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  4. A candle placed 25 cm from a lens forms an image on a screen placed 75 cm on the other side of the lens. The focal length and type of the lens should be

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Q. A candle placed 25 cm from a lens forms an image on a screen placed 75 cm on the other side of the lens. The focal length and type of the lens should be

KCETKCET 2018Ray Optics and Optical Instruments

Solution:

$\frac{1}{f} = \frac{1}{v} -\frac{1}{u} $
$ \frac{1}{f} = \frac{1}{75} + \frac{1}{25} $
$\frac{1}{f} = \frac{100}{75\times25} $
$ f = \frac{75}{4} $
$= 18.75 \, cm$