Why does a prism form a Spectrum, but a rectangular glass slab does not?
Answer: Prisms form a Spectrum, but rectangular glass slabs do not. Refraction in a prism results in different deviations of rays of different colours; Hence the spectrum is formed as a result of the scattering of white light. Two opposite surfaces of a rectangular glass slab are parallel. When light rays are refracted by this glass, the emitted rays are parallel to the incident rays. In this case, the light of different colours cannot be separated so there is no dispersion of light. As a result, the spectrum is not formed.