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

In summary, the light appears 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. So, wave propagation dissipates over distance becoming space "noise". There are many ways EMR can dissipate. Conservation of energy rules still apply.
  • #1
jaydnul
558
15
Other than nearby light pollution. Or is that the only reason?
 
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  • #2
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
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.
 
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  • #4
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
lundyjb said:
Other than nearby light pollution. Or is that the only reason?

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
completely prepetual motion is impossible.

the EMs rub against air friction to and that dulls the effect
 
  • #7
JKGlover said:
completely prepetual motion is impossible.

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

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
Drakkith said:
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.

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
lundyjb said:
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?

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
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
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
 

1. Why do things appear dimmer as they get further away from a light source?

The main reason for this is due to the inverse square law, which states that the intensity of light decreases as the distance from the source increases. This means that the same amount of light is spread out over a larger area, making it appear dimmer to our eyes.

2. Does this apply to all types of light sources?

Yes, this law applies to all types of light sources, including natural sources like the sun and artificial sources like light bulbs. It is a fundamental property of light and cannot be avoided.

3. Are there any other factors that contribute to this phenomenon?

Aside from the inverse square law, atmospheric conditions such as haze and fog can also play a role in making objects appear dimmer as they get further away. This is because these particles in the air scatter and absorb light, reducing its intensity.

4. Does this mean that light waves never truly travel forever?

While light waves do travel incredibly far, they do eventually lose their energy and dissipate. However, this distance is so vast that it can be considered "forever" in human terms. Additionally, light can also be reflected or refracted, allowing it to travel even further.

5. How does the color of light affect its intensity over distance?

The color of light does not significantly impact its intensity over distance. However, our eyes are more sensitive to certain colors, so objects of the same brightness may appear brighter or dimmer based on their color. This is why we perceive red objects as brighter than blue objects, even if they emit the same amount of light.

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