Question

Why a rectangular glass slab do not cause dispersion of light white light like prism?

Answer 1

In a rectangular glass slab opposite sides are parallel. But in triangular prism it is not parallel. So as light passes through the prism it first refracts and then gives the spectrum.

Hope it helps

Answer 2

Explanation:

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In a prism, refraction of light takes place at two lateral surfaces. The dispersion of white light occurs at the first surface of prism where the constituent colours of white light are deviated through different angles. Al second surface of prism, these colours get further separated due to refraction.

However, in a rectangular glass block, the refraction of light takes place at two paralle surfaces. Although the white light splits into its constituent colours refraction at the first surface of glass block, but there colours on suffering refraction at second surface emerge out in the form of a parallel beam, which gives an impression of white light.