Focal length of converging lens · PhysicsBowl 2019 #8
Physics · optics
Problem
A candle, a converging lens and a white screen are placed in a line with the lens between the candle and the screen. A distance of 72 cm separates the candle and screen. As the lens is moved to all points between the candle and the screen, only one focused image of the candle can be made on the screen. What is the focal length of the converging lens?
(A) 12 cm
(B) 18 cm
(C) 24 cm
(D) 36 cm
(E) It cannot be determined without knowing the location of the lens when the focused image is produced.