Question 17

An effective power of a combination of 5 identical convex lenses which are kept in contact along the principal axis is 25D. Focal length of each of the convex lens is:

For lenses in contact, the effective power is the sum of individual powers:

$$P_{total} = 5P$$ where P is the power of each lens.

$$25 = 5P \Rightarrow P = 5$$ D

Focal length: $$f = \frac{1}{P} = \frac{1}{5}$$ m = 20 cm

The correct answer is Option 1: 20 cm.

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