As shown in the figure, a combination of a thin plano-concave lens and a thin plano-convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is 30 cm and refraction index of the material for both the lenses is 1 . 75 . Both the lenses are placed at distance of 40 cm from each other. Due to the combination, the image of the object is formed at distance x = ______ cm , from concave lens.

Question

As shown in the figure, a combination of a thin plano-concave lens and a thin plano-convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is 30 cm and refraction index of the material for both the lenses is 1 . 75 . Both the lenses are placed at distance of 40 cm from each other. Due to the combination, the image of the object is formed at distance x = ______ cm , from concave lens.,

MARKS App · Accepted Answer

Using Lens Makers formula for the plano-concave part, we can write 1 f 1 = ( 1 . 75 - 1 ) - 1 30 ⇒ f 1 = - 40 cm Using Lens Makers formula for the plano-convex part, we can write 1 f 2 = 1 . 75 - 1 1 30 ⇒ f 2 = 40 cm Image from first lens will be virtual and on the left of first lens at focal length 40 cm . So the object for second lens will be 80 cm from second lens which is 2 f 2 . Final image is formed at 80 cm from second lens on the right. So, x = 120 .

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