CBSE Class 10 Science 2016 Term II Outside Delhi Set 2

Question : 11

Total: 12

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

Solution:

Magnification =−1 i.e., image is inverted and of same size. It means, object is at 2F and image is also at 2F.
Therefore, distance between u and v=4F =60‌cm.
∴‌‌F=60∕4=15‌cm
Object will be at =2F=30‌cm.
If the object is shifted towards the lens by 20 cm, the new object distance will be at 10‌cm from the lens. This distance is less than the focal length, so the image formed in this case would be virtual, erect and on the same side.

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