A laser is a device made out of a substance that will give off light when excited by a source of energy. There are many types of laser but the process of making a laser beam is the same for all of them.

The most common kind of laser is a gas laser. A gas laser is made up of a glass tube filled with a gas and this is stimulated by an electric current going through it. This electric current excites the atoms in the gas and they emit photons. Some of these photons hit other excited atoms which also give off photons, which hit other atoms and so on. The photons emitted by the atoms that are collided with are identical to those that did the colliding. So you end up with a heap of identical photons in terms of energy and phase. The gas tube has a mirror at each end which reflect the emitted photons back into the tube for further collision, but only photons travelling exactly parallel to the tube are reflected by the mirror, others just escape.

In addition, one of the mirrors is only partially reflective, it also lets some of the photons travelling parallel to the tube through. Thus out of one end of the laser a stream of parallel, identical photons is emitted - and this is what a laser is. In addition to gas lasers there are dye lasers which use a liquid coloured by a dye, semiconductor laser chips made from semiconductors, solid state lasers made from rods of solid, transparent materials, and chemical lasers.

Semiconductor lasers are the smallest and can now be made as small as a pin head. For about £100 it is possible to buy one the size of a little finger nail.