White light is a mixture of colours and a prism spreads them out

1. The white light from the Sun or from a filament lamp is not one colour on its own, it is many colours blended together.
2. Each of those colours is light of its own frequency, fixed by the source, and that frequency does not change when the light later crosses into glass or water.
3. Send a narrow beam of white light through a triangular glass prism and the light leaves as a band of colours lying side by side, called a spectrum.
   1. A lamp behind a card with a thin slit in it gives you the narrow beam, and a white screen about a metre past the prism catches the colours.
4. The prism has not manufactured anything: every colour you see on the screen was already travelling inside the white beam before it reached the glass.

Glass has a slightly different refractive index for each colour, so each colour bends by its own angle

1. Light refracts because it changes speed as it crosses into a new material, and the refractive index $n$ of the glass is the number that compares those speeds.
2. Glass does not have one single value of $n$: the value is slightly different for every colour that passes through it.
3. Glass slows violet light the most, so the glass has its largest refractive index for violet.
4. It slows red light the least, so the glass has its smallest refractive index for red.
   1. In a typical crown glass $n$ is roughly $1.53$ for violet and roughly $1.51$ for red, a gap of about one part in seventy.
5. For the same angle of incidence $i$, a larger $n$ gives a smaller angle of refraction $r$, so violet is turned further from its original direction than red at each surface.
6. The chain runs like this: white light meets the first face of the prism, each colour refracts by its own amount, the colours then travel through the glass in slightly different directions, and at the second face each one refracts again in the same order.
7. So violet emerges deviated most from the direction of the incoming beam and red deviated least, with orange, yellow, green, blue and indigo spread out in between.

A prism keeps the colours apart, a flat-sided block hands them back

1. In a rectangular glass block the two faces the light crosses are parallel to each other.
2. Each colour bends towards the normal by its own amount on the way in, then away from the normal by that same amount on the way out, so the two refractions cancel in direction.
3. Every colour therefore leaves the block travelling parallel to the beam that went in, and the light looks white again.
   1. The colours do fan apart a little inside the glass, so they come out side by side rather than on top of one another, but in a thin block that sideways offset is far too small for your eye to pick out.
4. The two refracting faces of a prism are not parallel, they meet at an angle, so the second refraction turns each colour further the same way as the first instead of undoing it.
5. The separation builds up rather than cancelling, and by the time the light has crossed the room the spectrum is wide enough to see clearly on a screen.

Red is the low-frequency end of the spectrum and violet the high-frequency end

1. In order of increasing frequency, the seven traditional colours run red, orange, yellow, green, blue, indigo, violet.
2. In order of increasing wavelength, the same seven run the other way: violet, indigo, blue, green, yellow, orange, red.
3. Red sits at one end with the lowest frequency and the longest wavelength of any light you can see.
4. Violet sits at the other end with the highest frequency and the shortest wavelength.
5. The two orders are mirror images of each other because all these colours travel at the same speed through a given material.
   1. $$v=f\lambda$$
   2. With $v$ effectively the same for all of them, a higher frequency $f$ has to come with a shorter wavelength $\lambda$.
6. These seven colours are one narrow band inside a much wider family of electromagnetic waves, and they are the only part of it your eyes respond to.

Light of a single frequency cannot be split any further

1. A monochromatic beam carries one frequency and nothing else, so it is a single pure colour rather than a mixture.
2. Send such a beam into a prism and it still refracts, but it leaves as one ray of that one colour with no spectrum, because there is nothing in it to separate.
   1. A red laser pointer and the orange-yellow glow of a low-pressure sodium street lamp are both close to monochromatic.
3. White light is the opposite case: it holds a whole range of frequencies, which is why a prism can disperse it.
4. Define the word using frequency rather than wavelength, since the frequency of the light stays the same in glass and water while its wavelength does not.