A thin converging lens has focal length f = 0.20 m. An object is placed at distance u = 0.30 m from the lens. Use the lens formula 1/f = 1/v + 1/u to find the image distance.

A thin converging lens has focal length f = 0.20 m. An object is placed at distance u = 0.30 m from the lens. Use the lens formula 1/f = 1/v + 1/u to find the image distance.

• A. 0.30 m
• B. 0.10 m
• C. 0.60 m
• D. 1.20 m

Answer: (C)

The lens equation links the focal length, object distance, and image distance for a thin lens. For a converging lens, use 1/f = 1/v + 1/u. Plug in f = 0.20 m and u = 0.30 m. Then 1/v = 1/f − 1/u = 1/0.20 − 1/0.30 = 5 − 3.333… = 1.666…. So v = 1 / 1.666… = 0.60 m. A positive image distance means the image sits on the far side of the lens from the object, i.e., a real image. Thus the image is 0.60 m from the lens. The other distances don’t satisfy the equation with these values.

The lens equation links the focal length, object distance, and image distance for a thin lens. For a converging lens, use 1/f = 1/v + 1/u.

Plug in f = 0.20 m and u = 0.30 m. Then 1/v = 1/f − 1/u = 1/0.20 − 1/0.30 = 5 − 3.333… = 1.666…. So v = 1 / 1.666… = 0.60 m.

A positive image distance means the image sits on the far side of the lens from the object, i.e., a real image. Thus the image is 0.60 m from the lens. The other distances don’t satisfy the equation with these values.