Question

Two thin lenses of power +3.5 D and -2.5 D are placed in contact. Find the focal length of the lens combination.

• A. 100cm
• B. -16.7 m
• C. +16.7 m
• D. 1cm

Answer 1

The correct option is A

100cm

For combination of lenses, the power of each lens is added to get the resultant power.

P = P₁ + P₂

= +3.5 - 2.5

= 1 D

Now, f (in m) = 1/P (in D)

So, f = 1/1 = 1 m = 100 cm