A parallel beam of monochromatic light falls on a thin converging lens, whose material has refractive index 1.5. After two internal reflections, one at each face of the lens, the light emerges. Prove that the beam will meet the optical axis at a distance f/7 from the lens, where f is the focal length.
Text Solution
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Sol. Refraction at air-glass interface (surface I)
Using 
– 
= 
, we get
– 
= 
R = f
Now, this image will act as an object for reflection at surface II
∴ 
+ 
= 
or 
= – 
Now, this image acts as an object for surface I
∴ 
+ 
= 
or 
= – 
Finally, considering refraction at surface II (Glass-air interface)
– 
= 
or v = 
Hence, the focus is at a distance 
from the lens.
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