Light in which phases of all electromagnetic waves in each point on the line of distribution make a right angle with the direction of a beam is called coherent. Such light, as a rule, monochromatic, and its the most widespread source for practical purposes is the laser.
Wave nature of light
Before entering a concept of coherence, it is necessary to understand what is light in terms of the wave theory. Light is the only type of electromagnetic waves which a human eye is capable to notice. Various frequencies of light waves are perceived by people as colors of the rainbow. At the same time red color has the biggest wavelength.
Colors it is accepted to have in process of reduction of wavelength. It looks so: red, orange, yellow, green, blue, blue, violet. Then there is invisible ultraviolet. The more wavelength, the lower it frequency. If the wave has length less, than a visible part of a range, then such radiation call infrared. White color turns out at simultaneous imposing of light waves of various frequency at each other.
Coherent waves
The white light bulb which at the same time radiates a set of various frequencies emits incoherent light. From such source waves which block proceed and extinguish each other and also have the uneven front of distribution. It is the best of all to visualize such case is to imagine the children's drawing consisting of tangled and wavy strips. In turn, coherent light waves of identical frequency are parallel each other. It means that they are not extinguished and, on the contrary, amplify. As a result of it coherent waves have big energy, in comparison with incoherent. Such waves remind the children's drawing of the ocean, with parallel wavy lines which are bent in the same points.
Principle of operation of the laser
Lasers are the most widespread application of coherent light waves in the equipment. Actually the name "laser" is an abbreviation from a phrase "strengthening of light by means of the forced radiation". During the operation of the laser the light waves made with its help repeatedly are reflected in a glass chamber. Also they amplify additional energy in special gas environment (for example, helium or neon) until they become coherent and are emitted in external space.
Holograms
Holographic images in Star Trek style are other application of coherent light waves. They are created by division of a laser beam into two parts. The first half is an object beam. It goes to an object which is scanned, and is reflected back on a film or the writing-down surface. Then there is an interaction to other half – a basic bunch. It creates an interferential picture which then registers. When the film is looked through by means of a coherent light source, in space it is projected 3D - the image.