Dispersion · the spectrum · and the eye that catches it
Newton's real experiment was not the first prism. It was the second one, which took the fan of colours and put them back together into white. That proved the colours had been in the sunlight all along, and the glass was only sorting them.
This page covers dispersion and the one fact behind it, the order of the colours and why it never changes, how it differs from scattering, and how the eye focuses and senses what reaches it. By the end you'll be able to:
Splitting light is how we learned the composition of the sun without going there. It is also why every camera lens is a stack of different glasses:
Searched as: dispersion of light, why does a prism split light, which colour bends most, why is the sky blue, human eye parts and functions, power of accommodation.
Predict how far the beam will be bent, lock it, then send white light through. Switch to the screen tab to see where each colour lands. 🟢 real Snell and Cauchy engine
Guess, then lock — the bench waits until you commit.
Set a prism angle and an entry angle, then look at the spread — and ask what in the glass produced it.
Three different causes get mixed together constantly. Six situations — name the cause before reaching for a formula.
A rainbow after rain. What is really going on?
Ask first: is anything being SPLIT here, or bounced, or is the question about what reaches the eye?
Only two of the six are dispersion, and that is the point of asking.
You are given a target fan between red and violet. Choose a prism angle that delivers it — 3 attempts.
Set the angle, then send the light through.
Each claim breaks something. Tap the broken idea.
The colours were in the sunlight before it arrived. Newton showed this by passing the fan through a second prism and recombining it into white — which would be impossible if the glass were adding anything.
The colours do separate inside the glass, but the two faces are parallel, so each colour is bent back by exactly the same amount on the way out and they recombine. A prism works because its faces are not parallel.
A camera focuses by moving its lens; an eye cannot. Both the lens and the retina are held in place, so the lens changes SHAPE instead, squeezed rounder by the ciliary muscle when you look at something near.
Thinking beats memorising. Sort every claim about light and colour into three boxes.
One situation, one decision. Everything about dispersion rests on this single ordering.
Ask which colour meets the larger refractive index — and whether anything is bending at all.
The retina sits 2.4 cm behind the lens and cannot move. Pick something to look at, then supply the power the eye needs.
P = 1/v + 1/u, with v fixed at the retina.
Read the scene. All three separate splitting from scattering from seeing.
Light enters a drop, reflects once off the back, and leaves at a narrow range of angles centred on about 42 degrees from its incoming direction. That angle is measured from the line through your own shadow, so the arc always sits opposite the sun.
This is scattering, not dispersion. Molecules much smaller than a wavelength redirect blue far more strongly than red, and they do it in every direction, so blue arrives from all over the sky rather than from one arc.
Colour is subtraction: the shirt absorbs everything except red and sends that back. A sodium lamp emits a very narrow band of yellow, so there is almost no red in the room for the shirt to return, and it looks nearly black.
In plain terms: light of different colours travels at slightly different speeds inside glass, so each one bends by a slightly different amount, and a single beam comes out as a fan.
The refractive index of a material is not a single value — it depends on the colour. For flint glass it is about 1.613 for red and 1.644 for violet. That difference of three hundredths is tiny, and it is the entire reason rainbows exist. Every colour was in the beam before it arrived; the glass simply sends each one off at its own angle.
Shorter wavelengths always meet a larger index in ordinary transparent materials, so violet is always bent most and red least. That ordering is why the inside of a rainbow is violet and the outside red, in every rainbow that has ever formed. A flat window splits light too, but its parallel faces undo the split on the way out.
The retina sits a fixed 2.4 cm behind the lens, so the eye cannot focus the way a camera does. Instead a ring of muscle squeezes the lens into a rounder shape for near objects, adding the extra dioptres needed. That is called accommodation, and the lens stiffening with age is exactly why reading glasses become necessary.
Push the angles far enough and the light never leaves the prism at all — total internal reflection traps it, which is what the bench reports rather than drawing a ray that cannot exist. Achromatic lenses pair two glasses chosen so their dispersions cancel while their bending does not. And the visible band, 400 to 700 nm, is less than one octave out of the sixty that the electromagnetic spectrum covers.
Exams reward the method. Work it one step at a time — read the thought, predict the line, then reveal it.
Three small labs, each one native to this idea.
Six wrong ideas, most of them about what the glass is doing. Tap a card to bust the myth.
Splitting light was not new. Newton's contribution was recombining the fan back into white, which proved the glass was sorting rather than colouring.
Newton named seven bands to match the seven notes of a scale. The spectrum itself is continuous, which is why indigo is so hard to point at.
Spreading sunlight far enough revealed hundreds of dark lines. Each one marks an element absorbing its own wavelength in the sun's atmosphere.
Thomas Young argued the eye could not possibly have a detector for every colour, and proposed three. He was right, and screens still exploit it.
In an optical fibre the same effect smears a pulse, so engineers spend real effort cancelling what Newton spent his effort demonstrating.
Millions of drops, each splitting sunlight and sending it back at about 42 degrees. You and the person beside you see different drops.
They supply the few dioptres an ageing lens can no longer add, putting close text back onto the retina where it belongs.
Several glasses stacked together, chosen so their dispersions cancel. Without that, every photograph would have coloured fringes.
One sentence each: what is being split, and who wanted it split.
Drag the slider. Six bands of the same wave, and one very narrow slice of it is everything you have ever seen.
Ten orders of magnitude of the same thing, and the eye opens a window onto one thin strip of it.
Each one is a build and a measurement — the measuring is what turns a demo into a project.
Build: a shallow bowl of water with a mirror leaning inside it, in direct sunlight.
Measure: the width of the spectrum on the wall, and how it changes with the mirror's angle.
Build: a cardboard tube with a slit at one end and a piece of CD at the other.
Measure: compare a filament bulb, an LED and a street lamp. The sodium lamp has almost nothing in it.
Build: a glass block on paper, with pins marking the ray in and the ray out.
Measure: several pairs of angles, then plot sin i against sin r. The gradient is n.
Build: a printed line of small text and a ruler.
Measure: the closest distance at which it is still sharp, for several people of different ages. The trend is very clear.
Build: a garden hose spray on a sunny day, with your back to the sun.
Measure: the angle from your own shadow to the arc, using a protractor and a straw. Expect about 42 degrees.
Build: a simple converging lens, then the same lens paired with a weak diverging one of a different glass.
Measure: photograph a black-and-white edge through each. Count the coloured fringes.
Straight answers, each one complete on its own:
Because the refractive index of glass is not one number: it depends slightly on the wavelength. Violet meets a larger index than red, so it slows more and bends more at each face. The colours were all present in the white light beforehand, and the prism simply sends each one out at its own angle.
Violet, in every ordinary transparent material. Short wavelengths interact more strongly with the electrons in the glass, which slows them slightly more, which shows up as a larger refractive index and therefore a larger bend. Red, at the long end, is bent least, and that ordering is why the inside of a rainbow is violet.
They are two different effects. A rainbow is dispersion: light split by bending inside water drops, which produces an arc at a specific angle. A blue sky is scattering: air molecules redirect short wavelengths far more strongly than long ones, in every direction at once, so blue arrives from the whole sky rather than from an arc.
It does split the light inside the glass, but its two faces are parallel. Whatever extra bend violet gets on the way in, it gets exactly the same amount back on the way out, so the colours recombine and emerge travelling in the original direction. A prism works precisely because its faces are not parallel.
The retina sits at a fixed distance behind the lens, so the eye cannot focus by moving anything. Instead the ciliary muscle squeezes the lens into a rounder shape, which increases its power, and relaxing lets it flatten again. Reading at 25 cm needs about four dioptres more than looking at a distant hill, and supplying those four is real muscular work.
The lens gradually stiffens and the muscle loses some of its grip, so the eye can no longer add the extra dioptres that close work demands. Distant vision is unaffected, because that is the relaxed state. Reading glasses simply supply the missing power from outside, putting close text back onto the retina.
No. The spectrum is continuous, with no boundaries anywhere in it, and you could name seventy bands as easily as seven. Newton chose seven to match the seven notes of a musical scale, which is why indigo sits in the list even though very few people can point to where it starts and ends.
Because a single lens focuses blue and red at slightly different points, which puts coloured fringes on every edge. Pairing a converging lens of crown glass with a weaker diverging one of flint glass cancels most of that difference while keeping most of the focusing, and stacking more elements cancels the remaining errors.
Spot these before the exam does.
Seven question formats, the way LLOS.ai serves them — multiple choice, multiple-correct, fill-in-the-blank, match, sequence, read-think-connect, and write-your-own. Every question has layered hints: a quick nudge, the reasoning, then a deeper connection — so a wrong answer opens a door, never a dead end. 🟢 received from a board-tagged question bank · seed toward 2,000
Pick your board — the set re-tunes to its wording and emphasis. Competitive draws the JEE / NEET / Olympiad lane.
Four transparent materials, each cut into a 60° prism. Tap them in order, widest fan of colours first. Then read the arithmetic.
The stuff rainbows are made of, held in a thin hollow prism.
Pure silica, the glass used where clarity matters more than anything.
Lead-bearing glass, the kind cut into chandelier drops.
A hard crystal with the highest refractive index of the four.
Tap the four in the order you believe, widest fan of colours first.
Newton's great move was not splitting the light. It was putting it back together, and thereby proving that the glass had never added anything at all.