Question
Sat March 14, 2015 By: Shagun Rastogi

i wanted to know about first principal focus and second principal focus . 

Expert Reply
Faiza Lambe
Mon March 16, 2015

First principal focus:

First principal focus of a lens is that point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity.

It is denoted by F1.


begin mathsize 14px style negative 1 over straight u plus 1 over straight v equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses end style
Putting, u=-f1 and v=begin mathsize 14px style infinity end style in the above equation, we get,
begin mathsize 14px style negative fraction numerator 1 over denominator negative straight f subscript 1 end fraction plus 1 over infinity equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses or 1 over straight f subscript 1 equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses end style
 
 
Second Principal Focus:
 
Second principal focus of a lens is that point on the principal axis of the lens where the image is formed when the object is at infinity.
It is denoted by F2.

begin mathsize 14px style negative 1 over straight u plus 1 over straight v equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses end style
Putting, u=-begin mathsize 14px style infinity end style and v=f2 in the above equation, we get,
begin mathsize 14px style negative fraction numerator 1 over denominator negative infinity end fraction plus 1 over straight f subscript 2 equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses or 1 over straight f subscript 2 equals left parenthesis straight mu minus 1 right parenthesis open parentheses 1 over straight R subscript 1 minus 1 over straight R subscript 2 close parentheses end style
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