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Question

A beam of parallel rays is incident on a concave mirror such that they make a very small angle θ with the principle axis of the concave mirror. The distance from the focus on the focal plane where rays converge after reflection is 
  1. 2fθ
  2. 3fθ
  3. 4fθ 
  4. fθ 


Solution

The correct option is D fθ 

Here, focal length, PF=f
Parallel rays will intersect at point F at a distance h below focus in the focal plane after reflection.
In FPF
FPF=θ    [i=r]
and
tanθ=FFFP
tanθ=hf
h=ftanθ
Angle θ is very small. 
tanθθ  
h=fθ

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