# If EM waves travel forever, why do things get dimmer as they get further away?

1. Oct 19, 2012

### jaydnul

Other than nearby light pollution. Or is that the only reason?

2. Oct 19, 2012

### Zula110100100

Things also appear dimmer from further because the waves are more 'spread out', imagine a sphere with lines coming straight from the center, the further out you get, the more distance between those lines.

3. Oct 19, 2012

### Physics_Kid

so, wave propagation dissipates over distance becoming space "noise". there are many ways EMR can dissipate. conservation of energy rules still apply.

you are questioning the intensity of the observation. infinite distance requires infinitely sized detector.

Last edited: Oct 19, 2012
4. Oct 20, 2012

### Staff: Mentor

The energy of any section of an EM wave falls off with the inverse square of the distance. IE as you double the distance from the emitter, the energy is now 1/4 of what it was. Quadruple the distance and it's 1/16th as much. The energy of the entire wavefront MUST stay equal, so as the wavefront expands it now takes up more area, so the energy per unit area MUST drop.

http://en.wikipedia.org/wiki/Inverse_square_law

5. Oct 20, 2012

### sophiecentaur

Nearby light pollution is very relevant along with the random noise in our eyes and in image sensors. They all serve to dilute the light energy reaching us from a distant source and reduce the information we get about it. The expression is Signal to Noise Ratio and it gets worse and worse as the received signal gets weaker.

6. Oct 20, 2012

### JKGlover

completely prepetual motion is impossible.

the EMs rub against air friction to and that dulls the effect

7. Oct 21, 2012

### Staff: Mentor

This is entirely incorrect. EM waves travel until they are absorbed by something but the wave loses energy as it spreads out.
The light can be scattered by air, but in the absence of something to scatter it the wave still loses intensity and appears dimmer.
This isn't air friction either. Air friction occurs when a physical material is moving through air or the air is moving over it. Light is not a physical material.

8. Oct 22, 2012

### jaydnul

When you say the light spreads out, do you mean the photons themselves are stretching out or the photons are spreading out away from each other?

9. Oct 22, 2012

### Staff: Mentor

If anything say the photons are spreading out away from each other. Remember that photons are not matter particles and you cannot associate a physical "size" to them. They don't stretch out or compress. They are simply the interaction of the EM wave with matter. Even when thinking about redshift/blueshift you cannot say that the photons are stretched or compressed, but that the EM wave itself is a different frequency.

10. Oct 22, 2012

### SHISHKABOB

imagine a bunch of marbles being dropped together onto the floor. Like you dumped them out all at once from a bag (no bouncing >.>)

they will at start at the middle, and spread out in all directions. At first they will all be pretty close to each other, but as they continue to head away from where you dropped them, they are further apart from each other.

Photons act in pretty much the same way, except there are A LOT of them. The light that you see is a constant stream of some bunch of photons that are hitting your eye.

In the marble case, your eye would be like a dust pan that is set on the floor in the way of the marbles. Closer to where you dumped them, the dust pan would pick up a bunch of them. As you move the dust pan further away, it would pick up fewer and fewer of them.

So as we get further away from a source of photons, our eyes pick up fewer and fewer of them.

11. Oct 22, 2012

### davenn

I like that explanation SHISHKABOB :)

hopefully that will get the idea across
I was thinking of waterdroplets out of a single hole sprinkler and how they spread out

Dave