Light of many colours can be derived from white light, e.g. from sunlight. A band of colours or a rainbow appears when direct sunlight is refracted by raindrops in the sky; and a similar band of colours is produced when a ray of white light from a tungsten filament lamp is refracted on passing through a triangular glass prism (Fig. 18.1). The band of colours displayed on the white screen is called a spectrum. It is not a pure spectrum, however, as some of the colours in the middle of the band overlap one another.
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