# I Constant frequency of light and connection to QM

1. Apr 17, 2017

### Medtner

Could someone provide a good explanation as to why light's frequency doesn't change under refraction? The textbook I'm using gave us a two step derivation and I don't really feel like it did a proper job in explaining anything (it's Haliday).

Also, a slightly *crackpot* follow up question - does it have somewhat of a deeper connection to QM, and how photons don't change their velocity in different mediums?

2. Apr 17, 2017

### rumborak

I think the most straightforward answer is, after all there has to be the same E field on both sides of the boundary. Imagine the E field oscillating with one frequency on one side, and with another frequency at the other. There would always be a constantly changing discontinuity.

3. Apr 17, 2017

### Staff: Mentor

But they do... Velocity and wavelength change, but frequency stays the same (see post above by @rumborak ).

4. Apr 17, 2017

### pixel

As not all of us have that book available, maybe you could reproduce it's explanation so we would know where to start.

As a handwaving argument involving a different type of wave, think of a rope that is attached to a second rope with different mass per unit length (hence different wave speed). If you start transverse waves in the first rope segment, as the peak amplitude entered the second segment it would induce a wave peak in that segment, one for one for each wave generated. So the number of waves per second has to be the same in both media.