Question

Assertion : A concave mirror and convex lens both have the same focal length in air- When they are submerged in water, they will have same focal length.
Reason : The refractive index of water is smaller than the refractive index of air.

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

Answer 1

Answer: (D)

If a mirror is placed in a medium other than air, its focal length doesnot change as $f=R / 2$. But for the lens
$\frac{1}{f_{a}}=\left({ }_{a} n^{g}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
and $\frac{1}{f_{w}}=\left(_{w} n^{g}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
As ${ }_{w} n^{g}<{ }_{a} n^{g}$, hence focal length of lens in water increase.
The refractive index of water is $4 / 3$ and that of air is $1$ .
Hence, $\mu_w > \mu_a$