Does Moon revolves itself ?

[Iggy]

The arithmetic is solid, Iggy, but I think you're off a little on the theory. It may be anti-intuitive, but it doesn't matter how big the light source is (the radius of the aperture), not once the measurement is taken at a distance 10+ times the source radius (5+ times the diameter). That's the minimum distance for the source to be considered a point source.

 

Nature does not follow your approximations. If you put an aperture on a distant star cutting its radius in half then a person on earth would find the star one quarter as bright by the action of the aperture--whether or not you consider it a point source at that distance. Your idea that nature gives up following physics once you approximate something as a point souse is not right.

 

The formula isn't r/d² (where r is the radius of the light source)

 

It is precisely right. r is the radius of the Lambertian radiating body, d is the distance to that body and (r/d)² is the factor by which brightness is reduced for the person viewing the object at distance d.

 

The inverse square formula is: i/d² (where i is the intensity of the photon flux)

 

Yes, but 1/d² gives only the factor by which flux per unit surface area gets smaller. The formula I gave accounts for the radius of the radiating body. John was precisely correct.

[John Cuthber]

All good stuff and I'm pleased to get some support.

The overriding consideration is that (f you will forgive the pun) the point is that they are all points.

The torch, a distant star etc. are all practically point sources and here's the bit that Akhenaten doesn't seem to get.

They don't give a point image on the retina.

In all cases they give a slodge- the so called point spread function, which is about the same size as the receptor cells there (which is, as Radical Edward pointed out a pretty neat fact).

Since all the photons from any "point like" source fall on the same small bunch of cells at the back of the eye the eye sees them just fine. A bit of twinkling and scattering doesn't help.