AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER
Visible light and colour
Waves · Lesson 11 of 12
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. What colours make up white light?
The colours of the rainbow
2. What is reflection?
A wave bouncing off a surface
3. What is absorption?
A wave's energy being taken in by a material
4. What is transparent?
Lets light pass through with no scattering
5. What is opaque?
Does not let light through
Learning Objectives
1. State that each colour of visible light has its own narrow band of wavelength and frequency.
2. Distinguish specular reflection from diffuse reflection.
3. Explain how colour filters work.
4. Explain 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.
Filters and Coloured Objects
|
Colour comes from which wavelengths are reflected or transmitted. |
White light through a red filter and reflecting from a red object, with other colours absorbed.
Specular or Diffuse Reflection?
|
SPECULAR REFLECTION |
DIFFUSE REFLECTION |
|
▸ From a smooth surface such as a mirror. ▸ Reflected in a single direction. ▸ Forms a clear image. |
▸ From a rough surface such as paper. ▸ Scattered in many directions. ▸ No clear image. |
Objects and Colours
What you see.
|
Object |
In white light |
Why |
|---|---|---|
|
Red shirt |
Red |
Reflects red, absorbs other colours |
|
Green leaf |
Green |
Reflects green, absorbs the rest |
|
White paper |
White |
Reflects all colours equally |
|
Black cloth |
Black |
Absorbs all colours |
A Colour Filter
|
White light passes through a green filter. What colour emerges? |
1. Filter
Transmits green and absorbs the other wavelengths
2. Light that emerges
Green
Answer: Green light.
Two Filters
|
White light passes through a red filter and then a blue filter. What emerges? |
1. Red filter
Transmits only red
2. Blue filter
Absorbs red and transmits blue
3. Result
No light
Answer: No light emerges (black).
Coloured Light on an Object
|
A red shirt is illuminated with green light only. What colour does it look? |
1. Red shirt
Reflects red but absorbs green
2. Green light
There is no red light to reflect
3. Result
The shirt looks black
Answer: Black.
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.
Key Terms
|
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. |
Your Task: Predict the Colour
10 minutes
|
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.
Note: Ask why a white shirt looks different under coloured lights.
Can I...?
☐ State that colours have wavelength bands.
☐ Explain specular reflection.
☐ Explain diffuse reflection.
☐ Explain filters.
☐ Explain object colour.
☐ Explain black and white.
☐ Define transparent, translucent, opaque.
☐ Predict colours under filters.
Summary
✓ 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) Reflects red, absorbs other colours. |
Exam Practice: Visible light and colour
Answer all questions. Use the mark allocation as a guide to how much to write. · 13 minutes
▸ Question 1 · 2 marks · Explain. A red book looks red in white light. Explain why.
▸ Question 2 · 2 marks · Describe. Describe the difference between specular reflection and diffuse reflection.
▸ Question 3 · 3 marks · Explain. White light passes through a green filter onto a white screen. Explain what colour the screen looks. Then explain what happens if the light…
▸ Question 4 · 3 marks · Explain. A blue shirt is seen in red light. Explain why it looks black.
▸ Question 5 · 2 marks · State. Explain the difference between a transparent and a translucent material.
Question 1 · 2 marks · Explain
|
“A red book looks red in white light. Explain why.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ Reflects red. 1 mark
▸ Absorbs other colours. 1 mark
▸ Model answer. The book reflects red light strongly and absorbs the other colours (wavelengths).
Question 2 · 2 marks · Describe
|
“Describe the difference between specular reflection and diffuse reflection.” |
HOW TO ANSWER IT Command word: Describe. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ 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.
Question 3 · 3 marks · Explain
|
“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.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ 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.
Question 4 · 3 marks · Explain
|
“A blue shirt is seen in red light. Explain why it looks black.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ 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.
Question 5 · 2 marks · State
|
“Explain the difference between a transparent and a translucent material.” |
HOW TO ANSWER IT Command word: State. Worth 2 marks, so plan before writing.
Question 5 · mark scheme
2 marks available. Award a mark for each point made.
▸ 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.