Optics 7 Question 12

12. A biconvex lens of focal length 15cm is in front of a plane mirror. The distance between the lens and the mirror is 10cm. A small object is kept at a distance of 30cm from the lens. The final image is

(2010)

(a) virtual and at a distance of 16cm from the mirror

(b) real and at a distance of 16cm from the mirror

(c) virtual and at a distance of 20cm from the mirror

(d) real and at a distance of 20cm from the mirror

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Solution:

  1. Object is placed at distance 2f from the lens. So first image I1 will be formed at distance 2f on other side. This image I1 will behave like a virtual object for mirror. The second image I2 will be formed at distance 20cm in front of the mirror, or at distance 10cm to the left hand side of the lens.

Now applying lens formula

1v1u=1f1v1+10=1+15 or v=6cm

Therefore, the final image is at distance 16cm from the mirror. But, this image will be real.

This is because ray of light is travelling from right to left.



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