Photons are tiny packets, or quanta, of light, and have energy in the form of electromagnetism. They do not have mass, but they do have momentum – a property in physics which is usually attributed to an object’s mass. However, the momentum of a photon is not dependent on mass but on its frequency. One way of imagining a photon is not to think of it as a particle, but as a little packet of energy made of oscillating electric and magnetic fields. Like any wave it has a frequency which determines the type of radiation it makes up. If it’s a low frequency it might be radio waves whereas a high frequency could be X-rays.
Another odd effect that makes light ‘appear’ to have mass is that it interacts with gravity. For instance, light follows curved paths around stars and cannot escape from a black hole, however this is not due to it having mass. Einstein’s Theory of General Relativity explains how stars and black holes have so much gravity that space and time around them becomes warped. In this scenario, the light is travelling in a straight line, however its path has become curved due to the bending of space-time.
Answered by Rik Sargent, HIW contributor.