The reason to collimate the light source before utilizing the prism is that if white light enters the prism at various angles, some blue light will exit the prism at the same angle.
Prisms' Dispersion of Light. When light source travels through a triangular prism, these hues are frequently seen. The prism separates the white light source divided into its individual colors, which are red, orange, yellow, green, blue, and violet. Dispersion is the process through which visible light is divided into its various colors.
Since blue light has a shorter wavelength than red light and refracts more than the latter, it is bent more when white light enters a prism than red light.
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