# Speed of light .

1. Jun 30, 2011

### quantizedzeus

Speed of light.....

It makes no sense to me that if the speed of light is always c......then in different mediums why this speed changes......?? I may have got it in a wrong way.....But can anyone help me out in detail......(thanks)

2. Jun 30, 2011

### Staff: Mentor

Re: Speed of light.....

The speed of light in a vacuum is always c.

3. Jun 30, 2011

### phinds

Re: Speed of light.....

Does it puzzle you that the speed of sound is different in air than in water?

4. Jun 30, 2011

### ZapperZ

Staff Emeritus
Re: Speed of light.....

Zz.

5. Jun 30, 2011

### srivi

Re: Speed of light.....

The speed of light is constant in vacuum and it changes in different mediums.
when light enters a denser medium (like from air to glass) the speed and wavelength of the
light wave decrease while the frequency stays the same.
How much light slows down depends on the new medium's refractive index, n.

6. Jul 3, 2011

### kevinfrankly

Re: Speed of light.....

speed of light (c = 3 x 10^8) is an constant while on vacuum
but c vary in other medium depend on its density. => n = c/v while n = refractive index.
higher refractive index means less density.
CMIIW

7. Jul 3, 2011

### danR

Re: Speed of light.....

Seems perfectly natural to me. But don't just say the speed changes. It only gets slower in a medium.

But the fact is that light is constantly absorbed and re-emitted in a medium. The light that goes out isn't the light that came in.

When a light-particle bangs into the electron cloud in an atom in a medium, it dies and is disappeared, and an impostor steals its clothes and runs out the same direction a bit later. Everyone is fooled. Even the cops.

8. Jul 5, 2011

### Keyur

Re: Speed of light.....

Speed of light is slower in all mediums except VACUUM.
v=frequency x wavelength

v changes but wavelength doesn't.

as v changes this causes refraction when light enters a different medium.

9. Jul 6, 2011

### danR

Re: Speed of light.....

I wouldn't include a vacuum as a medium, though. And strictly speaking, does c remain as a constant even with a medium, between re-emissions and re-absorptions?

10. Jul 6, 2011

### danR

Re: Speed of light.....

After reading the FAQ on transmission through a medium, I have to abandon the simple adsorbtion, re-emission model, although the wording at a key point in that article is a bit confusing.

11. Jul 8, 2011

### kevinfrankly

Re: Speed of light.....

so the light particle that come in will stay there ?? or what ?

12. Jul 17, 2011

### piroman

Re: Speed of light.....

If the light bulb is in the midle of the room and you turn it on the light reaches south wall same time as north wall. Does that mean you multiply the speed of light times 2?

13. Jul 17, 2011

### Staff: Mentor

Re: Speed of light.....

No, it doesn't. Speed is measured from one object to another. Not from one object to two others at the same time. That's not speed.

14. Jul 18, 2011

### Drakkith

Staff Emeritus
Re: Speed of light.....

You need to read up on what light is. Hit up wikipedia and the FAQ here on PF.