Question

Assertion (A): When a convex lens made of glass is immersed in water, its converging power increases.

Reason (R): The focal length of a lens depends only on the radii of curvature of its two faces.

पर्याय

• Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).

• Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).

• Assertion (A) is true, but Reason (R) is false.

• Both Assertion (A) and Reason (R) are false.

Answer 1

Both Assertion (A) and Reason (R) are false.

Explanation:

A lens’s power and focal length are determined by its geometry as well as the refractive index of its constituent materials and the surrounding medium.

Lens Maker’s Formula:

`1/f = (n_"lens"/n_"medium" - 1) (1/R_1 - 1/R_2)`

Power (P) = `1/f`

Assertion (A) is false. The relative refractive index `n_"lens"/n_"medium"` drops when a glass lens (n ≈ 1.5) is immersed in water (n ≈ 1.33) rather than air (n = 1). As a result, `1/f` becomes smaller as the term `(n_l/n_m - 1)` decreases. As a result, power decreases and focal length increases.

The reason (R) is false. The formula shows that the refractive indices of the lens and the medium, as well as the radii of curvature, affect focal length. The assertion is false because of the term “only.” Hence, both Assertion and Reason are false.