# Frequency- What is it?

## Main Question or Discussion Point

Some questions about frequency, doppler effect, and mass.

What exactly is frequency? I know its an intrinsic property, but there are a few things that dont make sence.

How can light waves be stretched out or compressed by the doppler effect if frequency is intrinsic? It makes sense with sound waves- the wave is composed of many air particles, so the wave can be compressed by moving the high pressure and low pressure areas together. But i see no way that an intrinsic property such as frequency can be affected by relative motion.
Also, high frequency photons are high energy, and low frequency photons are low energy. So when light is red shifted, what happens to the missing energy? And when light is blue shifted, where did the extra energy come from? nowhere? The doppler effect makes no sense!

Matter has frequency too. So, if you are moving toward a matter particle, then the frequency of the particle is increasing. And vice versa. So does the doppler effect exist with matter as well?

Another thing:
The energy of a lightwave is given by e=nhf
where n is number of photons, h is plancks constant, and f is frequency.

The conversion rate of matter to energy is e=mc^2

so, the relationship between mass-energy of a matter particle and light-energy of a photon is mc^2=nhf.

Now think about it. m is to f as h is to c^2.
Why? Because mass and frequency are measures of energy, while h-bar and the speed of light are constants. (n is simply the number of photons, which really doesnt matter because by reducing the number of photons to 1 and increasing frequency you would still see the relation between the energy of a single photon and the energy of a single matter particle of a given mass.)
Does anybody else find this very interesting?

Does this mean there is some kind of relationship between mass and frequency? Perhaps mass and frequency are different manifestations of the same thing.

## Answers and Replies

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Fredrik
Staff Emeritus
You probably shouldn't use the relativistic mass when you talk about photons. Use $E^2=\vec pc^2+m^2c^4$ instead, where m is the rest mass. The reason I don't like the idea of relativistic mass when we're talking about massless particles is that the only way to define it is to put the left-hand side of the equation above equal to "relativistic mass"2c4. That's pretty pointless. That concept makes more sense for massive particles when you can define it as $\gamma m$.