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A thin convex lens of focal length 25 cm and aperture 10 cm is cut to have a hole of aperture 5 cm at the centre. The power of lens now will be: 


1. 2 D
2. 4 D
3. 1 D
4. 8 D

1 Answer

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Best answer
Correct Answer - Option 2 : 4 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).

\(P = \frac{1}{f\,(m)}=\frac{100}{f\,(cm)}\)

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 - focal length f = 25 cm

  • Even if a hole of aperture 5 cm is made the focal length would remain the same
  • Then the power of the lens is given by

\(\Rightarrow P = \frac{100}{f}\)

\(\Rightarrow P = \frac{100}{25} = 4 D\)

  • Hence, option 2 is the answer

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