Question

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

Answer 1

As we know that

Focal length is inversely proportion to n

f=1/n-1

where n is refractive index

we know that red light has less refractive index then blue colour

so focal length of convex lens will increase when red light is used instead of blue light

Answer 2

A convex lens is inverted like a inverted prism. The phase velocity is different for each colour. When the light comes out from the other side, the bending is more for red and less is for blue. Therefore, the focal length, which is due to bending changes more as compared with the blue light,