Question

The power of a combination of two lenses x and y is 5d if the focal length of lens xb 15 cm calculate the focal length of a lens by

Answer 1

Given: Combined power of two lenses X & Y, (P) = 5 d
Let the combine focal length of lenses X & Y, be 'F'. 
Let F1 and F2 be the focal length of lens X and Y respectively. 
Here, the focal length of lens X is given as F1 = 15 cm
Relation between power and focal length is given as 
                  F = 1 / P
Since, the lens having power 1 d (diopter) always has focal length 100 cm
So,             F = [1 / ( 5 / 100)] cm
or,              F = 20 cm
Now, we shall calculate the focal length of lens 'Y'.
 Concept: when two lenses are in contact to each other, then the relation between their focal lengths and combined focal length is given as,
                 (1 / F) = (1/F1) + (1 / F2)
Or,           (1 /F2) = (1/F) - (1/ F1)
Or,           ( 1/ F2) = (1 / 20cm) - (1/15 cm)
Or,                  F2 = - 60 cm
Hence, the focal length of second lens 'Y' will be 60 cm and the nature of the lens will concave just because of having negative sign. 

Answer 2

Explanation:

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