Question

If lens has different focal length for different colours then how do we see things multicoloured?

Answer 1

When you are seeing a multicolored object,suppose a box with yellow,red and blue color present on its one surface like this and you are seeing it,

As,we know,lens have different focal length for different colour of light,so considering the lens you are using to be convex,we can say that the lens will form different focus,for different colour,so obviously,they will not meet on the same point.

But still we can see that the light rays of different colour are coming from the same plane.

Well that is something because of the way the brain interpretes it,and the concept is known as Horopter and Panum's area of fusion.

Horopter is an arc of a circle found by joining the external projection of light rays on retinal point,any two objects falling on the horopter is seen as flat surface,i.e no distance between them can be seen by the eyes,due to the brain processing.

Now,the relative difference between the focal points of individual light rays are so small that it falls on the horopter,and we see them as coming from the same plane,and can percept light rays of different colours as well.

Note the points `C,D,E` are the points where light meets after refraction through the lens you are holding,the concept is shown below,only for red light,see point`D`,so `C` and `E` will be formed thus,but their distance from the optical centre of the lens will be different.