A thin lens of focal length 20 cm is placed in contact with a 40-diopter thin lens.What is the power of the combined lenses?
A)1/4 diopter
B)35.0 diopters
C)45.0 diopters
D)80 diopters
E)4 diopters

Question

A thin lens of focal length 20 cm is placed in contact with a 40-diopter thin lens.What is the power of the combined lenses?
A)1/4 diopter
B)35.0 diopters
C)45.0 diopters
D)80 diopters
E)​4 diopters

Quizplus · Accepted Answer

The power of a lens is given by:

power (in diopters) = 1/focal length (in meters)

The focal length of the first lens is 20 cm = 0.2 m. Therefore, its power is:

P1 = 1/0.2 = 5 diopters

The second lens has a power of 40 diopters.

When two lenses are in contact, their powers add up. Therefore, the power of the combined lenses is:

P = P1 + P2 = 5 + 40 = 45 diopters

Hence, the answer is choice C: 45.0 diopters.

A Thin Lens of Focal Length 20 Cm Is Placed