What will be the focal length of a convex lens whose power is 1.5 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).

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 that P = 1.5 D 

Focal length (f) = 1/P = 1/(1.5) = 1/1.5 = 0.666 m = 66.6 cm

The focal length of a convex lens is positive.

So the correct answer is option 2.