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A screen is placed 80 cm from an object. The image of the object on the screen is formed by a convex lens placed between them at two different locations separated by a distance 20 cm. Determine the focal length of the lens. |
Answer» Solution :We can obtain image of a given point OBJECT O at another fixed point I for two different positions of a convex LENS provided that values of u and v are interchanged in two positions as SHOWN here.  Thus in PRESENT question `OI = |mu | + v|=80 cm =x +(x+20)=2x+20` `implies x =30 cm ` and so `mu_(1)` =- 30 cm and `v_(1)=+50` cm `:. (1)/(f) = (1)/(v) -(1)/(u) = (1)/(+50)-(1)/(-30)=(1)/(50)+(1)/(30) = (80)/(1500) implies f= (1500)/(80) = 18.75 cm =19` cm |
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