A laser, 'light amplification by the stimulated emission of radiation', produces an intense beam of light at a single wavelength. The simplest laser consists of a tube of gas through which electricity is passed. The atoms of the gas become excited. The atoms become de excited, giving out only certain wavelengths of light, characteristic of the gas atoms. If this emitted radiation hits a gas atom which is already excited, then it mat stimulate the excited atom to emit radiation of the same frequency exactly in phase with the wave which stimulated the emission. The light is amplified.
If most of the gas atoms are excited, then the light beam can be very intense. The light is reflected repeatedly inside the tube by the mirros at each end, a small amount of light escaping through the 99% mirror.
Lasers can now be made in a wide range of sizes, as small as a pen, to be used by lecturers to point at a part of a board, or as big as a car, to be used for industrial purposes like cutting metal. They can be made using liquids and metals – not just gases.