Q16 of 59 Page 1

The image of an object formed by a lens is of magnification -1. If the distance between the object and its image is 60 cm, what is the focal length of the lens? If the object is moved 20 cm towards the lens, where would the image be formed? State reason and also draw a ray diagram in support of your answer.

Since the object is same size as that of the object therefore the object is placed at 2f and image is formed at 2f’. Therefore,

4f = 60 cm


2f = 30 cm


F = 15 cm


The magnification of -1 is possible in only convex lens.


When the object is moved 20 cm towards the lens, so the new object distance will be = 30 cm - 20 cm = 10 cm.


Since the new object distance is less than focal length of the lens, thus the image will be virtual, erect and magnified. The image will be formed on the same side as that of the object.



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