Why a Prism Splits Light (The Colours Were Already There)

Everyone has seen a prism turn a beam of white light into a band of colours, and almost everyone has quietly assumed the glass is doing something to the light — adding the colours, or staining it somehow. It isn't. The colours were there the whole time, and the way one man proved that in the 1660s is still one of the cleanest experiments ever designed.

You Have Already Seen the Answer

A rainbow appears when sunlight passes through millions of falling water droplets. Each droplet bends the light entering it, bounces it off the inside of its back surface, and bends it again on the way out — and because different colours bend differently, they come out spread into a fan.

A glass prism is the same physics with better manners. Same bending, same separation, just in a block you can hold instead of a rainstorm you have to wait for.

So the real question isn't "what does the prism do to the light." It's: why does light bend when it goes into glass, and why doesn't every colour bend by the same amount?

Why Light Bends When It Changes Material

Light travels at its famous top speed — about 300,000 km per second — only in a vacuum. Push it through a material and it effectively slows down. In ordinary glass, light travels at roughly two-thirds of its vacuum speed.

If a beam hits the glass surface square-on, it slows down and carries straight on. But if it arrives at an angle, one side of the beam reaches the glass before the other — so one side slows down first, and the beam pivots.

The classic image is a car driving off tarmac onto sand at an angle. The wheel that touches sand first slows while the other is still on tarmac, and the car slews toward the slow side. Light does exactly this, for exactly this reason.

The relationship between the angle going in and the angle coming out is Snell's law. You don't need the formula to use the idea: the bigger the speed change, the bigger the bend. Each material has a number describing how much it slows light — its refractive index — and glass sits at about 1.5, meaning light moves 1.5 times slower inside it than in air.

If you want to put actual angles on it, a Snell's law refraction calculator does the trigonometry for any pair of materials.

The Bit That Makes Colours

Here's the crucial detail. That refractive index isn't one number — it's slightly different for each colour.

Colour, physically, is wavelength: red light has longer waves than blue light. And glass slows short waves marginally more than long ones. Look how marginal:

The difference between red and blue is 0.008 — about half a percent. That entire rainbow, every rainbow you have ever seen, comes out of a half-percent disagreement about how fast light travels through water. This spreading is called dispersion.

Wavelength and colour are two ways of saying the same thing, and a wavelength and frequency converter translates between wavelength, frequency and where a colour sits in the spectrum.

The angles are smaller than you'd think

Send light into glass at 45° and the colours refract to 27.84° (red) and 27.68° (blue). A difference of 0.16 of a degree — invisible.

A prism is shaped the way it is to make that tiny difference usable. Its two non-parallel faces mean the light bends twice in the same direction rather than bending back straight, and the separation accumulates. Through a standard 60° prism, red and blue come out about 0.71 degrees apart.

Still less than a degree. Which explains something odd about how prisms are demonstrated:

The colours leave the prism almost on top of each other, and only pull apart with distance. You need a long dark room to get a decent spectrum — which is exactly what Newton used.

Newton and the Experiment That Settled It

Prisms weren't new in the 1660s. They were sold as toys, and everybody had seen the colours. The argument was about where the colours came from.

The accepted view, going back to Aristotle, was that white light was pure and simple, and that colours were what you got when it became corrupted — darkened, weakened, modified by passing through the glass. Colour was damage.

Isaac Newton, working in his rooms at Cambridge around 1666, thought the colours were already in the white light. Which is a fine hypothesis, and completely useless unless you can tell the two ideas apart.

So he designed a test — later called the experimentum crucis, the crucial experiment. Split the light with one prism. Then take a single colour out of the spectrum by letting only that part through a small hole in a board, and send that one colour into a second prism.

The two theories predict different things, and the widget below lets you run both.

The single colour bent, and did nothing else. Green went in, green came out — just deflected a little further. Newton then went one better: he collected the whole spread-out spectrum with a second prism turned the other way and recombined it back into white light.

That pair of results killed the old theory outright. Glass cannot be manufacturing colour, because a colour that has already been through glass doesn't change when it goes through more of it. White light is a mixture, and a prism is a sorting machine. To play with the sorting yourself — different glass types, different prism angles, different incoming beams — there's a light dispersion simulator.

Newton published the finding in 1672 and spent years arguing about it with people who disliked the conclusion. He was also the one who chose to name seven colours in the spectrum — the boundaries between them are not physical, since the spectrum is continuous. He reportedly wanted seven to match the notes of a musical scale, which is why indigo is in the list at all and why nobody has ever confidently pointed at it.

Back to the Rainbow

Now the rainbow makes sense in detail. Sunlight enters a raindrop, refracts and separates, reflects off the inside of the back of the drop, and refracts again on the way out — separating a little more each time it crosses a surface.

Each colour emerges at its own characteristic angle:

• Red comes back at about 42.4° from the direction opposite the Sun.
• Violet comes back at about 40.6°.

Which explains a detail most people have never noticed a reason for: red is always on the outside of a rainbow and violet always on the inside. The drops sending you red light are the ones sitting slightly higher in the sky.

It also means a rainbow isn't an object in a location. It's an angle. Every observer sees a different rainbow made of different raindrops, centred on their own shadow — which is why you can never reach one, and why two people standing side by side are technically looking at two different rainbows.

If you ever see a fainter second bow above the first with the colours reversed, that's light that bounced twice inside the droplets before escaping. The extra reflection flips the order and loses brightness.

Where Else This Shows Up

• Cheap camera lenses. A lens bends light, so it disperses light too, and the colours focus at slightly different points — coloured fringing around high-contrast edges, called chromatic aberration. Expensive lenses fight it by combining glass types with opposite dispersion.
• Fibre optic cable. Different colours travelling at different speeds means a sharp pulse of mixed light smears out over long distances, limiting data rates. A genuine engineering headache.
• Diamond. Its "fire" — those flashes of colour — is dispersion. Diamond bends light unusually hard and disperses it strongly, and cuts are designed to exploit exactly that.

One thing that is not this: the colours in a soap bubble or an oil slick. Those come from light waves interfering with each other, a different mechanism entirely.

The Takeaway

A prism doesn't add anything to light. It slows light down, and slows blue slightly more than red, and that half-percent difference — accumulated over two surfaces and a few metres of room — pulls white light apart into everything it was made of.

The lasting lesson is arguably Newton's method rather than his result. Everyone could see the colours; the disagreement was about what they meant. What settled it was designing one experiment whose two possible outcomes pointed at two different explanations — and then going and looking.

Run the experiment yourself

Pick a colour with the hole, send it into the second prism, and compare what the two rival theories predicted — then recombine the spectrum back into white. Free, no sign-up, runs in your browser.