Question

we wish to obtain an equal sized inverted image of a candle flame on a screen kept at distance of 4 m from the candle flame. what should be the focal length of the lens and at what distance from the candle flame should the lens be placed

Answer 1

v = 4m

as it is given as equal sized image the object will also be of the same size.

u = - 4m

1/f = 1/v - 1/u

1/f = 1/4 - 1/-4

cross multiplying

1/f = - 4 - 4/ -16

= -8 / -16

minus gets cut

1/f = 1/2

therefore the focal length is 2m

and therefore as i already has mentioned u = -4m

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