Question

Define spectrum. Explain its formation with the help of a neat, labelled diagram.

Answer 1

The band of seven colours obtained on a screen when a beam of white light is passed through a glass prism is called a spectrum.

                        Dispersion of light through a prism.

The formation of a spectrum occurs due to the phenomenon of dispersion of light.

1. Varying Speeds: White light is a mixture of seven different colours (VIBGYOR). While all these colours travel at the exact same speed in a vacuum, they travel at different speeds inside a glass prism.
2. Differential Bending: Due to the difference in speeds, the prism offers a different refractive index to each colour. As a result, when white light enters the prism, each constituent colour bends (refracts) through a different angle.
3. The Result: Red light travels the fastest in glass, so it bends the least and appears at the top of the spectrum. Conversely, violet light travels the slowest in glass, so it bends the most and appears at the bottom. This differential bending separates the combined white light into its distinct colour bands on the screen.