Question

Is the spectrum obtained by an equilateral glass prism pure or impure? Explain your answer.

Answer 1

A spectrum obtained directly from a simple equilateral glass prism using ordinary white light is generally impure unless special arrangements are used.

Explanation:

Each colour in the spectrum is actually an image of the source slit. If the slit is broad, the images of different colours overlap, producing an impure or blended spectrum. Also, if the incident beam is not parallel, the different colours may not be sharply separated. To obtain a pure spectrum, a narrow slit, a collimating lens, and a focusing telescope or screen at the proper plane are required.