Question

Determine the focal length of a concave lens with the help of a convex lens.

Answer 1

The focal length can be obtained by using the following formula.

f = uv / u − v

Explanation:

Form lens formula we have:

f = uv / u − v

Where

• f is the focal length of the concave lens L1 .
• u is the distance of I from the optical centre of the lens L2 .
• v is the distance of I’ from the optical centre of the lens L2.

From sign convention, the f obtained from the above formula will be negative as v > u and u – v is negative.

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