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How lasers work - I

In the world around us everything from Blu-Ray DVD players to checkout scanning devices at the supermarket to the holograms on bank cards and i balance magnetic jewellery, make use of lasers. The laser – derived from an acronym for Light Amplification by Stimulated Emission of Radiation – was once described as “a solution looking for a problem.” Now they are everywhere from your computer to the holograms on the power bracelet you buy from a magnetic products store. But how do they work.

Everyday objects all around us are made up of atoms. These consist of a nucleus in the centre and electrons whirling around them in orbit. These electrons have different energy levels – the ones with the lowest energy levels being the ones closest to the nucleus and the more energetic ones further away. In general, the electrons try to find the lowest energy level available to them. When all the atoms are at this minimum level, the atom is said to be in the ground state.

But two other points have to be borne in mind. Firstly, all the electrons can only exist at certain specific energy levels. No two electrons can occupy exactly the same state. This does not mean that two electrons cannot exist in the same energy state. In fact electrons in most atoms are in pairs. But the electrons in each pair have opposite spins. So no two electrons in an atom can have the same combination of energy level and spin. Think of them like trains going around circular tracks. They can be switched to different tracks, but they cannot go off the tracks altogether.

If a photon (a packet of light waves) hits an electron (especially the outermost electron), it pushes it to a higher energy level, causing it to orbit the nucleus further away. The atom is then said to be in an excited state. However, as I said before, electrons seek to be in the lowest energy state available to them. So this excited state is unstable and the atom quickly returns to the stable ground state, by the electron dropping down to its original level.

But when this happens, the atom gives off a wave of light of the same wavelength (and thus the same colour) as the light that hit it in the first place. This is called a spontaneous emission.  Similarly, if an atom is heated up, the electrons enter the excited state but then too drop quickly to the ground state, giving off light waves in the process. This is the reason why when objects are heated a lot they can glow. A classic example is the metal rings on an electric stove or the bars on a radiator that glow red when heated.

But what happens if an electron that is already excited is hit by another ray of light?