Question

a bi convex lens converges the parallel rays on the principal axis at 10 cm, then find the focal length ??​

Answer 1

Answer:

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Explanation:

The parallel rays incident on the convex lens converge at the focus,F of the lens. At the mirror, after reflection forms an image at the optical centre of the lens, C.

Thus, the image of the virtual point object Fis formed at C.

f=CF

=CM+MF

=10+10

=20cm the focal length of the lens

Then n×10 = 20

n=2

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Answer 2

Answer:

The answer is 10cm.

Explanation:

• the rays which are coming to the principal axis are parallel.
• as per the rule the rays which are coming parallel to principal axis converges at focus.