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.
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.
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.