CHAPTER 18 : PHYSICAL OPTICS
18.1 Interference of light
The intricate phenomena like interference, diffraction
and polarization of light which occur due to wave nature of light
comprise the branch of optics known as Physical Optics.
When two "sources" of light that are
monochromatic, coherent, small in size, close to each other and
which produce waves of same amplitude are used, then on a screen
far away (compared to distance between the "sources")
a pattern of alternate bright and dark bands or fringes are observed.
This pattern is called the interference pattern.
The bright bands occur, due to reinforcements,
when crests (or troughs) of both the waves overlap or superimpose
on each other. The dark bands occur due to annulment when the crest
of one (or trough) over laps on the trough (or crest) of the other
The result of interference between waves in given by the superposition
principle : "The resultant displacement (vibration) of an ether
particle receiving two or more waves simultaneously is the vector
sum of displacements provided by each wave independently of the
When the crests (troughs) of two waves which have
the same wavelength, amplitude and velocity and which travel almost
along the same path and in the same direction, are superimposed
then according to the super position principle, the resultant amplitude
will be twice the amplitude of either of the two interfering waves;
hence intensity which is proportional to the square of amplitude
will be extremely large. The point on the screen where this happens
will therefore be very bright; this type of interference is called
The condition for constructive interference is
that the optical path difference ' D
' should be an integral multiple of wave length (see figure.1).