KS3 · Physics

The visible spectrum

Shine white light through a triangle of glass and a rainbow spills out. The glass didn't make those colours — they were hiding in the light all along.

Follow one beam of white light through a prism

Tap each stage to look closer. Watch for the moment the colours appear — and ask whether anything new was made there.

What colour is sunlight?

Pick up the yellow pencil?

You're drawing a sunny day. Almost everyone reaches for the yellow pencil to colour in the Sun.

Which idea about sunlight is closest to what you think right now?
How sure are you?

Put the prism on trial

Two experiments. If the prism really made colours, what would you expect? Commit before you look.

Test 1: a beam of pure red light goes into a prism. Test 2: the spectrum from one prism goes straight into a second prism turned upside down. What do you see?

The order that never changes

Line up the spectrum

Frequency of the light, lowest first

1 · Lowest frequency7 · Highest frequency

  1. Green

  2. Violet

  3. Red

  4. Blue

  5. Yellow

  6. Indigo

  7. Orange

Watch out: Frequency here is just 'lower' or 'higher' — you don't need any numbers, only the direction it goes.

Seeing colour

?

Reason it through

How do your eyes and brain turn light into the colours you see?

Link 1 of 4

First link · your turn

Light from the scene goes into your eye. Where does it need to land before anything is detected?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Colours your brain makes

Which cones does each light trigger?

Using the simple three-cone model, tick the cones each light triggers. Then tick the last column if the colour you would see appears anywhere in the spectrum.

Pure yellow light
Red light + green light (equal amounts)
Red light + blue light together
White light
Pure blue light

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Ordinary white light has been carrying a rainbow all along — here's how we know, and how your eyes pull the colours back out.

What you need to know

  • White light (sunlight, daylight, light from many bulbs) is a mixture of all the colours of the visible spectrum. It is not one pure colour and not an absence of light.
  • A triangular prism splits white light into a spectrum because each colour changes direction by a slightly different angle. It separates colours that were already there; it adds none.
  • The spectrum always runs red, orange, yellow, green, blue, indigo, violet, and the frequency of the light increases from red to violet.
  • Rod cells in the retina sense brightness; three kinds of cone cell (R, G and B) detect colour. The colour we see depends on which cones are triggered and how strongly.
  • Some colours, such as magenta and white, can only be seen when colours of light are mixed — they appear nowhere in the spectrum.

The big picture

White light, such as sunlight, is a mixture of every colour of the visible spectrum. A triangular prism separates those colours because each one changes direction by a slightly different angle — it sorts them and adds none. The colours always run red, orange, yellow, green, blue, indigo, violet, with frequency increasing from red to violet. We see colour with three kinds of cone cell (R, G and B), and the colour depends on which cones are triggered and how strongly — so red-plus-green light can look exactly like yellow, and magenta appears in no rainbow.

Key points

1A prism is a sorter, not a paint box: the colours were in the white light from the start.
2Red light through a prism is bent but stays red — a single colour has nothing to split.
3A second prism, upside down, reverses the changes in direction and turns the spectrum back into white light.
4A rainbow forms in a similar way: raindrops split white sunlight into the colours of the spectrum.
5Sunlight is white; the Sun looks orange at sunset because its light passes through more of the atmosphere.
6Pure yellow light and red-plus-green light look identical because they trigger the same cones (R and G).

Worked example

Problem

A phone screen is showing a plain white page. Through a magnifying glass you can see that the screen is made only of tiny red, green and blue lights. Explain why the page still looks white to you.

⚠ Watch out

Saying the prism 'adds', 'makes' or 'creates' the colours. It doesn't — the colours were already mixed together in the white light. The prism only sends each colour off in a slightly different direction, so they separate.

🧠

Memory hook

Richard Of York Gave Battle In Vain gives you the order, red to violet, with frequency climbing all the way. Then two rules: the prism only sorts — it never makes a colour — and magenta is made up by your brain, so you'll never find it in a rainbow.

✓

Check yourself

Cover the page and explain, in three sentences, how you could prove to a friend that white light is a mixture of colours. Use the red-light test and the upside-down second prism in your answer.

Flashcards

(15)
What is white light?
A mixture of all the colours of the visible spectrum — not one pure colour, and not an absence of light.
Why does a prism split white light into a spectrum?
Each colour changes direction by a slightly different angle in the prism, so the colours come out travelling in slightly different directions and separate.
Does a prism add colour to light?
No. It separates the colours that were already in the white light.
What order do the colours of the spectrum come in?
Red, orange, yellow, green, blue, indigo, violet — the same order every time.
Which end of the visible spectrum has the highest frequency?
Violet. Frequency increases from red to violet.
What happens to a beam of pure red light that goes through a prism?
It changes direction but is not split and stays red, because it is a single colour, not a mixture.
What does a second prism, placed upside down after the first, do to the spectrum?
It reverses the changes in direction, so the colours travel together again and recombine into white light.
How does a rainbow form?
White light from the Sun enters raindrops and is split into the colours of the visible spectrum.
Why can the Sun look orange at sunset?
When the Sun is low, its light passes through more of the atmosphere. The Sun itself has not changed colour.
Is the visible spectrum really just seven separate colours?
No. It is a continuous range of colours; the seven names are the traditional way of describing it.
What do rod cells do?
They sense brightness.
What do cone cells do, and how many kinds are there?
They detect colour. There are three kinds: R, G and B, each sensitive to a range of colours.
What decides the colour you see?
Which cone cells are triggered, and how strongly. Triggered cones send signals along a nerve to the brain.
Why do pure yellow light and equal red and green light look the same?
Both trigger the R and G cones, so the brain receives the same signals.
Why is there no magenta in a rainbow?
Magenta is only seen when red and blue light enter the eye at the same time. No single spectrum colour does that.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More KS3 Physics topics

See the full KS3 Physics curriculum →

How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.