A convex lens with a focal length of 10 cm and a concave lens with a focal length of 20 cm are placed in the same axis. The combined lens acts as a:

Correct option is B

The correct answer is option (b) Convex lens.

Explanation

When a convex lens and a concave lens are placed along the same axis, the combined power of the system is the sum of the powers of each lens. The power of a lens is the reciprocal of its focal length.

Given:

• The focal length of the convex lens is 10 cm (positive value).

• The focal length of the concave lens is -20 cm (negative value).

To calculate the power:

• The power of the convex lens is 1/0.10 = +10 diopters.

• The power of the concave lens is 1/(-0.20) = -5 diopters.

Now, the combined power of the system is:

Combined power = Power of convex lens + Power of concave lens
Combined power = +10 + (-5) = +5 diopters.

Since the combined power is positive, the system behaves like a convex lens.

Focal length of the combined lens:

The focal length is the reciprocal of the combined power.
Focal length = 1 / Combined power = 1 / 5 = +20 cm.

Therefore, the combined system behaves like a convex lens with a focal length of +20 cm.