Question

The focal length of a double convex lens is 25.0 cm, its power is
1. 4.0 D
2. - 4.0 D
3. O.04 D
4. -0.04 D

Answer 1

Correct Answer - Option 1 : 4.0 D

CONCEPT:

Power of Lens: 

• ​The inverse of the focal length is known as the power of the lens.
• It shows the bending strength for the light ray of the lens.
• The unit of power of a lens is Dioptre when the focal length of the lens is taken in meter (m).

\(P = \frac{1}{f} \)

where P is the power of the lens and f is the focal length of the lens.

Concave lens: 

• It is a diverging lens that diverges the parallel beam of light.
• It can also gather light from all directions and project it as a parallel beam.
• The focal length of the concave lens is negative.
• It has a virtual focus from the diverging rays of light that seem to converge.

Convex lens:

• The lens whose refracting surface is upside is called a convex lens.
• The convex lens is also called a converging lens.
• The focal length of a convex lens is positive.

CALCULATION:

Given:

f = 25 cm  = 0.25 m

• The power of the lens can be written as

\(\Rightarrow P = \frac{1}{0.25} = 4 D\)

• Hence, option 1 is the answer.