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An object of height 4 cm is placed at a distance of 20 cm from a concave lens of focal length 10 cm. Use lens formula to determine the position of the image formed.

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#### Solution

Object distance, *u* = 20 cm

Image distance, *v* = ?

Focal length, *f* = 10 cm

As per the sign conventions used,

*f* = −10 cm

*u* = −20 cm

According to the lens formula,

`1/f=1/v-1/u`

`1/v=1/f+1/u`

`1/v=1/-10+1/-20=-3/20`

`rArrv=-20/3=-6.6" cm"`

The image is formed at a distance of 6.6 cm from the lens at the same side where the object is placed.

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