Question

Define power of a lens. A convex lens has a focal length of 10 cm. What is its 5

Power? Two thin lenses of focal length +10cm and -5cm are kept in contact. What

Is the focal length and power of the combinations?​

Answer 1

Answer :-

The Power of a lens is defined as the reciprocal of its focal length.

• The SI unit of Power is Dioptre .

• It is denoted by D .

_______________________________________

Given:-

• Focal length ,f = 10 cm

To Find:-

• Power of lens ,P

Solution:-

• P = 1/f

Substitute the value we get

→ P = 1/10

→ P = 0.1 D

Therefore the power of lens is 0.1 Dioptre.

_______________________________________

Given:-

• Focal length ,f = +10 cm

• Focal length ,f' = -5 cm

To Find:-

• Powe of combination P

Solution:-

Power of combination is calculated by this formula.

• P = 1/f + 1/f'

Substitute the value we get

→ P = 1/10 + 1/(-5)

→ P = 1/10 -1/5

→ P = 1-2/10

→ P = -1/10

→ P = -0.1

→ P = -0.1/100m

→ P = - 0. 001 D

here Negative sign show that the lens is concave lens.

Therefore the power of lens is -0.001 Dioptre .