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A convex lens of focal length 10 cm is placed coaxially 5 cm away from a concave lens of focal length 10 cm. If an object is placed 30 cm in front of the convex lens, find the position of the final image formed by the combined system. |
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Answer» Solution :For convex lens `f_(1) = 10`cm and `u_1 = - 30 cm`, hence from lens formula `1/v -1/u =1/f`,we have: `1/v_(1) = 1/u_(1) + 1/f_(1) =1/(-30)+ 1/10 =1/15` or `v_(1) = 15 cm`  Thus, as shown in Fig. 9.43, convex lens `L_(1)`forms REAL image `I_(1)` , of given object O at a distance 15 cm from `L_(1)`or 15 - 5 = 10 cm from concave lens `L_2`. For concave lens,behaves as a virtual object i.e.,`u_2 = + 10` cm and `f_2 = - 10 cm`. Hence, we have, `1/v_(2) -1/(+10) = 1/(-10) rArr 1/v_(2) =-1/10 +1/10 =0` or `v_(2) = infty` Thus, final image is FORMED at infinity only. |
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