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Physics · Waves

Visible light and colour

Explain specular and diffuse reflection, the effect of colour filters, and why objects have particular colours (Physics only).

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What colours make up white light?

    Show answerHide answer

    The colours of the rainbow

  2. 2

    What is reflection?

    Show answerHide answer

    A wave bouncing off a surface

  3. 3

    What is absorption?

    Show answerHide answer

    A wave's energy being taken in by a material

  4. 4

    What is transparent?

    Show answerHide answer

    Lets light pass through with no scattering

  5. 5

    What is opaque?

    Show answerHide answer

    Does not let light through

Learning Objectives

  1. 1State that each colour of visible light has its own narrow band of wavelength and frequency.
  2. 2Distinguish specular reflection from diffuse reflection.
  3. 3Explain how colour filters work.
  4. 4Explain why an opaque object has a particular colour.

COLOUR

Each colour within the visible spectrum has its own narrow band of wavelength and frequency. The colour of an opaque object depends on which wavelengths it reflects most strongly.

Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object looks white; if all are absorbed it looks black.

Specular or Diffuse Reflection?

Specular reflection

  • From a smooth surface such as a mirror.
  • Reflected in a single direction.
  • Forms a clear image.

Diffuse reflection

  • From a rough surface such as paper.
  • Scattered in many directions.
  • No clear image.

Objects and Colours

What you see.

  • Red shirt

    In white light: Red. Why: Reflects red, absorbs other colours

  • Green leaf

    In white light: Green. Why: Reflects green, absorbs the rest

  • White paper

    In white light: White. Why: Reflects all colours equally

  • Black cloth

    In white light: Black. Why: Absorbs all colours

A Colour Filter

White light passes through a green filter. What colour emerges?

Show the solutionHide the solution
  1. 1 Filter Transmits green and absorbs the other wavelengths
  2. 2 Light that emerges Green

AnswerGreen light.

Two Filters

White light passes through a red filter and then a blue filter. What emerges?

Show the solutionHide the solution
  1. 1 Red filter Transmits only red
  2. 2 Blue filter Absorbs red and transmits blue
  3. 3 Result No light

AnswerNo light emerges (black).

Coloured Light on an Object

A red shirt is illuminated with green light only. What colour does it look?

Show the solutionHide the solution
  1. 1 Red shirt Reflects red but absorbs green
  2. 2 Green light There is no red light to reflect
  3. 3 Result The shirt looks black

AnswerBlack.

Key Terms

Transparent, translucent and opaque.

  • Transparent

    Transmits light; you can see clearly through it.

  • Translucent

    Transmits light but scatters it; you cannot see a clear image.

  • Opaque

    Does not transmit light.

  • Filters

    Absorb certain wavelengths and transmit others.

Predict the Colour

What colour does a blue book look in (a) white light (b) red light? What does a white page look in blue light?

1. What wavelengths does it reflect?

2. What light is available?

A good answer shows: (a) Blue. (b) Black, because it absorbs red. A white page in blue light looks blue.

Can I...?

  1. 1State that colours have wavelength bands.
  2. 2Explain specular reflection.
  3. 3Explain diffuse reflection.
  4. 4Explain filters.
  5. 5Explain object colour.
  6. 6Explain black and white.
  7. 7Define transparent, translucent, opaque.
  8. 8Predict colours under filters.

Summary & Exam Focus

  • Colours: narrow bands of wavelength and frequency.
  • Smooth: specular; rough: diffuse.
  • Filters transmit some wavelengths and absorb others.
  • Object colour from the wavelengths it reflects.

Exam focus

Explain why a red book looks red in white light. (2 marks) (2 marks)

Reflects red, absorbs other colours.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Specular reflection
Reflection from a smooth surface in a single direction.
Diffuse reflection
Reflection from a rough surface in many directions.
Colour filter
Material that transmits some wavelengths and absorbs others.
Transparent
Lets light pass through with no scattering.
Translucent
Lets light through but scatters it.
Opaque
Does not let light through.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Explain 2 marks Easier

A red book looks red in white light. Explain why.

Mark scheme — 2 marks available

  • Reflects red — 1 mark
  • Absorbs other colours — 1 mark

Model answer

The book reflects red light strongly and absorbs the other colours (wavelengths).

2. Exam question Describe 2 marks Easier

Describe the difference between specular reflection and diffuse reflection.

Mark scheme — 2 marks available

  • Specular: smooth surface, one direction — 1 mark
  • Diffuse: rough surface, scattered — 1 mark

Model answer

Specular reflection is from a smooth surface and goes in one direction; diffuse reflection is from a rough surface and scatters in many directions.

3. Exam question Explain 3 marks Easier

White light passes through a green filter onto a white screen. Explain what colour the screen looks. Then explain what happens if the light from the green filter goes through a red filter.

Mark scheme — 3 marks available

  • Green filter transmits green — 1 mark
  • Screen looks green — 1 mark
  • Red filter absorbs green: no light — 1 mark

Model answer

The green filter transmits green and absorbs the other colours, so the screen looks green. A red filter then absorbs the green light, so no light is transmitted and the screen looks black.

4. Exam question Explain 3 marks Easier

A blue shirt is seen in red light. Explain why it looks black.

Mark scheme — 3 marks available

  • Shirt reflects blue — 1 mark
  • Absorbs red light — 1 mark
  • So no light reflected: looks black — 1 mark

Model answer

The shirt reflects blue light and absorbs other colours. Red light contains no blue, so the shirt absorbs all the red light and no light is reflected.

5. Exam question State 2 marks Easier

Explain the difference between a transparent and a translucent material.

Mark scheme — 2 marks available

  • Transparent: clear view — 1 mark
  • Translucent: light scattered — 1 mark

Model answer

A transparent material transmits light without scattering so you can see clearly through it. A translucent material transmits light but scatters it, so you cannot see a clear image.

6. Multiple choice 1 mark Easier

Reflection from a smooth surface is called...

  1. A diffuse
  2. B absorption
  3. C refraction
  4. D specular Correct

Why: One direction.

7. Multiple choice 1 mark Core

A white object reflects...

  1. A all colours Correct
  2. B no colours
  3. C only red
  4. D only blue

Why: All wavelengths equally.

8. Multiple choice 1 mark Core

A black object...

  1. A reflects all
  2. B absorbs all colours Correct
  3. C transmits light
  4. D is transparent

Why: No reflection.

9. Multiple choice 1 mark Core

A green filter transmits...

  1. A red light
  2. B all light
  3. C green light Correct
  4. D no light

Why: It absorbs other colours.

10. Multiple choice 1 mark Stretch

A material that scatters the light it transmits is...

  1. A transparent
  2. B opaque
  3. C a mirror
  4. D translucent Correct

Why: You cannot see clearly through it.