Question

15. Assertion: When a concave mirror is held
under water, its focal length will increase.
Reason: The focal length of a concave
mirror is independent of the medium in
which it is placed.
1. Both assertion and reason are true
O reason is the correct explanation of
assertion
2. Both assertion and reason are true, but
O reason is not the correct explanation of
assertion.
3. Assertion is true, but reason is false
O 4. Assertion is false, but reason is true​

Answer 1

Answer:

A is false but R is true

Explanation:

correct assertion will be:

focal length of convex mirror change on dipping in water

Answer 2

Assertion: When a concave mirror is held under water, its focal length will increase.

Reason: The focal length of a concave mirror is independent of the medium in which it is placed.

Assertion is false, but Reason is true.

But why?

• Focal length of mirror is a measure of the radius of curvature of the sphere from which the spherical mirror was obtained.

• However, the focal length and radius of curvature of a concave mirror is totally independent of the medium in which the mirror is placed. So, Assertion is false and Reason is true.

• So, whether we place it in water or oil, the focal length will remain the same.

____________________

• Please note that this is not the case with lens. Focal length of lenses do change with respect to the medium in which they are kept.

$\star$ Hope It Helps.