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CBSE Class 10 Answered

An image formed on a screen is three times of the object .The object and screen are 80 cm apart when the image is sharply focused.Calculate the focal length of the lens.
Asked by Rajendra Beniwal | 11 Jan, 2018, 04:20: PM
Expert Answer
magnification m is given by , m = v/ u, v is the distance between lense and image, u is the distance between object and lens.
 
magnification m = 3 = v/u, i.e., v = 3u
 
object to screen distance u+v = 80 cm;
solving for u and v, we get u = 20 cm, v = 60 cm.
 
applying to lens formula, (1/u) +(1/v) = (1/f), we get focal length =15 cm
Answered by Thiyagarajan K | 12 Jan, 2018, 11:21: AM
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