A lens of focal length f = 40 cm is cut along the diameter into two identical halves. In this process, a layer of the lens t = 1mm in thickness is lost, then the halves are put together to form a composite lens. In between the focal plane and the composite lens, a narrow slit is placed, near the focal plane. The slit is emitting monochromatic light with wavelength λ = 0.6 µm. Behind the lens a screen is located at a distance L = 0.5 m from it. Find the fringe width and number of visible maxima.
Text Solution
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Sol. From lens equation, we have
v = 
(v lines on the left side)
d = 2 
= t 
D = L + v = L + 
Fringe width,
ω = 
= 
or ω = 
as u → f. ω = 
From figure, 
= 
⇒ y = 
(L – f)
Distance of the point upto which interference occurs,
y 0 = 
(L – f) + 
= 
Number of visible maxima = 
= 
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