Thank you everybody for the comments and encouragement. Ok. I accept that: I can
not see the light only it's reflection I can see.
I create a mirror (ambient 0 diffuse 0 reflection 1 -perfect mirror?) point
spotlight to the mirror at 45deg and look at the place where it reflected with
camera (at -45deg).

I do indeed see bright circular spot but ...
I defined spot light with sharp cut-off: radius=falloff= 30(deg)
With light source being 283(mm) from mirror I expect circle to be 327mm dia
spot. And spot I see is much narrower.
What's more, when I change beamwidth of spotlight from 30 to 15 deg, diameter of
spot depicted by camera does not change.
https://imgur.com/2ofeH7W
?
I create 2d mirror to reflect the beam of light one more time. I expect to see
spot twice as wide with intensity diminished 4 times
Instead i see spot of smaller diameter.
https://imgur.com/hnjxNTU
??

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