the energy of the light travelling
>through water would be 0.
>  How is this possible?

Hard question that one. I kept wondering, but now I got the solution! The
equation for m that I gave:
m = m0 / sqrt(1 - (v*v/c*c))
cannot be used. AFAIK is photons travelling at the speed of light a special
case.This equation is to be used with bodies travelling at speed v
*relative* to the observer, but light *always* travels at speed c *relative*
to the observer. Actually, the factor 1/sqrt(1-v*v/c*c) doesn't give meaning
to bodies (photons) travelling at the speed of light. It is exactly the
same! It is just another view of the photons not having resting mass, but
relativistic mass only. It's not really hard to accept that if light travels
at the speed of light (naturally) and *only* that, we must in some way have
a special case. (It is the light itself that makes the equations possible.)

Thus, the energy of light travelling through water is not 0. When the light
goes out in the vacuum again, it's speed also increases. Thus, it is
impossible that light loses energy on the way in and gains on the way out.
(Where should the photons take their energy from?) What actually happens, is
that the wavelength og the light changes with refraction. You can see this
if you send white thru a prism: Different wavelengths refracts differently
and produces a spectre on the other side. (There is a path for POV-ray to
simulate this, i think.)

In physics I learnt that when a wave on water hits a shallower area, the
wavelength and speed must change in order to preserve energy. You can see
waves on water refract like light, hitting a diagonal, shallow area. It's
exactly the same with light: speed and wavelength changes during refraction
in order to preserve energy. The rules for index of refraction
(sin(alpha1)/alpha1=sin(alpha2)/alpha2) is also true for waves on water!


>
>:>reason why light bends near massive objects is because the space is
curved
>:>there and the light tends to move along the geodesic lines of the space
>:>(which are actually the shortest way from one point to another). The
>:>massive object doesn't attract the light, it's just bending the space.
>:>
>
>: Correct, afaik.
>
>  Is the bending of the light caused by the curvature of the space,
>the mass of the light (which the massive object attracts), or both?
>  If both, then it would seem that the light would not stay in the
>geodesic line because the massive object is attracting it out of that line.
>


This is a very advanced topic, and I know absolutely nothing about it. In
fact, I think it is two sides of the same case. But I don't know at all.

>: Okay. I was wrong there. But light has mass! The solar winds was just a
>: (very) bad example. You can verify that photons actually may cause
pressure!
>: This is the empirical prof of the mass of light. You cannot have pressure
>: without mass.
>
>  How can I verify that photons may cause pressure?
>

This is a simple experiment:
1. Take a glass jar (empty).
2. Take two small sheets of aluminium foil and create a kind of windmill.
4. Use a fire and darken every other side of the mill's wings.
3. Fasten it to a thin thread and hang it into the glass jar.
4. Put in sunlight.

(I'll post a rendered picture in a minute)

You can now see that the windmill turns from the heat of the sunlight! The
black sides absorb much more energy (pressure from the light) and the mill
starts turning.

This is an experiment we did in physics class some years ago, and it works!
This shows that photons cause pressure.

(It should not be very difficult to accept, though. Mass traveling at a
given velocity does indeed create pressure. Soap-bubbles, wind, the shower,
rain, et.c.)

>--
>main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
>):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

What does this code do? I've tried it with Borland, but I get some error
messages. "Cannot call main from main" et.c.

Simen.

Post a reply to this message

On 30 Sep 1999 10:29:58 -0400, Nieminen Juha wrote:
>Ron Parker <par### [at] fwicom> wrote:
>  The speed of the photon must slow down at least somewhere so that
>the average speed could be less than c.

In the period of time between absorption and re-emission, the photon 
doesn't exist.  While it exists, its speed is exactly c.  While it 
doesn't, it doesn't have a speed but you can make a convincing argument
for assuming a value of zero.  The time during which it doesn't exist
is small but nonzero, and it happens many many times while the photon 
is traveling in a medium.

>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

This is something I don't know offhand.  I suppose that the ability to do 
that over a given frequency range is what makes it a transparent molecule 
rather than an opaque one, however it comes by that ability.  Something to
do with dielectric constants and such, but I changed majors before we got
into any of the heavy stuff.

It's also possible that it was an oversimplistic abstraction given to 
first-year physics students. :)

Post a reply to this message

Look in binaries.images for an illustration to the experiment i mentioned.

Simen.

Post a reply to this message

Simen Kvaal <sim### [at] studentmatnatuiono> wrote in message
news:37f37cd8@news.povray.org...
> Look in binaries.images for an illustration to the experiment i mentioned.

I've got one of these specially made actually.  It's a specially blown glass
"bubble" in a shape similar to a light bulb... (bulb at the top, a pin goes
up through the middle point upwards and the mill sits down on top of it on
the point) it is pressure sealed and thus there is not very much resistance
inside against the "mills".

If I leave it in the sun it gets up to about 120 RPM.  I find it quite
interesting to observe :)

My opinion, light creates pressure, therefore photons have mass (which makes
sense when you think about how light reacts with things like liquids, or
gravitational lens etc).  But I'm not well enough advanced to be able to
prove any of that yet.

BTW, this should be moved to p.o-t

Post a reply to this message

Lance Birch wrote:
> 
> BTW, this should be moved to p.o-t

I agree on that 8)

Besides, I tried to point at that there's more to colour then the physical
apprach and see what happens here... :o

Remco

Post a reply to this message

>This shows that photons cause pressure.<

No, it doesn't. If you put it in a vacuum, it won't turn. It turns
because the black side heats the ambient air more than the white side,
which causes the molecules of gas on that side to hit the vanes harder
than the white side. This causes the vane to turn-sort of a macroscopic
variation of Brownian movement.
Note: I am not saying whether light has mass or not, just that the vanes
don't turn because of pressure from photons.

Post a reply to this message

On 30 Sep 1999 10:29:58 -0400, Nieminen Juha <war### [at] cctutfi> wrote:

>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

My father tried to explain this to me a lot of times and never
understood it. I only know that this phenomenon is the reason that a
laser beam is virtually non-divergent.


Peter Popov
ICQ: 15002700

Post a reply to this message

On 30 Sep 1999 10:29:58 -0400, Nieminen Juha <war### [at] cctutfi> wrote:
>Ron Parker <par### [at] fwicom> wrote:
>: The explanation I always got is that the photon is being absorbed and
>: re-emitted each time it hits a water molecule, but it is traveling at
>: c in the interstices between molecules.
>
>  The speed of the photon must slow down at least somewhere so that
>the average speed could be less than c.
>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

Okay, new data.  This is from Feynman's _Lectures on Physics_, which 
everyone should own.  He does a non-differential-equation derivation
in chapter 31 of volume 1 for gases (because if the atoms are too 
close together you get interactions that are too hard to account for
without differential equations) which he then summarises in chapter
32 of volume 2 thusly:

  The electric field of the light wave polarizes the molecules
  of the gas, producing oscillating dipole moments.  The acceleration
  of the oscillating charges radiates new waves of the field.  This
  new field, interfering with the old field, produces a changed field
  which is equivalent to a phase shift of the original wave.  Because
  this phase shift is proportional to the thickness of the material,
  the effect is equivalent to having a different phase velocity in
  the material.

This is the wave-based version rather than the particle-based version,
but it makes more sense, I think.

Post a reply to this message

On 30 Sep 1999 19:04:55 -0400, par### [at] fwicom (Ron Parker) wrote:

>Okay, new data.  This is from Feynman's _Lectures on Physics_, which 
>everyone should own.  

I do, but they are in Bulgarian. Just so that you know :)


Peter Popov
ICQ: 15002700

Post a reply to this message

> No, it doesn't. If you put it in a vacuum, it won't turn. It turns

Yes, it does work in a vacuum actually.

Post a reply to this message

Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
: This equation is to be used with bodies travelling at speed v
: *relative* to the observer, but light *always* travels at speed c *relative*
: to the observer.

  There is an effect known as Cherenkov radiation which happens usually in
nuclar reactors when neutrons travel faster than light in water.
  Those neutrons of course travel slower than c but faster than the speed
of the light in water. This causes a radiation which is an equivalent of
the sonic boom for light.

: You can now see that the windmill turns from the heat of the sunlight! The
: black sides absorb much more energy (pressure from the light) and the mill
: starts turning.

  You said it right: from the _heat_ of the sunlight.
  Of course the electromagnetic waves carry energy. I have never said
anything else. You can convert this energy to anything. You can boil water
with sunlight and make a steam engine run. You can use solar panels to
generate electricity and run a car if you want.
  But that's not due to the mass of the light.
  I have read about the solar windmill, and the reason for it to move is
not the mass of the light, but the heat caused by the energy it carries.
Even the theoretical amount of mass of the light would not be enough to
move an object because it's so small.

  The theory says that there's no difference between mass and energy. All
mass is just energy. Everything is just energy.
  This doesn't mean that everything is mass. The fact that everything is
made of atoms doesn't mean that everything is milk just because milk is
made of atoms. Every object has a color, but that doesn't mean that
every object is blue because blue is a color. Every computer program is
made of bits, but that doesn't mean that every computer program is povray
because povray is made of bits.

  (Cool, I succeeded in introducing povray somewhere in this off-topic
thread... :) )

:>main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
:>):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

: What does this code do? I've tried it with Borland, but I get some error
: messages. "Cannot call main from main" et.c.

  You have to compile it as C, not C++.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Lance Birch <lan### [at] usanet> wrote:
: I've got one of these specially made actually.  It's a specially blown glass
: "bubble" in a shape similar to a light bulb... (bulb at the top, a pin goes
: up through the middle point upwards and the mill sits down on top of it on
: the point) it is pressure sealed and thus there is not very much resistance
: inside against the "mills".

: If I leave it in the sun it gets up to about 120 RPM.  I find it quite
: interesting to observe :)

: My opinion, light creates pressure, therefore photons have mass

  Have you ever thought why it has to be a pressure sealed bulb? Why it
doesn't work without it?
  If the movement was caused by massive particles hitting the object, then
it shouldn't matter if it's inside a pressure sealed bulb or not.
  Have you ever thought that light can be used to power electric devices,
boil water, etc? Do you see any connection between these two things?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Nieminen Juha wrote:

> : You can now see that the windmill turns from the heat of the sunlight! The
> : black sides absorb much more energy (pressure from the light) and the mill
> : starts turning.
> 
>   You said it right: from the _heat_ of the sunlight.
>   Of course the electromagnetic waves carry energy. I have never said
> anything else. You can convert this energy to anything. You can boil water
> with sunlight and make a steam engine run. You can use solar panels to
> generate electricity and run a car if you want.
>   But that's not due to the mass of the light.
>   I have read about the solar windmill, and the reason for it to move is
> not the mass of the light, but the heat caused by the energy it carries.
> Even the theoretical amount of mass of the light would not be enough to
> move an object because it's so small.

See also the off topic group for an continuation of this thread.

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

Post a reply to this message

Why it seems to be so hard to understand, that even if we consider the
theoretical mass of the light as real mass, it would never be massive enough
to affect such a tiny object?
  Perhaps if we had a windmill of the size of the earth which weights about
0.1 grams and which doesn't get warm and is put in vacuum near the Sun, it
would theoretically begin to turn in a couple of hundred of years.
  The reason why it turns the tiny windmill is the same as why it is able
to run a car. It carries lots of energy.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

Post a reply to this message

Yes I do, and especially after reading the other messages you've written :)

I don't know enough about it at the moment, sorry.  But I feel you're right,
it makes more sense.

AAAAAAANNNNNNNYYYYYYYWWWWWWWAAAAAAAAAAYYYYYY, over to p.o-t I go.

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