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A slide projector has a convex lens with a focal length of 20 cm. The slide is placed upside down 21 cm from the lens. How far away should the screen be placed from the slide projector’s lens so that the slide is in focus?

Rohit Sharma February 14, 2023

A slide projector has a convex lens with a focal length of 20 cm. The slide is placed upside down 21 cm from the lens. How far away should the screen be placed from the slide projector’s lens so that the slide is in focus?

Question:

A slide projector has a convex lens with a focal length of 20 cm. The slide is placed upside down 21 cm from the lens. How far away should the screen be placed from the slide projector’s lens so that the slide is in focus?

The correct answer is :

(c) 1/f= 1/v- 1/u 1/20 = 1/v- 1/-20 1/v = 1/20 - 1/21 = (21 - 20)/420 = 1/420 v= 420 cm (2 marks)