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Essay on color, para. 2, reality in physics (continued)
If white light is composed of many wavelengths, then we should be able to show this with means of physics. Indeed this is possible. Breaking down white (or colored) light into its component wavelengths is called "dispersion". Prisms and gratings can perform it.
2.5.1 Dispersing Light with a Prism
With a very distant light source and two narrow slit apertures, Newton made a bundle of white light that had the cross-sectional shape of a very slim rectangle.
This bundle was refracted by a roof-shaped glass prism.
You certainly know that
* propagating from air to glass, light is refracted towards the glass surface normal;
* propagating from glass to air, light is refracted away from the glass surface normal;
* the angle of refraction depends on the step in the index of refraction.
This causes effectively the ray bundle to be refracted towards the prism base.
The trick is that the glass index of refraction (round about 1,5) decreases with increasing light wavelength. Hence blue light is refracted more than red light and a spectrum is spread out.
On the screen, every monochromatic color is displayed as wide as the white bundle would be after travelling the same distance. But the monochromatic color strip of the adjacent wavelength is not only nearby, but even butted. That's why
the narrower you choose the slit apertures, the better the wavelengths are separated, the better the colors are saturated. (And, admitted, the less light intensity will be available.)