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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Interference of Light Know More

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Huygen's theory assumes that light propagates in the form of waves.

When the interference takes place with two non-coherent sources, I = I1 + I2

When two waves having amplitudes a1 and a2 and a phase difference f superpose at a point, the resultant amplitude is given by
A2 =
a12 + a22 + 2a1a2cosq
\ I = I1 + I2 + cos q

If a ray of light is incident on a thin film of thickness t and refractive index m, the refracted rays interfere with each other.

The unit of luminous intensity is candela.

For an interference pattern using white light, the central fringe is white while the first few fringes are coloured.
Beyond a few coloured fringes, the intensity of maxima of different wavelengths overlap resulting in white light.

Colours and Wavelengths


3,900 - 3,600 A


4,300 - 3,900 A


5,000 - 4,300 A


5,600 - 5,000 A


6,100 - 5,600 A


6,500 - 6,100 A


7,500 - 6,500 A


Fringe visibility

Fringe contrast

If the Young's double slit experiment is performed in a medium of refractive index 'm', the fringe width is given by

X =

When one of the slit is partially closed, the contrast between the fringes decreases.
If each of the slit is illuminated by two independent sources, no interference pattern is obtained.
If one is illuminated by red light and other is illuminated by violet light, no interference takes place.

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