Question

A concave lens of focal length 15 cm form an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram also.

Solution

Given that, Focal length $$f=15\,cm$$ Distance of the image $$v=-10\,cm$$ We know that,   $$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$$  $$\dfrac{1}{-15}=\dfrac{1}{-10}-\dfrac{1}{u}$$  $$-\dfrac{1}{u}=\dfrac{1}{-15}+\dfrac{1}{10}$$  $$-\dfrac{1}{u}=\dfrac{1}{30}$$  $$u=-30\,cm$$ Hence, the object is placed $$30\ cm$$ away from the concave lansPhysics

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