# Question about frequency versus wavelength

1. Mar 7, 2008

### noospace

This is something I really should know but found I was unable to explain it to myself. When a ray of light passes from one medium to another its frequency remains invariant, but it slows down, forcing the wavelength to decrease according to $c = \nu\lambda$.

The frequency of the wave will correspond to the frequency of the elementary charged oscillators responsible for its production.
What is it about the frequency of the wave that leaves it unaffected, as opposed to the wavelength which is free to change? This leads me to a related question: is it practically possible to change the frequency of a ray of light, how would one do that anyway?

2. Mar 7, 2008

### dst

Because the energy of a photon is dictated by its frequency, and not its wavelength, it must remain unchanged. As far as I know, anyway.

3. Mar 7, 2008

### noospace

Conservation of energy eh? I like that explanation.

What assurance do we have that the photon does not exchange energy with its surroundings in passing from one medium to another?

Is it it possible to `bump up' the energy of a photon that is part of a self-propagating electromagnetic wave, thereby changing its frequency?

4. Mar 7, 2008

### DaveC426913

I speak out of turn here but -

Seems to me, the photon would have to be absorbed and then another emitted at a different freq.

Which is fluorescence. Or is it phosphorescence?

5. Mar 8, 2008

### pam

It is really a simple classical effect. In going from one medium to another the wave boundary conditions at the interface must hold at all times. This requires the time dependence of the transmitted wave to be the same as that of the incident wave.

6. Mar 8, 2008

### Staff: Mentor

The wave has to be continuous across the boundary at all times. This can be true only if the wave oscillates at the same frequency on both sides.

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7. Mar 10, 2008

### Claude Bile

Nonlinear optical materials are used to convert laser light from one frequency to another. This is done by exploiting that fact that in a nonlinear material, the frequencies you get out do not necessarily equal the frequencies you put in.

Essentially, by shaking atoms hard enough, you can get them to wobble with additional frequency components, such as the second and third harmonics.

One widespread use of this phenomenon is in the use of Nd:YAG lasers (which have a wavelength of 1064 nm) to generate 532 nm (green) light via the generation of a second harmonic within a crystal such as Lithium Niobate.

Claude.

8. Mar 10, 2008

### Lee_Loveridge

Normally you change medium at a fixed place and continuously in time. If you could change the index of refraction of an extended area, but at the same time, then the wavelength would stay fixed and the frequency would change. A cute thought problem, but I'm not certain if it has ever been done experimentally.