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A beam of light converges of at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge, if the lens is (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16 cm? |
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Answer» Solution :Here the lens has been placed in the path of the convergent BEAM before its actual converging. It means that the object is situated on the right side of lens, i.e., u = + 12 cm (a)If the lens is convex, then f= + 20 cm `therefore 1/V -1/12 =1/20` or `1/v = 1/12 + 1/20 = (5+3)/60 = 8/60 rArr v = 60/8 = 7.5` cm (b) If the lens is concave, then f= - 16 cm, `1/v -1/12 =1/(-16)` or `1/v = 1/12 - 1/16 = (4-3)/48 rArr v= 48 cm` Thus, the image is REAL and situated at 48 cm from the lens. |
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