Dispersion, Colour and the Visible Spectrum
White light is a mixture of colours. A prism separates them because each colour refracts by a different amount — and that single fact explains prisms, rainbows and the order of the spectrum.
1. Dispersion
Dispersion is the splitting of white light into its component colours.
How a prism does it:
- White light enters the prism and refracts at the first surface
- Each colour travels at a slightly different speed in the glass, so each refracts by a different amount
- The colours separate and spread out
- They refract again on leaving, spreading further
- A spectrum appears on a screen
The colours separate because they refract by DIFFERENT AMOUNTS, not because the prism “adds” colour. The colours were always present in the white light.
2. The visible spectrum
Red, Orange, Yellow, Green, Blue, Indigo, Violet — ROYGBIV
| End | Colour | Wavelength | Frequency | Refraction |
|---|---|---|---|---|
| Top of the spectrum | RED | longest | lowest | refracts LEAST |
| Bottom | VIOLET | shortest | highest | refracts MOST |
Red bends least; violet bends most. This is the fact every dispersion question turns on. A memory hook: red is at the “relaxed” end — it barely bends.
Why: violet light travels more slowly in glass than red, so the glass has a slightly higher refractive index for violet — and a bigger change in speed means a bigger change in direction.
In a vacuum (and near enough in air) ALL colours travel at the same speed — 3 × 10⁸ m/s. They only separate inside the glass, which is why dispersion needs a medium.
3. Monochromatic light
Monochromatic light is light of a single frequency (and therefore a single colour and wavelength).
A laser is the standard example.
Monochromatic light does NOT disperse — there is only one colour present, so there is nothing to separate. It still refracts, but it emerges as a single beam.
White light is the opposite: a mixture of all visible frequencies.
4. Recombining the spectrum
Passing the spectrum through a second, inverted prism brings the colours back together to form white light again — proving the prism separates existing colours rather than creating them.
5. The rainbow
A rainbow is dispersion in water droplets:
- Sunlight refracts entering each droplet, and disperses
- It totally internally reflects off the back of the droplet
- It refracts again on leaving, spreading the colours further
- Red appears on the outside of the arc, violet on the inside
6. Where this sits in the electromagnetic spectrum
Visible light is a narrow band in the middle of the electromagnetic spectrum.
Just beyond red is infrared; just beyond violet is ultraviolet. Both are invisible.
The same wavelength–frequency trend continues in both directions: moving towards violet and beyond means shorter wavelength, higher frequency, higher energy.
7. Mistakes that cost marks
Saying the prism adds or creates colour.
Getting the spectrum order backwards.
Saying red refracts most — it refracts least.
Saying colours travel at different speeds in a vacuum.
Saying monochromatic light disperses.
Confusing the visible spectrum with the whole electromagnetic spectrum.
Putting violet on the outside of a rainbow.
Frequently asked questions
What is dispersion? The splitting of white light into its component colours.
Why does a prism split white light? Each colour refracts by a different amount, because each travels at a slightly different speed in the glass.
What is the order of the visible spectrum? Red, orange, yellow, green, blue, indigo, violet.
Which colour refracts the most? Violet — it has the shortest wavelength and highest frequency.
Which refracts the least? Red.
Do all colours travel at the same speed? In a vacuum, yes. In glass they differ slightly, which is what causes dispersion.
What is monochromatic light? Light of a single frequency — for example from a laser.
Does monochromatic light disperse? No — there is only one colour to begin with.
How can you prove a prism doesn’t add colour? Use a second, inverted prism to recombine the spectrum into white light.
Which colour is on the outside of a rainbow? Red.
Quick revision checklist
- I can define dispersion
- I can explain why a prism separates colours
- I know the spectrum order ROYGBIV
- I know red refracts least, violet most
- I can link this to wavelength, frequency and speed in glass
- I know all colours travel at the same speed in a vacuum
- I know what monochromatic means
- I know monochromatic light does not disperse
- I can describe recombining the spectrum
- I can explain a rainbow and the colour order
- I know infrared and ultraviolet lie either side of the visible band
These notes cover dispersion, colour and the visible spectrum in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons. This was one of the least-covered subtopics in that set, so the page follows the syllabus closely and is deliberately concise rather than padded. Always check the current syllabus for your own exam series.
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