Question

When monochromatic red light is used instead of blue light in a convex lens, its focal length will ______.

Options

• increase  

• decrease

• remain same 

• does not depend on colour of light.

Answer 1

When monochromatic red light is used instead of blue light in a convex lens, its focal length will increase.

Explanation:

From the Cauchy

Formula, μ = `"A"+"B"/lambda^2+"C"/lambda^4`   ∴ μ ∝ `1/lambda`

As, `lambda_"blue" < lambda_"red"`  ∴ `mu_"blue" > mu_"red"`

From lens maker's formula

and `1/"f"=(mu-1)(1/"R"_1-1/"R"_2)`

⇒ `1/"f"_"B"> 1/"f"_"R"`

⇒ `"f"_"R">"f"_"B"`.