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.

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.

Tardigrade Admin · Accepted Answer

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}} = (_{a} n^{g} - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})

and

\frac{1}{f _{w}} = (_{w} n^{g} - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})

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,

μ_{w} > μ_{a}

Q. 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.

If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.

If both Assertion and Reason are true but the Reason is not the correct explanation of the Assertion.

If Assertion is true but Reason is false.

If both Assertion and Reason are false.

Q. 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.