LASER LIGHT

Imagine a crowded market place or a railway platform with people entering a gate and going towards all directions. Their footsteps are random and there is no phase correlation between them. On other hand, think of a large number of soldiers in a regulated march. Their footsteps are very well correlated. See figure here.

This is similar to the difference between light emitted by an ordinary source like a candle or a bulb and that emitted by a laser. The acronym LASER stands for light  Amplification by stimulated Emission of Radiation. Since its development in 1960, it has entered into all areas of science and technology. It has found applications in physics, chemistry, biology, medicine, surgery, engineering, etc. There are low power lasers, with a power of  0.5 mW, called pencil lasers, which serve as pointers. There are also lasers of different power, suitable for delicate surgery of eye or glands in the stomach. Finally, there are lasers which can cut or weld steel.

Light emitted from a source in the form of packets of waves. Light coming out from an ordinary source contains a mixture of many wavelengths. There is also no phase relation between the various waves. There fore, such light, even if it is passed through an aperture, spreads very fast and the beam size increases rapidly with distance. In the case of laser light, the wave length of each packet is almost the same. Also the average length of packet of waves is much larger. This means that there is better phase correlation over a longer duration of time. This results in reducing the divergence of laser beam substantially.

If there are N atoms in a source, each emitting light with intensity I, then the total intensity produced by an ordinary source is proportional to NI, where as in a laser source, it is proportional to NNI (NN means N square). Considering that N is very large, we see that the light from laser can be much stronger than that from an ordinary source.

When astronauts of the Apollo  missions visited the moon, they place a mirror on its surface, facing the Earth. Then scientists on the Earth sent a strong laser beam, which was reflected by the mirror on the moon and received back on the Earth. the size of the reflected laser beam and the time taken for round trip were measured. This allowed a very accurate determination of (a) the extremely small divergence of a laser beam and (b) the distance of the moon from the earth.