The radius of the convex surface of plano-convex lens is 20 cm and the refractive index of the material of the lens is 1.5 . The focal length is :

Question

The radius of the convex surface of plano-convex lens is 20 cm and the refractive index of the material of the lens is 1.5 . The focal length is : (A) 30 cm (B) 50 cm (C) 20 cm (D) 40 cm

Tardigrade Admin · Accepted Answer

Using the relation for focal length of plano-convex lens

\frac{1}{f} = (μ - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})

(Refractive index of material of lens

μ = 1.5

R_{1} = 20 c m, R_{2} = \infty

)

\frac{1}{f} = (1.5 - 1) (\frac{1}{20} - \frac{1}{\infty})

\frac{1}{f} = \frac{1}{40}

or

f = 40 c m

Q. The radius of the convex surface of plano-convex lens is 20cm and the refractive index of the material of the lens is 1.5. The focal length is :

30 cm

50 cm

20 cm

40 cm

Using the relation for focal length of plano-convex lens f1​=(μ−1)(R1​1​−R2​1​) (Refractive index of material of lens μ=1.5 R1​=20cm,R2​=∞) f1​=(1.5−1)(201​−∞1​) f1​=401​ or f=40cm

Q. The radius of the convex surface of plano-convex lens is $20 c m$ and the refractive index of the material of the lens is $1.5.$ The focal length is :