When two, or more waves, travelling through a medium, arrive at a point of the
medium simultaneously, each wave produces its own displacement at that
point, independently of the other wave.
resultant displacement at that point is equal to the vector sum of the
displacements due to all the waves.
of Light: The physical effect (i.e., change in intensity of light)
produced due to the superposition of the waves of light is called the
interference of light.
If two waves of light, arriving simultaneously at a
point, are in phase, (crest of one wave coincides with the crest
of the other wave, trough of one wave coincides with the trough of the
other wave), the resultant displacement at that point is maximum, giving
rise to maximum intensity and hence a point of brightness.
This phenomenon is called constructive
two waves of light, arriving at a point at the same time, are in opposite
phase (crest of one wave coincides with the trough of the other wave
and vice versa), the resultant intensity at that point is minimum
and hence that point is dark.
This phenomenon is called destructive
Steady Interference Pattern: When the
resultant maximum and minimum intensity of two interfering light waves is
observed on a screen, the alternate bright and dark bands are termed as
an interference pattern.
The pattern is said to be steady
if the intensity of light at any given point always remains constant.
In a steady interference pattern, dark points remain
dark and bright points remain bright.
bright and dark bands or rings are observed (depending on the
geometry of the source). These bands or rings are called interference
Conditions Necessary for
Obtaining a Steady Interference Pattern:
The following conditions must be
satisfied in order to observe a steady or stationary interference
two sources of light must be coherent, i.e., there is a
constant phase difference between the light waves emitted by the two
This condition is met only if the
two sources are derived from the same source.
two sources of light must be monochromatic, i.e., the light
emitted is of the same wavelength. (Though light waves of different colours interfere, the resulting interference
pattern is not well defined).
light waves emitted by the two sources must be of equal
amplitudes or the sources must be of same intensity.
For example, The interference pattern produced by
the waves (1) and (2) is
two sources must be narrow. Point sources of light in the
form of illuminated slits are used.
two sources should be close to (near) each other.
For example, the distance between the coherent sources ~10-3
to 10-4 m while the interference pattern is observed on a
screen placed about 1 m away from the source.
two sources should emit waves in nearly the same direction.