Question

Assertion : The focal length of an equiconvex lens placed in air is equal to radius of curvature of either face.
Reason: For an equiconvex lens radius of curvature of both the faces is same.

• A. If both assertion and reason are true and reason is the correct explanation of assertion
• B. If both assertion and reason are true but reason is not the correct explanation of assertion
• C. If assertion is true but reason is false
• D. If both assertion and reason are false

Answer 1

Answer: (B)

For an equiconvex lens, $R_1 = R_2 = R$
From $\frac{1}{f} = (\mu -1) (\frac{1}{R_1}-\frac{1}{R_2})$
[using lens makers formula]
For lens, $\mu = 1.5$ placed in air.
Therefore, $\frac{1}{f} = (1 .5 -1 )\frac{2}{R}$
$ \Rightarrow f = R$,