Question

A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with each other. What is the power of this combination? What is the focal length of the combination? (1D, 1m)

Answer 1

Given that Focal length of a convex lens,F1=20 Focal length of a concave lens,F2=25

power of a convex lens,P1=100/F1 =100/20

=+5D

now,

power of a concave lens,P2=100/25

=-4D

therefore,

i)power of combination,P3=P1+P2 =5+(-4)

=1

ii)focal length of combination 3D100/P3

=100/1

=100cm

therefore the ans of power of combination and focal length are 1 & 100 cm

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