What is spectrum and how is …

The band of seven colours dispersed by a glass prism is known as a spectrum.  

Visible light or white light is made up of seven colours. The light waves of different colours vary in their frequency and wavelengths. When a white light is made incident on a glass prism, it undergoes refraction. The prism splits the incident white light into a band of colours. The higher frequency (lower wavelength) violet light travels slower or refracts more than the lower frequency (higher wavelength) red light. Likewise the other colour light waves bend with respect to the incident white light as they pass through the prism. 

Thus, the rays of different colours emerge alone  and become distinct. This band of the coloured components of a light beam  is called its spectrum. The sequence of colours from bottom to top is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red).