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AQA GCSE Physics · Topic 4.6 Waves · Physics only
Colour, Filters and Black Body Radiation
Physics only Combined Science students don't need this page.
In this lesson you'll learn to: explain why an opaque object looks the colour it does, explain what a colour filter does using absorb and transmit, describe a perfect black body, use the graph to link intensity and peak wavelength to temperature, and explain how the rates of absorbing and emitting radiation decide the temperature of an object and of the Earth.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
In this lesson you'll learn to: explain why an opaque object looks the colour it does, explain what a colour filter does using absorb and transmit, describe a perfect black body, use the graph to link intensity and peak wavelength to temperature, and explain how the rates of absorbing and emitting radiation decide the temperature of an object and of the Earth.
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Start here — have a play
Don't read anything yet. On tab 1, shine white light on each object and see which colour reaches your eye. On tab 2, look at the objects through a filter: predict what a blue object looks like through a red filter before you try it. Tab 3 compares three stars, and on tab 4 you can melt the ice and add greenhouse gases.
Object
Filter
Show
Reading the graph
Time
Absorbed and emitted
Everything emits and absorbs radiation all the time. Absorbs at the same rate as it emits: the temperature stays the same. Absorbs faster than it emits: it gets hotter. Emits faster than it absorbs: it gets colder.
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▲ Four tabs: what colour it looks, filters, the black body graph, absorbing and emitting
Got it to look black? Now learn the words examiners want.
The notes below give you reflect, absorb and transmit, the black body graph, and the rates rule for the Earth's temperature.
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Revision notes
Reflect, absorb, transmit
When light meets an object it can be reflected, absorbed or transmitted (passed through). An opaque object doesn't transmit any light. A transparent object transmits most of it. Each colour of visible light has its own narrow band of wavelengths, from violet (about 400 nm) to red (about 700 nm).
A reflected · B absorbed · C transmitted
Each colour has its own wavelength.
Why an opaque object has a colour
⭐ The colour you see is the colour that is reflected
An opaque object absorbs some colours of white light and reflects the rest. The reflected light reaches your eye, and that's the colour you see.
| The object looks… | Reflects | Absorbs |
|---|---|---|
| red | red | green and blue (all the other colours) |
| green | green | red and blue (all the other colours) |
| blue | blue | red and green (all the other colours) |
| white | all colours equally | none |
| black | none | all colours |
Colour filters
⭐ A filter absorbs some colours and transmits others
A red filter transmits only red light and absorbs the rest. A green filter transmits only green; a blue filter transmits only blue.
Looking at an object through a filter: only light that the object reflects and the filter transmits reaches your eye.
| Object | Filter | Looks | Why |
|---|---|---|---|
| blue | blue | blue | it reflects blue, and the blue filter transmits blue |
| blue | red | black | it reflects only blue, and the red filter absorbs blue |
| white | green | green | it reflects all colours; only green gets through |
| green leaf | red | black | it reflects only green, and the red filter absorbs green |
Black body radiation
⭐ A perfect black body
An object that absorbs all the radiation that hits it. It doesn't reflect or transmit any. A good absorber is also a good emitter, so a perfect black body is the best possible emitter.
All objects emit and absorb infrared radiation all the time, whatever their temperature. The hotter the object, the more radiation it emits each second.
Temperature, intensity and peak wavelength
- An object gives out a spread of wavelengths. The one with the highest intensity is the peak wavelength.
- Hotter object → higher intensity at every wavelength, and a shorter peak wavelength.
- A person (37 °C) peaks in the infrared. The Sun (about 5500 °C) peaks in the visible.
- Stars: a bluish-white star has a shorter peak wavelength than our yellowish Sun, so it is hotter. An orange or red star has a longer peak wavelength, so it is cooler.
The Sun and a person (not to scale).
Absorbing, emitting and temperature
| If an object… | its temperature… | Earth example |
|---|---|---|
| absorbs radiation at the same rate as it emits it | stays constant | the average over a year |
| absorbs faster than it emits | rises | the Earth in the day |
| emits faster than it absorbs | falls | the Earth at night |
The temperature of the Earth Higher
- Radiation from the Sun reaches the Earth. Some is reflected by clouds, ice and snow; the rest is absorbed.
- The Earth emits infrared radiation into space.
- Greenhouse gases absorb some of the infrared the Earth emits, so less escapes to space.
- If less is reflected (melting ice) or less escapes (more greenhouse gas), the Earth absorbs faster than it emits and its temperature rises, until a hotter Earth emits enough to balance again.
A person in a warm room: absorbed = emitted.
A incident · B reflected · C absorbed · D emitted
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How to write it in the exam
Every colour answer, the same shape
It looks [colour] because only [colour] light is
reflected (or transmitted). The other colours are
absorbed.
the colourreflected by an object, transmitted by a filterabsorbed
Workbook page 17 · opaque objects in white light
Looks red: only red is reflected; green and blue are absorbed.
Looks green: only green is reflected; red and blue are absorbed.
Looks blue: only blue is reflected; red and green are absorbed.
Workbook page 17 · filters
A filter absorbs some colours and transmits others.
Red filter: transmits red, absorbs green and blue. The white room looks red.
Green filter: the white room looks green.
Blue filter: the white room looks blue.
Exam-style question · page 17 · 7 marks
(a) A shirt looks white. Explain why.
- All wavelengths (colours) of visible light (1) are reflected equally (1).
(b) A pair of shoes looks black. Explain why.
- All wavelengths are absorbed (1), so no light is reflected to the eye (1).
(c) A green leaf is seen through a red filter. State what colour it looks and explain.
- The leaf looks black (1).
- The leaf only reflects green light (1).
- The red filter absorbs green light (it only transmits red), so no light reaches the eye (1).
"It looks red" scores 0: there is no red light coming off a green leaf for the filter to let through.
Workbook page 18 · the black body graph
- A perfect black body absorbs all the radiation incident on it; it doesn't reflect or transmit any; a good absorber is a good emitter, so it is the best possible emitter.
- A person's peak is in the infrared. The Sun's peak is in the visible.
- The hotter the object, the shorter the peak wavelength and the higher the intensity.
Exam-style question · the Earth's temperature · page 18 · Higher 9 marks
Average rates for each square metre of the Earth.
(a) Calculate the rate at which radiation is absorbed by each square metre.
- 340 − 100 = 240 W/m² (1)
(b) Explain why the Earth's average temperature stays constant.
- Radiation is absorbed at 240 W/m² and emitted at 240 W/m² (1): absorbed at the same rate as emitted, so the temperature doesn't change (1).
(c) The polar ice is melting and greenhouse gases are increasing. Explain how these change the Earth's temperature. (6)
- Ice and snow reflect radiation from the Sun. Less ice, so less is reflected (below 100 W/m²) and more is absorbed (above 240 W/m²).
- Greenhouse gases absorb the infrared emitted by the Earth's surface, so less is emitted into space (below 240 W/m²).
- Radiation is now absorbed faster than it is emitted, so the Earth's temperature rises.
- A hotter Earth emits more, until emitted = absorbed again at a higher temperature.
For level 3: link both changes to the two rates and to a rise in temperature.
📘 Now do it in your workbook
Page 17: reflected, absorbed or transmitted, what decides an object's colour, filters, and a 7-mark exam question. Page 18: the perfect black body, the intensity graph, absorbing and emitting, and the Earth's temperature with a 9-mark exam question.
AQA GCSE Physics Workbook · WAVES · pages 17 and 18The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
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Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 17 and 18 of the workbook have colour, filters, black body radiation and the Earth's temperature, with two exam questions and answers at the back.
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