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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 wave.

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 other."

Constructive Interference

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 constructive interference.

The condition for constructive interference is that the optical path difference ' D ' should be an integral multiple of wave length (see figure.1).

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18.1 Interference of light
18.2 Diffraction of Light
18.3 Polarization of light

Chapter 19

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