Colour, filters and the visible spectrum

What you'll learn

This revision guide covers how white light splits into different colours, why objects appear coloured, and how filters work. You'll learn about the visible spectrum, primary and secondary colours of light, and how to predict what colour objects appear under different lighting conditions—all essential content for AQA GCSE Physics Paper 2.

Key terms and definitions

Visible spectrum — the range of electromagnetic waves that can be detected by the human eye, with wavelengths from approximately 400 nm (violet) to 700 nm (red)

White light — light containing all the colours of the visible spectrum in roughly equal amounts

Primary colours of light — red, green and blue; these cannot be made by mixing other colours of light and can combine to produce all other colours

Secondary colours of light — cyan, magenta and yellow; each is produced by mixing two primary colours of light

Absorption — when an object takes in light energy and does not reflect or transmit it

Transmission — when light passes through a material without being absorbed or reflected

Filter — a transparent material that absorbs certain wavelengths of light while transmitting others

Dispersion — the separation of white light into different colours due to refraction through a prism or similar medium

Core concepts

The visible spectrum and wavelength

The visible spectrum forms a small section of the electromagnetic spectrum. White light from the Sun or a light bulb contains all visible wavelengths mixed together.

When white light passes through a prism, it undergoes dispersion. Different wavelengths refract by different amounts:

• Violet light (shortest wavelength, ~400 nm) refracts the most
• Red light (longest wavelength, ~700 nm) refracts the least
• The result is a spectrum showing: red, orange, yellow, green, blue, indigo, violet (often abbreviated as ROYGBIV)

Each colour corresponds to a specific range of wavelengths. The boundaries between colours are not sharp but gradually merge into one another.

Key exam point: In questions about prisms, remember that violet bends more than red because it has a shorter wavelength and higher refractive index in glass.

Primary and secondary colours of light

Understanding colour mixing is crucial for filter and object colour questions.

Primary colours of light:

• Red
• Green
• Blue

These are called additive primaries because combining them adds wavelengths together. Different combinations produce:

Secondary colours of light:

• Red + Green = Yellow
• Red + Blue = Magenta
• Green + Blue = Cyan

Important: Red + Green + Blue = White light

This is different from mixing paints or pigments, which use subtractive mixing. At GCSE, you only need to know additive mixing of light.

Complementary colours: Each secondary colour is complementary to the primary colour not used to make it:

• Cyan is complementary to red
• Magenta is complementary to green
• Yellow is complementary to blue

When complementary colours combine, they produce white light.

How coloured objects appear

The colour an object appears depends on:

1. The wavelengths of light hitting it
2. Which wavelengths it reflects, absorbs or transmits

Opaque objects (you cannot see through them):

A red object in white light:

• Reflects red wavelengths
• Absorbs all other wavelengths (green, blue, yellow, etc.)
• Appears red because only red light reaches your eyes

A white object in white light:

• Reflects all wavelengths equally
• Absorbs very little light
• Appears white

A black object in white light:

• Absorbs all wavelengths
• Reflects very little or no light
• Appears black

Transparent and translucent objects:

These allow light to pass through them (transmit light). The colour of a transparent object depends on which wavelengths it transmits and which it absorbs.

Objects under coloured light

This is a common exam topic that requires careful thinking.

Method: Consider which wavelengths are available in the incident light, then determine which of those wavelengths the object can reflect.

A red object under different lighting:

Under white light: Reflects red, absorbs other colours → appears red

Under red light: Reflects red → appears red

Under green light: No red light available to reflect, absorbs the green → appears black

Under blue light: No red light available to reflect, absorbs the blue → appears black

A white object under different lighting:

Under white light: Reflects all colours → appears white

Under red light: Reflects red (no other colours available) → appears red

Under blue light: Reflects blue → appears blue

Critical rule: An object can only reflect wavelengths that are present in the light hitting it. If the required wavelength is absent, the object appears black (or much darker).

How colour filters work

A filter is a transparent material that selectively transmits certain wavelengths while absorbing others.

Red filter:

• Transmits red light
• Absorbs green and blue light

Green filter:

• Transmits green light
• Absorbs red and blue light

Blue filter:

• Transmits blue light
• Absorbs red and green light

White light through a filter:

When white light passes through a red filter:

• Red wavelengths are transmitted
• All other wavelengths are absorbed
• Light emerging from the filter is red

Coloured light through a filter:

Red light through a red filter:

• Red light is transmitted
• Light emerges as red

Red light through a green filter:

• Green filter cannot transmit red light
• Red light is absorbed
• No light (or very little light) emerges → appears black

Multiple filters in sequence:

White light → red filter → green filter:

1. After red filter: only red light remains
2. Red light reaches green filter
3. Green filter absorbs red light
4. No light transmitted → appears black

For light to pass through multiple filters, each filter must be able to transmit the wavelengths that passed through the previous filter.

Secondary colour filters

Cyan filter:

• Transmits blue and green light (because cyan = blue + green)
• Absorbs red light

Magenta filter:

• Transmits red and blue light (because magenta = red + blue)
• Absorbs green light

Yellow filter:

• Transmits red and green light (because yellow = red + green)
• Absorbs blue light

Exam tip: If asked about secondary colour filters, break them down into their primary components to determine what they transmit.

Example: White light through a yellow filter then a blue filter:

1. Yellow filter transmits red and green, absorbs blue
2. Only red and green light reach the blue filter
3. Blue filter cannot transmit red or green
4. All light absorbed → appears black

Worked examples

Example 1: Object colour under different lighting (3 marks)

Question: A pair of jeans appears blue in white light. Explain why the jeans would appear black when viewed in red light.

Mark scheme answer:

• The jeans appear blue because they reflect blue light and absorb other colours (1 mark)
• Red light contains no blue wavelengths (1 mark)
• The jeans absorb the red light and have no blue light to reflect, so appear black (1 mark)

Examiner note: Students must explain both what happens normally (in white light) and why it's different under red light. Simply stating "no blue light to reflect" without explaining absorption scores fewer marks.

Example 2: Filter combinations (4 marks)

Question: White light passes through a magenta filter followed by a green filter.

(a) State what colour light emerges from the magenta filter. (1 mark)

(b) Explain why no light emerges from the green filter. (3 marks)

Mark scheme answer:

(a) Magenta (1 mark)

Alternative acceptable: Red and blue, or a mixture of red and blue

(b)

• The magenta filter transmits red and blue light (and absorbs green) (1 mark)
• The green filter only transmits green light (1 mark)
• No green light reaches the green filter, so all light (red and blue) is absorbed (1 mark)

Examiner note: Part (b) requires an explanation showing the sequence of events. Stating only the final result without explaining the transmission and absorption process would not earn full marks.

Example 3: Predicting object appearance (2 marks)

Question: A white T-shirt is viewed through a cyan filter under white light. State and explain what colour the T-shirt appears.

Mark scheme answer:

• The T-shirt appears cyan (1 mark)
• The white T-shirt reflects all colours, and the cyan filter only transmits blue and green light, which combine to make cyan (1 mark)

Alternative explanation: The cyan filter absorbs red light but transmits blue and green, which the white shirt reflects, so it appears cyan.

Examiner note: The question asks to "state and explain"—both parts are needed for full marks. Students often forget to state the colour clearly at the start of their answer.

Common mistakes and how to avoid them

• Confusing light mixing with paint mixing: Paint mixing is subtractive (red + blue paint = purple), but light mixing is additive (red + blue light = magenta). GCSE only tests light mixing. Remember: red + green light = yellow, which seems counterintuitive if you think about paints.

• Saying a filter "lets through its own colour only": This works for primary colour filters but not secondary. A yellow filter transmits both red AND green light. Break secondary colours into their primary components.

• Forgetting objects can only reflect what's available: A red object under green light appears black, not red. If the wavelength isn't in the incident light, it cannot be reflected. Always check what light is actually hitting the object.

• Not explaining both absorption and reflection: Exam answers often need both. For example: "The apple appears red because it reflects red light and absorbs all other wavelengths." Missing either part may lose marks.

• Mixing up transmission and reflection: Filters transmit light (let it pass through). Opaque objects reflect light. Use the correct term for the correct situation—using "reflects" when discussing filters will lose marks.

• Assuming some light always gets through filters: When a red filter receives only green light, no light emerges—it appears black, not "dark green" or "a bit of light." Be definitive when appropriate.

Exam technique for "Colour, filters and the visible spectrum"

• Command word "Explain": You must give reasons, not just describe what happens. For 3-mark "explain" questions, your answer needs a clear logical sequence: state what happens, give the reason, then state the result. Example: "The ball reflects green light (what), because green wavelengths are not absorbed (why), so only green light reaches the eye (result)."

• Drawing ray diagrams with filters/prisms: Use a ruler and draw clearly. Label incident light, transmitted/emerging light, and absorbed light where relevant. For prisms, show red deviating least and violet deviating most—many students draw this backwards.

• Questions involving "appears black": This is a positive statement requiring explanation. State explicitly that no light is reflected/transmitted, not just that certain colours are absorbed. "All wavelengths are absorbed" or "no light available to reflect" scores marks.

• Multi-step filter questions: Work through systematically. Write down what emerges from each filter in sequence. This prevents errors and shows clear working that can earn method marks even if the final answer is wrong.

Quick revision summary

The visible spectrum ranges from red (~700 nm) to violet (~400 nm). White light contains all visible wavelengths. Primary colours of light are red, green and blue; secondary colours are cyan, magenta and yellow. Objects appear coloured by reflecting certain wavelengths and absorbing others. Filters transmit specific wavelengths and absorb the rest. An object can only reflect wavelengths present in the incident light—if those wavelengths are absent, it appears black. Secondary colour filters transmit two primary colours.