Colours and spectra

Before we can finish our exploration of spin, we need to understand a bit more about light.

We learned in an earlier post that light is made of photons, that photons are tiny ripples of electromagnetism, that each photon vibrates at a particular rate or frequency, and that the frequency of a photon determines the colour it appears to be when it interacts with our eye.

Well before the development of quantum theory scientists had found that each element had a distinctive set of colours which it would absorb when light shone upon it. The element would give off exactly the same set of colours if it was heated up sufficiently.  The set of colours is called the element’s spectrum. When we talk about the colours being given off by an element we use the phrase ‘emission spectrum’. When we talk about the colours being absorbed by the element we say ‘absorption spectrum.’ The actual colours are the same in both cases.

The pictures below show the emission spectra of hydrogen and iron, as well as the spectrum of all the frequencies of light.