# I do not live in a vacuum

1. May 20, 2006

### eggman

So how fast is light moving ?

....does atmosphere slow it down?

or when it deflects off and object does it slow down?

....does it slow over distance like a rifle bullet ?

If I walk into the back yard under a tree...how fast light be?

2. May 20, 2006

### eggman

They can slow light according to wikapedia..(i just found out)

Why can't it..apparently being a 'thing' be captured and held onto ?

3. May 20, 2006

### Hootenanny

Staff Emeritus
Yes, the speed of light is reduced in the atmosphere. Reflection is slightly more complex, however it is simple enough to say that when the light ray leaves the mirror it is travelling at the same speed it 'struck' the mirror. No it doesn't slow over distance.

~H

4. May 20, 2006

### Hootenanny

Staff Emeritus
Congratulations, you have discovered the principle behind refraction.

~H

5. May 20, 2006

### eggman

Thanks...wikapedia is good

6. May 20, 2006

### Hurkyl

Staff Emeritus
Light can be "captured" too. In some sense, that's exactly what's happening when you are warmed by sunlight! But they have done experiments where they capture light and then release it in roughly its original form.

But the words "captured" and "slow" are describing the macroscopic behaviors of light, and not individual photons which always travel at c throughout their entire existence.

7. May 20, 2006

### Hootenanny

Staff Emeritus
Further to what Hurkyl said. The observed speed of light in a medium is different to the actual speed of light. And to reiterate what Hurkly said, photons always travel at c throughout their lifetime. Wiki explains it in easy to understand terms;

So yes, light does change its speed when considered on the macroscopic scale, however, photons always travel at c. Therefore, the speed of photons is always constant.

~H

8. May 20, 2006

### eggman

Where does light come from ?

What is its origin ?

it appears to be the constant...everything else is measured in relation to it..correct?

Light is a very fascinating subject...without it...nothing would be...

(no i am not a philosopher if u are wondering)

9. May 20, 2006

### eggman

Is'nt light a mystery to a physicist?

...yeah...a mad rush to nowhere : )

10. May 20, 2006

### ZapperZ

Staff Emeritus
I know that in all the "excitement" of finding a new forum, you want to ask all of these questions. May I suggest you get use to how this forum is run first, and what have already been asked by looking through a few threads in several of our forums. You'll do yourself a lot of favor by discovering that a lot of what you want to know have already been answered.

Start first with our FAQ in the General Physics forum.

Zz.

11. May 20, 2006

### rcgldr

Is there any evidence that light changes frequency over time, like the frequency getting lower over long periods of time (an optional explanation for red-shift of light from very far away stars)?

12. May 20, 2006

### eggman

do u mean is light subject to a 'decay' ?

...thats a good question...if i interpret it correctly

13. May 20, 2006

### Lyuokdea

That would violate conservation of energy:

For a single photon:

$$E = hf = \frac{hc}{\lambda}$$

If the frequency or wavelength were to shift the amount of energy carried by the photon would change accordingly, and energy wouldn't be conserved.

~Lyuokdea

14. May 20, 2006

### kmarinas86

This definition of energy does not take into account potential energy.

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

Total energy is conserved.

15. May 20, 2006

### Lyuokdea

Good call, I didn't stop to think about the gravitational dependence of the question.

~Lyuokdea

16. May 20, 2006

### Mk

I've been impressed several times by Lyuokdea's posts. Very good.