If light is constant and doesn't end, then how come a light source dims with distance?

Question

Wouldn't the beam be at a constant considering light is constant, so therefore should not dim?

Or should I lay off the booze and go to bed?

Answer 1

Light 'dimming' is due to a decrease in the intensity of the light that reaches your eyes. This is different from the waves loosing energy or becoming 'shorter.'

Think of it this way... When you shoot a shotgun, the pellets are very dense, grouped closely together, when the exit the barrel. As they get farther away from the barrel, they get farther away from each other. This is kind of what happens with a decrease in intensity.

Here's another way, one that is more applicable to your question. Think of the surface of a star. It is, roughly, a sphere. To make this simpler, lets just think of the star as a circle. (It may help to draw this out as I'm explaining it) Let's assume that all of the light that is emitted comes straight out of the surface. A way to picture this is to draw straight lines from the center of the circle outwards. Do this for a few lines that start out very close together and are almost going in the same direction. Extend these lines out very far from the center. Do you see how, as they get farther from the center, they get farther away from each other?

You can also draw a tiny dot some distance away from the circle. This can represent the earth. How many lines that initially start out close together actually intersect this dot? Now can you see why the light gets dimmer? It is because only a small fraction of the light that is sent in our direction actually reaches us.

While it is true that light waves do extend forever, they do not exist everywhere. To describe where something is located, you can only talk about how probable it is to be located somewhere. This is a consequence of the Uncertainty Principle.

If you want to know more, I suggest looking at wikipedia or googling Schroedinger's Equation. There are probably other people who can explain it far better than I could.

Answer 2

In a light flux , if all rays would be moving in parallel in there would be no reduction of intensity. We never receive a ray from the sun directly because is a moving point source and the rays we receive are at angle. An angle spreads out with distance so is the light ray. That is the reason why light atenuates as per inverse proportion to the distance from the source.

What is meant by light being constant is still not understood by
science.I dont believe that constants are really constants in the Universe, except in order to solve an equation it must have at least one constant relative to the other variables..
So constants are relative.

Answer 3

Most sources of light emit the light in all directions so as you move further away from the light source you recieve a smaller proportion of the light (it follows the inverse square law i.e. twice the distance a quarter of the light, three times the distance a ninth of the light). For a coherent beam of light (e.g. a laser) in a vacuum there would be no dimming as the beam doesn't spread.

Answer 4

Conservation of energy man. Only a certain amount of energy started the "beam". Maybe you mean a "point source" in which case the light will spread spherically (1/r^2) in intensity per distance from the source.
If you do mean a beam, the light will still spread out (unless the aperature is only 1 photon in width) due to the same spherical spreading but will attenuate in intensity much slower because it is focused in a certain area.

Answer 5

Light from most objects, like stars, radiates in all directions. Because of that, light becomes more and more scattered with distance. Even the needle-thin laser beam will be 4 miles in diameter when it reaches the Moon.

So, when you look at a star in the sky, you only see a fraction of the light it emmited, because all other light was sent into other directions.

For objects on earth, the atmosphere, mist and pollution in air can diminish light with distance.

Answer 6

Imagine a certain amount of light leaving the bulb - a certain amount of photons. Nearby the bulb, you draw a sphere with the bulb at the center. All the light passes through this surface. As you move further away, all the light is still passing through the surface of the sphere, but the sphere increases in size and the amount of light remains constant, so the flux (amount of light passing through a small surface element) gets smaller as you go further away - you have more surface area of a larger sphere and only the same amount of light you started with. The surface area of the sphere goes as 1/r^2, so the flux goes as 1/r^2 - light decreases with distance as 1/r^2.

Answer 7

I have no real idea but i think the light is constant but it gets filtered by dust particles and so on therefore the further away from the source the more dust and so on it has to travel through and subsequently the more filtering that takes place the dimmer the light. So have another beer !

Answer 8

well the light beams like a torch light will surly fade at a distance.
The light doesint really fade. the ligth is just not brigth enough for u to see the beam of the light . A normal torch ligth. its light waves will spread every where thats another reason thats u caant seee the beam at distance.

Answer 9

if we were in a perfect vacuum, the light would not diminish, which is why we can see stars at night...on a clear night, because particles in the atmosphere, or say, clouds, get in the way. Now lay off the booze and go to bed.

Answer 10

Eri has it right. I would add that in the atmosphere, there is dust in suspension that will diffuse the light.

The question that really make my students think is: If blue light has a shorter wave light and therefore a higher energy, how come it's red light that we can see from the fartest, and thus using it for tail lights and stop signs?