Question

When white light passes through a glass slab it gets refracted two times but it does not splits into 7 colours but in prism it white light disperses into 7 colours.why glass slab cannot split white light into 7 colours while a prism can do it?Explain.

Answer 1

A glass slab is usually in the shape of a rectangle. A rectangle can be divided equally by one of its diagonal.
On dividing a rectangular glass slab, along its diagonal, we get two right angled prisms.
Prisms have the ability to disperse white light in 7 colours and also, recombine 7 colours into a white light.
Hence, when a white light is allowed to fall on a rectangular glass slab, the white light undergoes dispersion due to the first half of the rectangular glass slab.
When the dispersed light falls on the second half of the glass slab, they are recombined to form a white light.
Thus when white light is passed through a glass slab, though there are two refractions, we do not observe the effect of dispersion due to the fact that the dispersion of white light in to 7 colours by first half of the glass slab is nullified by the recombination of 7 colours into white light by second half of the glass slab.