The spectrum in which the colours are separated using a narrow source of light is called:
a) Pure spectrum
b) Rainbow
c) Impure spectrum
d) none of these

Hint : A spectrum of the beam in which no two wavelengths (colours) overlap is a pure spectrum. A pure spectrum is made when white light is transferred through a thin slit, and the incident beams are parallel to each other. The collection of colours resting side by side has described a spectrum.

Complete step-by-step solution:
A spectrum is a state that is not restricted to a specific collection of values but can change, without steps, beyond a continuum. The word was first utilized scientifically in optics to explain the rainbow of colors in apparent light after moving through a prism. A systematic understanding of light forward came to apply to the whole electromagnetic spectrum. It thereby displayed a mapping of a range of wavelengths to a range of features, which are the recognized "colors of the rainbow" and other characteristics related to wavelengths that extend outside the visible radiation spectrum.
The spectrum can be pure spectrum and impure spectrum.
The spectrum in which the colours are distinct using a narrow light source is called pure spectrum.
Condition of the pure spectrum: The slit should be minute. A widespread slit is comparable to several slits arranged side by side. Pure spectra achieved due to -different parts of a wider slit may overlay each other. To avoid this, the slit is formed as small as possible. A convex lens is used to give the incident beams parallel so that all the lights have an equal incidence angle. Prism to be located in the minimum deviation state. Rate of rising deviation varies for different colors. So risks of overlapping of colors are least if the prism is located in a minimum deviation condition. Since various colors are inclined oppositely to the convex lens, they can be concentrated at different points by utilizing the lens.
Option (a) is correct.

Note: A spectrum that contains overlapping of various colors is described as an impure Spectrum. In different words, in an impure spectrum, the colors are not separate and independent, but they flap. On the opposite hand, a spectrum with every color separate and independent is named a pure spectrum.