Properties of Light

I have been thinking about something and I just can't wrap my head around it. Maybe someone here can tell me what I'm missing. Here goes...

We all know the Inverse Square Law. If I shine a flashlight or fire a strobe, the amount of light will drop off by a factor of four every time the distance is doubled.

Now lets say I'm looking at a picture hanging on a wall. The picture is illuminated by a light above the picture. I move away from the picture, i.e., I double the distance between me and the picture. But the picture does not appear darker by a factor of four.

Why does reflected light act differently than projected light?

I feel like I'm mixing apples and oranges but I can't explain it (to myself). Can someone illuminate me?

Maybe the image does not seem darker because your eyes are adjusting to the lighting conditions. If you were to meter reflected light I'm fairly certain that you would see the change. By the nature of basic particle dynamics, light reacts the same whether it is projected, reflected, or refracted, it may bend and bounce, but the properties of the light enegry do not change. If you were able to control the dialation of your pupils, and keep them at a constant, I'm sure that there would be an obvious change in luminance strength in regards to reflected light.

With projected light, the unchoherent beam of light diverges (lasers don't diverge like uncoherent light). Because it diverges, as it goes farther it spreads out more, so less light per unit is reaching the target, so 2x the source-to-target distance, 1/4 of the light hits per unit area.

The Inverse Square Law works only for a POINT of light as the source. Anything that is larger than a point source doesn't function the same. I think this may be what is confusing the issue for you.

My brain which is not fully coherent right now. Seems to think part of your problem is
In the flashlight example by moving a foot away the light is now traveling 2 feet farther. one extra foot from you to target and one extra foot from target back to you. So if you double the distance between you and target you are really quadrupling the distance.

With the illuminated picture if you move a foot away the light only has to go an extra foot in distance. So if you double the distance you only doubled the distance.

or so I seem to remember from radar theory class. same principles.

All other effects are haze, spread, diffusion, wavelength and aperture (pupil) related.

At least I think that is it.

RT, Skip is on the right track except is doesn't have to be a point source of light. You are confusing light with brightness. As you move away from the picture on the wall, it is still illuminated by the same source of light so it will be just as bright viewed from 4 feet as viewed from 8 feet. The same light is falling on the picture from the same location.

Try this......stand at a fixed distance from the picture on the wall and have someone move a lamp from 4 feet away from the picture to 8 feet and watch what happens to the brightness of the picture.

Somehow I missed the bit about the picture with the fixed light source. Yes, that is completely different from moving the light source relative to the subject.

Anyhow, back to the inverse square law for a moment. To get the precise numbers from the inverse square formula, you need to have a light source that is a point source. A light with a tightly focussed beam, for example, will act differently from the formula calculations. A light with a huge softbox in front of it will likewise not follow the rule.

Yup, that's what I remember, too, from my Electromagnetics class in college. For a point light source, the intensity is inversely proportional to the square of the distance of the light source. For an infinitely long line light source (i.e. a very, very long florescent bulb), the light intensity is inversely proportional to the distance of the source and for an planar light source of infinite size (i.e. a very, very huge light box), the intensity is constant and does not vary with distance.

The light that is illuminating the picture is subject to the same effect - in a downward motion. Your eye is catching the reflected light from the picture because it is being illuminated by a constant amount of light at any given distance from the light source.

another explanation --

when you move away from the picture, you indeed see less of its light according to the inverse square law. But the apparent size (area) of the picture is also decreasing by an inverse square law. For example, if you double the distance, the amount of light you see from the picture decreases by 1/4, and the apparent area of the picture decreases by 1/4. So the light per unit area remains constant.

Try this -- stand near the illuminated picture in an otherwise dark room, and hold a piece of white paper in front of you. See how much light the paper receives from the bounce off the picture. Now move away, and you'll see the paper getting darker. That demonstrates the total amount of light you're seeing. An incident light meter, if you have one, would confirm the phenomenon.

kind regards,

The light source isn't moving! Only you are. If you took the light with you (like you do with a flash attached to your camera), it would appear alot darker. Only a fraction of the flash from your camera is falling on the picture. The rest is illuminating the wall surrounding the picture.

The "inverse square law" for light is countered by "inverse square law" for your retina.

What's the "inverse square law" for your retina (or sensor for that matter)? The area of the image projected on your retina from an object is inversely proportional to the square of the distance to the object.

So you double the distance to the object. You eye gets four times less light. But the area of the object is four times less. The brightness remains the same.

Anybody ever took physics?

No it doesn't. It demonstrates the reflective light thrown from the painting which is different from what your eye is looking at. Your test would be valuable if the painting it self was a light source you were trying to use. However, as most people here have said, the answer to the original question is that the source illuminating the painting is not moving, so it (the painting) has a constant amount of light hitting it. Your eyes are focusing on the painting at the distance of the painting, not some distance that is between you and the painting (did that make sense?)