Interference

Coherent sources are those which emit light waves of the same wavelengths or frequency which are always in phase with each other or have a constant phase difference. Two coherent sources can produce the phenomenon of interference.           

The colors that we see when sunlight falls on a soap bubble, a spot of oil on a wet pavement or a ruby throated hummingbird are caused by the interference of light waves reflected from the front and back surfaces of thin transparent films. It depends on the fact that two sets of waves arriving at the same plane will add up their effects if they arrive in step but will cancel each other out if they arrive out of step. Their combined effect is obtained by adding algebraically the displacements at the point due to sources individually. This is known as the principle of superposition. Thomas Young discovered this interference principle around 1800.The film thickness is typically of the order of magnitude of the wavelength of light. Thin films deposited on optical components such as camera lenses can reduce reflection and enhance the intensity of the transmitted light. Thin coatings on windows can enhance the reflectivity for infrared radiation while having less effect on the visible radiation. In this way it is possible to reduce the heating effect of sunlight on a building.

Depending on its thickness, a thin film can be perfectly reflecting or perfectly transmitting for light of a given wavelength as shown in Figure 11.These effects result from constructive or destructive interference.