Causes of Colour

Introduction

Colour is a fundamental part of our visual perception, but the underlying causes are often complex and diverse. The spectrum of colour is the result of a wide range of electronic interactions in matter, all of which can be traced back to the properties of electrons. In this article, we will examine the main mechanisms responsible for generating colour in nature and in man-made materials.

Electronic Transitions

The most common cause of colour is the interaction between light and the electronic structure of atoms and molecules. Atoms have a specific set of allowed energy levels that electrons can occupy, comparable to the rungs of a ladder. When an electron is excited to a higher level, it can emit a photon upon falling back to a lower level, with an energy corresponding to the difference between the levels. The precise energy differences determine the colour of the emitted light.

This principle underlies the characteristic emission colours of gas discharge lamps, such as the yellow of sodium vapour lamps and the blue of mercury lamps (Cassidy et al., 2017). The vivid colours of certain gemstones, such as ruby and emerald, are also caused by electronic transitions in the chromium ions they contain (Nassau, 1983).