# What determines the colour? Frequency or wavelenght?

1. Jul 25, 2004

### darkar

What determines the colour? Frequency or wavelenght?

2. Jul 25, 2004

### marlon

Normally high frequencies correspond to small wavelengths, and this correspond to colours that are blue-shifted. In the EM-spectrum blue is high energy, and red is low energy. these colors come from the EM-variant of the Doppler-effect. An EM-spectrum can be "made" when reflected sunlight from a planet undergoed emission or absorption processes when passing through our atmosphere. So we will see lines that move to blue when a planet is moving towards us. lines will be moved to red when the planet moves away from us. This is the EM-Dopplereffect.

3. Jul 25, 2004

Staff Emeritus
Frequency times wavelength is the speed of light, so they are inversely related and either of them can be used.

4. Jul 31, 2004

### salsero

That's not a complete answer: the wavelength depends also on the medium through which the light passes. The color we see is determined by the frequency.

5. Aug 4, 2004

### Gonzolo

It depends on the energy of the photons our retina cells receive. This energy would be $$E = hf$$ or $$E = hv/\lambda_{eye fluid}$$, with h plank's constant, and $$v = c/n_{eye fluid}$$ is the speed of light through the fluid in our eye.

6. Aug 4, 2004

### Staff: Mentor

Just a note: The energy of a photon depends on its frequency, which does not change when light passes from one medium to another.