# Is the value of c ( speed of light ) is constant ?

1. May 17, 2012

### kevinsetiono

is the value of c is constant , although the light through other media , like glasses or water or vacuum ?

2. May 17, 2012

### Staff: Mentor

c is the speed of light in a vacuum. That value is an invariant: it doesn't depend on how fast the source or the observer is moving.

3. May 17, 2012

### DrGreg

...and, to clarify, the speed of light through non-vacuum media isn't constant, but usually isn't written c either.

4. May 17, 2012

### Emilyjoint

Not written as c !!!!! what is it written as?.... do you mean it has to have a suffix like 'a' for air, 'g' for glass etc?

5. May 17, 2012

### Whovian

Do remember that the symbols are arbitrary symbols given to certain constants. What it's written as probably varies.

6. May 17, 2012

### Emilyjoint

so you can use whatever you want in physics?
our books have c as the speed of light. We have even found that c is being used as the normal speed symbolk for waves.
Why is it that information given here is usually the opposite of what we do in school

7. May 17, 2012

### DrGreg

Well, I did say "usually". Not all books use the same conventions, so maybe your books use a different convention than other books use. Sometimes even in the same book the same symbol is used with different meanings. So it's important to read the words that accompany the equations as well as the equations themselves. And if you are reading several different books you need to check whether they use the same symbols or not.

8. May 17, 2012

### Steely Dan

The speed of light through any particular medium is usually written as $c / n$, where $n$ is the index of refraction of the medium.

9. May 17, 2012

### Drakkith

Staff Emeritus
Your book is most likely talking about the speed of light in a vacuum. That is what c stands for. In a medium light does not travel at c, so you cannot use it unless you say 0.9c or whatever it's velocity is in that medium.
Edit: (Or c/n as steely dan pointed out)

10. May 18, 2012

### kevinsetiono

so , the c will be change if light walk through some media , will it increased or decreased ?

11. May 18, 2012

### jssamp

It decreases, it can't increase. c is the speed limit.

12. May 18, 2012

### Drakkith

Staff Emeritus
The constant c has a specific value equal to the speed that light travels in a vacuum. Light traveling through a medium does not travel at c but at a slower velocity. c never changes, it is a constant.