(a) What is meant by dispersion of white light? (b) State the colours in the spectrum of white light in ascending order of their wavelengths (c) Which colou...

(a) Dispersion of white light is the separation of white light into its constituent colours when it passes through a refracting medium such as a glass prism.

(b) In ascending order of wavelength:

Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).

(c)

1. The colour deviated least is red.
2. The colour deviated most is violet.

(d) White light is made up of colours of different wavelengths. The refractive index of glass is different for the different colours. Consequently, the colours travel at different speeds in the prism and are refracted through different angles. Violet light is refracted most, while red light is refracted least; hence the colours spread out to form a spectrum.

(e) Production of a pure spectrum of white light

An illuminated narrow slit is placed at the principal focus of a converging lens, L₁. The lens produces a parallel beam of white light which falls on a glass prism. The prism disperses the light into separate parallel beams of different colours. A second converging lens, L₂, brings each coloured beam to a separate focus on a white screen placed in its focal plane. Thus, sharp non-overlapping images of the slit form a pure spectrum.