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

Options

A. not depend on colour in light.
B. increase
C. decrease
D. remain same

Answer

A, B, C

Explanation

For red light, and blue light `n_r lt n_b`
Now `1/f=(n-1)((1)/(R_1)-(1)/(R_2))`
`1/f prop n`
`therefore (f_2)/(f_b)=(n_b)/(n_r)`
`therefore (f_r)/(f_b) gt 1 " "thereforef_r gt f_b`
`therefore` Focal length will increase.