# Quick Question

1. Feb 12, 2008

### Alex48674

I know that as space expands it cause light to force itself to increase its wavelength, but I can't seem to remember how it relates to a decrease in energy, and thus a decrease in temperature. I know that in this respect energy is related to wavelength as opposed to how it classically related to amplitude.

Thanks, I hope I've got that much right so far =]

2. Feb 12, 2008

### Wallace

From quantum mechanics, the energy of a photon is given by

$$E=hf$$

where h is a constant (Plancks constant) and f is the frequency of the wave. Using the basic wave equation

$$v=f\lambda$$

where $$\lambda$$ is the wavelength, and taking the velocity of light to be c we get

$$E=\frac{hc}{\lambda}$$

So, as the 'classical' wavelength increases, the energy of the photon decreases.

I hope that helps, let me know if I'm being too equationy!

3. Feb 12, 2008

### Alex48674

Perfect thanks =]