Q16 of 38 Page 1

A student has to project a three times magnified image of a candle flame on a wall. Name the type of the lens (converging/ diverging) required for the purpose. If the candle flame is at a distance of 6 m from the wall, find the focal length of the lens.

Converging or convex lens.

As image is real


m = -3


hi=3ho


Image distance = 6 + object distance


v = 6 + u


we know that magnification=m=v/u


m = v/u


[since image is magnifed three times ]


-3u = 6 + u


4u=-6


u=-6/4


u=-1.5 m


v=6 - (-1.5)


v=7.5m


by using lens formula,



⇒


⇒


⇒


∴ f =75/60=1.25m


Thus, Focal length of a given convex lens is 1.25m


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