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Image in Thin Lens by Ray Tracing

A screen is p...

Question

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.

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Solution

Distance between the image (screen) and the object, D = 90 cm

Distance between two locations of the convex lens, d = 20 cm

Focal length of the lens = f

Focal length is related to d and D as:

Therefore, the focal length of the convex lens is 21.39 cm.

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