Question

A glass prism is able to produce a spectrum when white light passes through it but a rectangular block of same transparent glass does not produce any spectrum. Why?

Answer 1

The reason as to why we can observe spectrum in case of Glass prism but not in case of a rectangular glass slab can be understood if we look at the basic differences between the two, that is, in case of a prism the refracting surfaces are parallel to each other, whereas in case of rectangular slab, they are parallel to each other.

Complete answer:
We know that spectrum is observed in case of glass prism due to Refraction. Refraction can be defined as the bending of light rays when it passes from one medium to another.

In the given question, white light is incident on the glass prism, which results in observation of spectrum at emergence. Now, we know that which light comprises seven different coloured light acronyms as VIBGYOR, which can be expanded to Violet, Indigo, Blue, Green, Yellow, Orange, and Red.

Let us consider rectangular glass slab, we know here the two refracting surfaces runs parallel to each other, therefore, when white light (all its components) falls on the first interface, it refracts as since this interface is parallel to the second interface, each component gets refracted through same angles and thus emerges as white light.

Whereas, in case of glass prism, the two refracting surfaces are inclined at an angle, thus the white light when incident of first interface gets refracted at different angles, and separates into its different components as it leaves the second surface.

Hence, Spectrum is observed as a case of prism but not in cases of rectangular glass slab.

Note: This splitting of white light into different components is referred to as dispersion through prism, as the coloured lights observed is known as the spectrum.
Violet light has the least wavelength as it has maximum frequency, whereas red has the largest wavelength as it has minimum frequency.