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An object is placed at 20 cm from the pole of a concave mirror. It forms real image at a distance of 60 cm from the pole. Find the focal length of the concave mirror.

Answer» In the given problem,
Object distance, u = - 20 cm (using Cartesian sign convention )
image distance , v =- 60 cm (using Cartesian sign convention for real image)
Focal length of the mirror is given by,
`1/f = 1/u + 1/v`
`1 /f=(-1)/20 + (-1)/60`
`1/f = (-(60+20))/(60 xx 20) (-80)/(1200)`
` f = (-1200)/80 =- 15` cm
The focal length of the given concave mirror is 15 cm.


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