Question

What is the focal length of combination of a convex lens of focal lenghth 30cm?

Answer 1

Focal length of convex lens is positive

Therefore the focal length of the combination

=30+30 cm=60 cm

Answer 2

F1=30 cm , F2=30 cm and F=?.
1/F=1/F1 +1/F2
1/F= 1/30 + 1/30
1/F=1+1/30
1/F=2/30
1/F=1/15
F=15 cm
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