Question

A prism can disperse white light into seven colors but glass slab can't.Why

Answer 1

In case of prism , after final refraction , the incident ray and the emergent ray makes an angle with the incident ray calledthe angle of deviation . Every colour in the white has different wavelength .The refractive index of prism is different for different colours and therefore different colours deviate through different angles .

However, in case of rectangular glass slab , after refraction at the two parallel faces of the glass slab , a ray of light emerges in a direction parallel to the direction of the incidence of white light on the slab. As rays of all colours emerge in the same direction ( of incidence of white light ) , hence there is no dispersion ..

So we can't see dispersion of white light.

Answer 2

because glass slab has two parallel surfaces and when light enter from one surface it refracts but when ot gets out from another surface it undo's the refraction and the light doesn't seem to dispersed