Question

A concave lens has focal length of 15 CM At what distance should the Object from the lens be placed so that it form an image at 10cm from the lens? also, find the magnification produced by the lens ?
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Answer 1

Explanation:

Solution : 

Here, focal length of concave lens, f=−15cmf=-15cm. 

object distance, u=?u=?, image distance, v=−10cmv=-10cm, magnification of lens, m=?m=? 

As 1f=1v−1u,1u=1v−1f=1−10+115=−130oru=−30cm1f=1v-1u,1u=1v-1f=1-10+115=-130oru=-30cm

Thus the object should be placed at a distance of 30 cm on the left side of the concave lens. 

Linear magnification, m=vu=−10−30=13m=vu=-10-30=13. 

The positive sign of m shows that the image is virtual and erect, and its size is (1/3)(1/3) of the size of the object.