# Spectral index

Sanjay87
Hi,

Can anybody please explain to me (in everyday language) what spectral index is? The wikipedia entry doesn't shed much light, and a lot of the textbooks give very convoluted explanations for its derivation.

Thanks.

In radio terms (in my experience), spectral index applies to synchrotron sources which have brightness, S, which changes with for different frequencies, $$\nu$$, such that $$S \propto \nu^{\alpha}$$. For a negative $$\alpha$$ this means that when a synchrotron source is observed at higher frequencies, it will be fainter. The opposite is true for positive $$\alpha$$.