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A convex lens produces an image of a condle flame upon a screen whose distance from candle is D. When the lens is displaced through a distance x, (the distance between the candle and the screen is kept constant), it is found that a sharp image is again produced upon the screen. Find the focal length of the lens in terms of D and x . |
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Answer» `b =(D+x)//2A=(D-x)//2` `(1)/(b)-(1)/(-a)=(1)/(f) RARR (2)/(D+x)+(2)/(D-x)=(1)/(f)` `_(2).(D-x+D+x)/(D^(2)-x^(2))=(1)/(f) rArr(4D)/(D^(2)-x^(2))=(1)/(f) rArr f=(D^(2)-x^(2))//4D Ans.] 
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