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

1. Increase
2. Decrease
3. remain same
4. doesn’t depend on color of light

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,