As shown in the figure a combination of a thin plane concave lens and a thin plane convex lens is used to form the image of an object placed at infinity. The radius of curvature of both lenses is 30 cm and refractive index of the material for both lenses 1.75. Both lenses are placed at a distance of 40 cm from each other. Due to the combination the image of the object is formed at a distance x cm, from concave lens then x is

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answer is 120.

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Detailed Solution

$\frac{1}{f_{1}} = (1.75 - 1) - \frac{1}{30} f_{1} = - 40 cm$
$\frac{1}{f_{2}} = (1.75 - 1) \frac{1}{30} = 40 cm$
Image from L₁ will be virtual and on the left of L₁ at focal length 40 cm so the object for L₂ will be 80 cm from 'L₂' which is 2f.
Final image is formed at 80 cm from L₂ on the right so x = 120 cm