Science · Year 9 · Waves · Summer Term

The Visible Spectrum and Colour

Discover how white light is composed of a spectrum of colours and learn how objects appear coloured due to the reflection and absorption of specific wavelengths.

TL;DR:Illuminate your students' understanding of the world by exploring the science of colour. This topic uncovers the secrets hidden within a simple beam of white light and explains why our world is so vibrant.

• National Curriculum Attainment Targets
• KS3 National Curriculum in England: Science - Physics: Light waves: the transmission of light through materials: absorption, diffuse scattering and specular reflection at a surface
• use of colour filters

About This Topic

This topic delves into the physics of light and colour, a core component of the Key Stage 3 Science curriculum in Great Britain. It builds upon earlier concepts of light travelling in straight lines and introduces the idea that white light is not fundamental but is a composite of the colours of the visible spectrum. The first key concept is dispersion, the process by which a prism separates white light into its constituent colours, ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet), due to the different wavelengths of light refracting at slightly different angles. This provides a tangible demonstration that underpins the entire topic.

The second major area explores how we perceive the colour of objects. Students will learn that the colour of an opaque object is determined by the wavelengths of light it reflects, while it absorbs all other wavelengths. This is a crucial distinction from the misconception that colour is an intrinsic property of an object. The topic culminates in contrasting the two types of colour mixing: additive mixing of light (where the primary colours are red, green, and blue) and subtractive mixing of pigments or inks (where the primary colours are cyan, magenta, and yellow). Understanding this difference is key to explaining real-world applications like television screens (additive) and colour printing (subtractive).

Key Questions

1. Explain how a prism separates white light into the colours of the spectrum.
2. Analyse why a red object appears red in white light and black in blue light.
3. Compare the additive mixing of coloured lights with the subtractive mixing of coloured pigments.

Learning Objectives

• Recall that white light is a mixture of different colours.
• Describe how a prism can be used to disperse white light into a spectrum.
• Explain how the colour of an opaque object is determined by which wavelengths of light are reflected and which are absorbed.
• Compare and contrast the additive mixing of coloured lights with the subtractive mixing of coloured pigments.
• Predict the apparent colour of an object under different lighting conditions.

Key Vocabulary

Dispersion	The splitting of white light into its constituent colours, as seen when it passes through a prism.
Spectrum	The range of colours (red, orange, yellow, green, blue, indigo, violet) that is produced when white light is dispersed.
Wavelength	The distance between two successive crests of a wave. In light, wavelength corresponds to colour.
Reflection	The bouncing back of light from a surface.
Absorption	The process by which light energy is taken in by an object and converted into another form, usually heat.
Pigment	A substance that gives colour to other materials by absorbing specific wavelengths of light.

Watch Out for These Misconceptions

Common Misconception

Colour is a physical property of an object, like its mass or texture.

What to Teach Instead

An object's colour depends on the light it reflects. A red apple appears red because its surface reflects red wavelengths of light and absorbs all other colours. In the absence of red light, it cannot reflect red and will appear black.

Common Misconception

Mixing coloured paints and mixing coloured lights follow the same rules.

What to Teach Instead

Mixing light is additive: adding colours makes things brighter, with red, green and blue light combining to make white. Mixing paint is subtractive: pigments absorb light, so adding colours makes things darker, with cyan, magenta and yellow combining to make a dark brown or black.

Common Misconception

White is the absence of colour, and black is a colour.

What to Teach Instead

White light is a mixture of all the colours of the visible spectrum. Black is the absence of visible light; an object appears black when it absorbs all wavelengths of light that fall on it.

Active Learning Ideas

Experiential Learning

Prism Rainbows

In a darkened room, students use a ray box and a glass prism to split a beam of white light into the visible spectrum. They can project the spectrum onto a white card and label the colours in the correct order.

20 min·Pairs

Experiential Learning

Colour Filter Investigation

Students observe various coloured objects through red, green, and blue filters (e.g., cellophane). They record the apparent colour of each object and explain their observations based on which colours of light are transmitted by the filter and reflected by the object.

25 min·Small Groups

Experiential Learning

Additive Light Mixing

Using three torches or ray boxes with red, green, and blue filters, students overlap the beams of light on a white screen. They can identify the secondary colours produced (cyan, magenta, yellow) and the effect of mixing all three.

15 min·Whole Class

Real-World Connections

• The operation of digital displays on phones, computers, and televisions, which use tiny red, green, and blue pixels to create millions of colours.
• Colour printing for books, magazines, and photographs, which layers cyan, magenta, yellow, and black inks to produce a full-colour image.
• The use of coloured filters in theatre lighting to create specific moods and effects on stage.
• The way artists and painters mix pigments to achieve the exact shades they need for their work.
• The biological adaptation of camouflage in animals, where their skin or fur reflects the colours of their natural habitat to avoid predators.

Assessment Ideas

Quick Check

Use mini-whiteboards for students to draw a diagram showing what happens when white light hits a green leaf, labelling the reflected and absorbed colours.

Quick Check

A short, structured quiz at the end of the topic featuring questions that require students to apply their knowledge, for example, 'Explain why a red snooker ball appears black under a blue light'.

Peer Assessment

Students work in pairs to create a concept map linking key vocabulary like 'dispersion', 'reflection', 'pigment', and 'spectrum', then review each other's maps for accuracy and completeness.

Frequently Asked Questions

Why is the sky blue on a clear day?

Sunlight is scattered by the tiny molecules of gas and dust in the Earth's atmosphere. Blue light has a shorter wavelength and is scattered more effectively than the other colours, so more blue light reaches our eyes from all directions.

How do rainbows form?

Rainbows are formed when sunlight shines through raindrops. Each raindrop acts like a tiny prism, refracting and dispersing the sunlight, which splits it into the colours of the spectrum.

Why do computer screens use RGB while printers use CMYK?

Screens emit light, so they use the additive primary colours: Red, Green, and Blue (RGB). Combining these creates lighter colours, eventually white. Paper reflects light, so printers use subtractive primary colours: Cyan, Magenta, Yellow, and Black (Key) (CMYK). The inks absorb light, so adding more ink creates darker colours.