No embarrassment because I think you have it, but you're expressing it in an unconventional way. Actually this is a lot easier to explain with a pencil and paper, but the rule is normally expressed by saying that the total amount of light that a given surface of constant area will receive from a point source of light will diminish with the inverse square of distance, because light radiates out from a point source at a constant angle. The further the surface from the source, the less actual light energy falls on the surface, either to be reflected or absorbed.Quote:
Originally Posted by trish
(If the surface maintained its angular relation to the light rays, ie expanded proportionally to distance, then the total amount of light falling on it would remain the same. This does not mean that the surface will necessarily be evenly illuminated, however, and it won't be if it is flat, but if the surface is part of a sphere it will, which BTW is why the images from Hasselblad SWC lenses show some light fall-off in the corners. The same effect is seen looking at the inside of a cylindrical lamp-shade, which will appear evenly lit, but if we take off the shade and put the lamp at the same distance from a white wall, we see a hot-spot. But I digress.)
However the apparent brightness of a source of light or a light-reflecting surface remains the same, irrespective of distance, as long as we maintain the angle of view. (Assuming ideal conditions; over very long distances light is absorbed and reflected by intervening matter, such as dust or vapour.) You can test this if you have access to a spot light-meter, by reading from an object 1 metre away and then again much further away; the reading will not change.
What this means is that how bright an object reflecting light appears is related to the distance from the light source to the object, not the object to the viewer. This is why distant objects, mountains etc, appear as brightly lit as near objects. The sun is so far away it can be considered as at infinity, which is why the tops of mountains do not appear brighter than the valleys below them.
Since a diminution by the inverse square of distance is actually quite a lot, then if the way you put it initially were true we would all be walking around at the centre of brightly-illuminated pools of light, surrounded by total darkness.
(Yes I know some people here ARE surrounded by total darkness, but Trish, you are definitely not one of them.)
Nice to talk to you, as always.
