# B Speed of light -- Why is it constant?

1. Mar 24, 2016

### Docscientist

In an electromagnetic wave like light,both wavelength and frequency changes but it's speed always remains constant.Why?

2. Mar 24, 2016

### Staff: Mentor

Are you familiar at all with Special Relativity?

3. Mar 24, 2016

### Docscientist

I'm sorry.I'm not.I just need a short and subtle answer for my question.The Wikipedia page confuses me with a lot of new terms I haven't yet learned.

4. Mar 24, 2016

### Staff: Mentor

Unlike, say, sound, light was discovered to behave as if it is independent of any medium (when in a vacuum). It is measured to travel at C by observers, regardless of inertial reference frame.

That said, it can be forced to travel through a medium like glass or water at a different speed.

5. Mar 24, 2016

### Staff: Mentor

6. Mar 25, 2016

### Docscientist

And that should mean that speed of light is not constant,right?

7. Mar 25, 2016

### Docscientist

8. Mar 25, 2016

### Ibix

Einstein formulated special relativity from two postulates, or two assumptions. One, that the laws of physics are the same in all inertial frames of reference. Two, that the speed of light is constant in all inertial frames of reference. In other words, Einstein offered no explanation for the constancy of light speed. He just showed that if it is constant he could explain the preceding fifty years of confusion in ten pages.

The modern approach is to start from a set of rules of geometry that are slightly different from Euclid's rules, then show that the constant speed of light falls out of them. That approach works well for going onto general relativity, but is a bit abstract. However, it brings out an important point. There is a speed which is invariant - it is the same in all frames. Light happens to travel at this speed (not a coincidence) but nothing changes about relativity if it turns out not to.

Don't worry about the speed of light in a medium. Light and matter interacting is complicated, and light only travels at the invariant speed in free space. In free space, a light pulse will always pass you at c, whatever you are doing. In a medium you aren't talking about just light anymore, so what happens depends on what the medium is doing.