In Young's experiment monochromatic light is used to illuminate the two slits 
and 
. Interference fringes are observed on a screen placed in front of the slits. Now, if a thin glass plate is placed normally in the path of the beam coming from the slit 
, then
Text Solution
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Suppose 
and 
arc the slits at a distance 
from each other.
Distance of screen be 
. Let 
be a point where there is a bright fringe.
A glass plate is placed in the path of the ray from 
to 
.
We know that the path difference between the rays in absence of glass plate is 
In presence of the glass plate, the optical path length of the ray from 
to 
will be different.
The total optical path length for this ray is given by
Where 
is the refractive index of the glass platc and 
is its thickness.
Hence the new path difference is given by
For a bright fringe, 
and 
distance of the bright fringe from the central fringe
Hence the fringe width remains constant.
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