Question

Determine the focal length of the lens with refractive index is given 2 and radius of curvature of two surfaces respectively 20 cm and -35 cm.

Answer 1

Answer :

Refractive index of lens = 2

Radius of curvature of two surfaces are 20cm and -35cm respectively.

We have to find focal length of the lens$.$

For a spherical, thin lens having the same medium on both sides :

◈ 1/f = (u - 1) (1/R¹ - 1/R²)

• f denotes focal length
• u denotes refractive index
• R¹ and R² are radii of curavature

⇒ 1/f = (2 - 1) [1/20 - 1/(-35)]

⇒ 1/f = (1) [1/20 + 1/35]

⇒ 1/f = (35 + 20) / (35 × 20)

⇒ 1/f = 55 / 700

⇒ f = 12.72 cm