Spectra of Light

The principle of obtaining the spectra of light is basically refraction.  Simple, single wavelength refraction consists of the incident beam, the reflected beam and a single refracted beam.  

However, to obtain the spectra of light, white light is used, which is made up of the whole visible spectrum.  Now, as the graph below shows, different wavelengths have different refraction coefficients (n values).  And since white light has many wavelengths incorporated into it, it can easily be seen that there will be many refracted rays.  This is called chromatic dispersion, in which "chromatic" refers to a colour associated with each wavelength and "dispersion" refers to the separation of the wavelengths of colours.  This then produces the pretty spectra.  The figure aside shows the blue and red rays, all others, green, yellow, violet are in between.

Generally, the index of refraction in a medium is larger for a shorter wavelength (blue light) than for a longer wavelength (red)  This means that when white light refracts through a surface, the blue components bend more than the red components, with the intermediate colours undergoing intermediate bending.
The index of refraction as a function of wavelength for fused quartz.  Light with a short wavelength, corresponding to a higher index of refraction, is bent more upon entering quartz than light with a long wavelength.
Actual representation of the prism spectra experiment

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Data from Fourth Edition of Fundamentals of Physics