# Does speed of light depend on the wavelength or frequency of it?

• nineteen
In summary, the speed of light is a constant in vacuum and does not depend on the wavelength or frequency of it. This means that for a light wave with a certain frequency, the wavelength is fixed at a specific value. In a medium, such as glass or water, the speed of light may vary with frequency, but this still means that the wavelength is determined by the chosen frequency and medium.

#### nineteen

I heard that the speed of light doesn't depend on the wavelength or frequency of it, how does it happen? I don't understand it because the ultimate equation is c = fλ (where c is the speed of velocity of light, f is the frequency and λ is the wavelength of it...).

As the frequency increases the wavelength decreases, so their product doesn’t change.

nineteen and davenn
nineteen said:
I heard that the speed of light doesn't depend on the wavelength or frequency of it
Where did you hear this? It's wrong in general.

The speed of light is a constant in vacuum. There isn't really an answer to why that is at a B level - it's just the way the world works. In terms of your ##c=f\lambda##, it means that a light wave of frequency ##f## must have wavelength ##c/f##. You don't get to choose both ##f## and ##\lambda##.

For light in a medium, such as glass or water, the speed of light does vary with frequency. But again, this means that once you've chosen a frequency and a medium the wavelength is fixed, because you are not free to choose the speed and the frequency independently.

nineteen, vanhees71 and davenn