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A diverging lens has a focal length of 4 cm. Its power is______.
1. +25 D
2. -25D
3. -0.25D
4. +0.25 D

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Correct Answer - Option 2 : -25D


  • 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.

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

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

  • The unit of power of a lens is Dioptre when the focal length of the lens is taken in meter (m).
  • The focal length of the convex lens (converging) is positive and that of the concave lens (diverging) is negative. 


Given that lens is diverging. so focal length is negative.

f = -4 cm = -0.04 m

Power of lens \(P = \frac{1}{f} =\frac{1}{-0.04} = -25 D\)

P = -25 D

So the correct answer is option 2.

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