Dispersion of white light by a glass prism
Hold a glass prism up to a beam of sunlight and something magical happens on the wall behind it: a single white beam turns into a glowing ribbon of seven colours, in the same order every time — violet at one edge, red at the other. This is dispersion, and it is the reason rainbows exist, the reason a prism looks like it is "making" colour even though it never does, and one of the simplest, most visually convincing experiments in all of school physics. The explanation does not need a single formula: it rests entirely on one fact, that glass bends different colours of light by very slightly different amounts. This section builds that idea from the ground up — what white light actually is, why the bending differs colour by colour, what order the spectrum comes out in, and how a second prism can undo the whole effect and rebuild white light from its rainbow.
A glass prism bends each of the seven colours making up white light by a slightly different amount because glass has a slightly different refractive index for each colour, so the colours fan out into a spectrum called VIBGYOR, with violet bent most and red bent least.
The core idea
White light is not one colour — it is seven, travelling together. Ordinary sunlight, and the light from a filament bulb, looks colourless only because seven different colours are mixed uniformly inside it. Passing that mixed beam through a glass prism separates the mixture into its parts, exactly the way a sieve separates a mixed handful of grains by size. The resulting band of colours is called a spectrum, and the process of splitting is called dispersion.
The order never changes. The seven colours always emerge in the same sequence, remembered by the acronym VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red. Violet sits at one end of the band, bent away from the original beam direction the most; red sits at the other end, bent the least; the rest fall in between in that fixed order.
Why the bending differs: refractive index depends on colour. Refraction happens because light slows down on entering a denser medium like glass, and the amount of bending at a surface is governed by the medium's refractive index. The subtlety a Class 10 student must know is that glass does not have one single refractive index — it has a slightly different value for each colour of light. Violet light slows down the most inside glass, so glass has its highest refractive index for violet; red light slows down the least, so glass has its lowest refractive index for red. Since greater refractive index means greater bending, violet is deviated the most and red the least, and the six colours in between line up smoothly according to their own refractive indices.
A prism has two refracting surfaces, and both surfaces disperse. Light bends once entering the glass and again leaving it, and at each of those two surfaces every colour is nudged apart a little further from every other colour by this refractive-index difference. By the time the light exits the second face, the tiny differences built up at both surfaces have separated the once-overlapping colours into a visibly fanned-out band.
where: "deviation" is how far a colour's exit path bends away from the original straight-line direction of the incoming white beam, and "refractive index" is the standard optical property of glass for that colour — this is a qualitative proportionality only, used to explain order, not a formula to calculate numbers with; CBSE Class 10 requires no numerical relation between refractive index and colour.
Dispersion is reversible. Nothing is added and nothing is destroyed inside the glass — the prism only sorts colours that were already mixed together. Proof of this comes from a second, identical prism held upside down (inverted) just after the first: the second prism refracts each colour back the opposite way, and by the time the rays leave its second face, all seven colours have converged again into a single beam of white light. This recombination experiment was first performed by Isaac Newton, and it settles the question decisively — a prism disperses colour, it never creates it.
Why it works
Dispersion has no numerical formula at the Class 10 level, so instead of a calculation, this section reasons through why the physics must come out this way — the kind of explanation an examiner expects you to write out in full sentences.
Step 1. Start from what refraction already tells you: light bends at a surface because it changes speed when it crosses from one medium into another, and the amount of bending is set by the refractive index of the second medium for that particular light. This is true for glass exactly as it is true for water or any other transparent medium.
Step 2. Recognise that "the refractive index of glass" is not one fixed number — it is a property that depends on the colour (equivalently, the wavelength) of the light passing through. Violet light is slowed down by glass more than red light is, so glass presents a higher refractive index to violet than it does to red, with the other five colours arranged in between in the fixed VIBGYOR order.
Step 3. Apply the refraction rule colour by colour. Since bending increases with refractive index, and refractive index increases from red toward violet, the bending (deviation) of each colour must also increase from red toward violet. Every one of the seven colours obeys the same refraction rule, but because each sees a slightly different refractive index, each is bent a slightly different amount.
Step 4. Add up the effect across the whole beam. Before entering the prism, all seven colours travel together along one path, so they overlap and the eye sees only white. After passing through both faces of the prism, the small but different deviations have separated the colours' paths enough that they no longer overlap when they land on a screen — the fixed VIBGYOR order results directly from the fixed order of refractive indices from red (lowest) to violet (highest).
Step 5. Confirm the logic by reversing it. If dispersion is purely a sorting process and creates nothing new, then feeding the already-sorted spectrum into a second, inverted, identical prism should undo the sorting and rebuild white light — which is exactly what Newton's recombination experiment shows, closing the argument.
Worked example
Question: A beam of white sunlight enters a glass prism and forms a spectrum on a screen. A student notices the red band lies closest to the direction the original beam was travelling, while the violet band lies farthest away, and is then asked to also predict where a beam of pure yellow light (with no other colours mixed in) would land if sent through the same prism alone.
Step 1. Recall that white light contains all seven VIBGYOR colours, and that glass bends each one by a different amount because its refractive index is different for each colour — highest for violet, lowest for red.
Step 2. Explain the observed spectrum: since red has the lowest refractive index in glass, it is bent least and lands closest to the original beam direction; since violet has the highest refractive index, it is bent most and lands farthest away. This matches exactly what the student observed.
Step 3. Now consider the new case — a beam of pure yellow light, with no other colours present. There is nothing here to disperse, because dispersion requires a mixture of colours to be sorted apart; a single-colour (monochromatic) beam has nothing to separate from.
Step 4. Predict the outcome: the pure yellow beam still refracts (bends) at the prism's two faces, by an amount corresponding to glass's refractive index for yellow light — but it emerges as a single yellow spot, in a position between where the yellow band sits inside a full spectrum. No spreading into other colours occurs.
Answer: the white-light spectrum shows red closest to the original direction and violet farthest because red has the least and violet the most refractive index in glass, while a pure yellow beam alone would simply refract to one position and remain a single yellow spot, since dispersion cannot occur without a mixture of colours to sort.
Competitive stretch
If the same white beam were sent instead through a container of pure water shaped like a prism, the same VIBGYOR order would appear, because water's refractive index also rises gently from red to violet — dispersion is a general property of transparent refracting media, not something unique to glass; only the amount of spreading (glass typically disperses more than water) would differ.
Common traps
- Thinking the prism "creates" or "adds" colour to white light — it only sorts colours that are already present and mixed together.
- Getting the VIBGYOR order backwards — violet is always bent the most, red always the least; the mnemonic itself already lists them from most-bent to least-bent.
- Assuming all seven colours travel through glass at the same speed — they do not, and this speed difference is the entire cause of dispersion.
- Believing a single, pure colour of light (monochromatic light) can be "dispersed" into a spectrum — dispersion needs a mixture of colours to sort; one colour alone just refracts, it does not spread into a band.
Ask the tutor: Curious why glass bends violet more than red at the deeper refractive-index level, or want the AI tutor to sketch the recombination experiment with two prisms step by step?
Why does a prism split light into colours but a plain glass slab does not seem to?
A rectangular glass slab does disperse light very slightly at each of its two parallel faces, but the two dispersions happen in opposite directions and almost exactly cancel by the time light exits the second, parallel face, so the emerging beam looks white again with only a tiny, hard-to-see colour fringe. A prism's two faces are not parallel — they meet at an angle — so the bending at the second face adds to, rather than cancels, the bending at the first, and the small colour separations accumulate into a clearly visible spectrum.
Is the order VIBGYOR the same as the order of wavelengths?
Yes. Violet has the shortest wavelength of visible light and red has the longest, with indigo, blue, green, yellow and orange lying in between in decreasing wavelength from violet toward red. Shorter-wavelength light is slowed down more by glass, so it has a higher refractive index and bends more — which is exactly why violet, the shortest wavelength, is deviated the most, and red, the longest wavelength, is deviated the least.
Does a rainbow in the sky work the same way as a prism spectrum?
Yes, in essence. A rainbow forms when sunlight enters countless tiny spherical raindrops, refracts on entering, reflects once off the inside back of each drop, and refracts again on leaving — and at both refractions, the raindrop's water disperses the colours exactly as a glass prism does, because water's refractive index also depends slightly on colour. Millions of drops each send out their own tiny spectrum, and only one colour from each drop reaches a given observer's eye at the right angle, so the sky-wide arc you see is built from countless individual drop-spectra stacked together.