Simple Light Physics Question?

Question

How does an illuminated area on a screen change as a point light source moves away?

Now at first, i would guess that it gets larger, but when i think about it, wouldnt the illuminated area be the EXACT same? Since light comes out in infinite rays around a point source, the whole screen would be lit up close AND far away right?

either from close up or far, you can ALWAYS draw a ray connecting the point and any part of the screen right? does that mean the whole screen will be lit up no matter where the point light source is?

I dont know, i drew a ray diagram, but im still so confused Can someone help?

So the change is, that the light gets fainter? or more spread out right? the illuminated AREA is still the same, but the concentration is less?

i no the rays spread out, but the area illuminated should still be the same?

Answer 1

Let's try looking at this from a different perspective.

Your light source will continue to put out the same amount of light, reguardless of the distance to the screen. In other words, the same number of photons.

Let's start with a screen 5 feet away from your source, say we have a 6 inch diameter circle that is illuminated. Now we count the photons that are striking this area, let see, 1, 2, 3, 4, 5, 6, 7, 8, 9. . . well since one of my fingers hurt, I don't want to use it, and since I don't feel like taking off my shoes, let's say 9 photons.

Now we go back to 10 feet. Suddenly we are illumiating a 12 inch diameter circle, but the number of photons has remained constant, we have the same number of photons illuminating twice the area. WRONG. We have the same number of photons illuminating 4 time the area. If you double the diameter, the then area increases by 4 (the square of the difference).

So, it looks like the intensity (energy) of light decreases with the square of the distance!

WOW, seems like I have heard that before.

You would still have the same number of photons hitting the screen convering 4 times the area, so you would be illuminating a greater area, but it would only be 1/4 as bright.

Gee, guess my high school physics teacher knew what he was talking about after all. . .

Answer 2

If the light comes from a point the image would get smaller as the point moves away.
T think of a circle paralel to the screen.
The point and the light rays form a cone which is cut by the screen.
As more away is the light point the smaller would be the image.

If the poit moves to infinity,we considerer that all light ray s are parallel and the image would be of the same size as the oject

Answer 3

it changes becasue the light rays spread out more try drawing the ray diagram at one point then take the same rays and draw them from ferther back and you will se that it will change!!!!!
hope that helps

Answer 4

complicated issue. query into search engines like google. that will can assist!