A convex lens forms an image of the same size as the object at a distance of 30 cm from the lens. Find the total length of the lens. Also find power of the lens. What is the distance of the object from the lens?

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Solution

A convex lens forms the image of the same size as the object only when the object is placed at a distance 2f from the lens. In this case the image is also equal to 2f from the lens.

Hence, 2f = 30 cm

or f = 15 cm = 0.15 m

Power of the lens, P = $\frac{1}{f}$ = $\frac{1}{0.15}$ D = 6.6 D

The distance of the object from the lens is also 2f = 30 cm

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