Brian Elliott wrote:
> "Alain" <ele### [at] netscapenet> wrote in message 
> news:47274dcc@news.povray.org...
>> ... Gamma radiation is light. The kynetic energy mass gets transformed 
>> into light. And remember: light can push objects. It's just that this 
>> push is normaly to small for you to notice, but it can easily be 
>> demonstrated with a very simple experiment.
>> Take an empty globe of glass, place a needle holding a light rotor 
>> made from a glass axis and holding 3 or 4 blades, white on one side, 
>> black on the other. Place the rotor on the needle. Remove all air from 
>> the globe. Have ANY light shining on that rotor, even a candle light, 
>> and the rotor will spinn. That experiment is over 100 years old!
> 
> I saw that experiment in a high-school physics lab three decades ago.  I 
> was mightily impressed -- in fact it was one of my first "WOW" moments 
> in science.  It completely turned my mind to suddenly realise that:
> 
> a.  Light actually has a mass that exerts force on "solid" matter.
> 
> b.  Even at typical levels, light's force on macro-scale objects is 
> strong enough that I could stand in an average lab in an average 
> country-town high-school and watch a finely-balanced vane turn in a 
> vacuum, impelled by nothing but light.
> 
> Until then, I'd thought that the effects of things down at quantum 
> dimensions could not be observed so simply at our size.
> 

It is impressive but the understanding that the rotation is caused by 
light pressure in the way you recall is incorrect.

The commonly known instrument is a Crookes Radiometer 
(http://en.wikipedia.org/wiki/Crookes_radiometer) and the rotation is 
due to a thermal effect in the thin gas near the vanes.  It is 
definitely not a vacuum inside.  In fact if the gas pressure is reduced 
to approach a vacuum the effect disappears.

Light pressure would not produce a strong enough effect to show up here.

Also you'll find that the white/silver sides rotate towards the light 
rather than the reverse that you would expect if the cause was momentum 
being transferred by light bouncing off the more reflective side of the 
vanes.

The proper explanation can be found in the Wikipedia article.

A true light pressure effect was measured using a Nichols radiometer 
(http://en.wikipedia.org/wiki/Nichols_radiometer).  Not nearly as 
spectacular or easy to produce in the typical classrooms.

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