AQA GCSE Physics Foundation (8463), Foundation tier · Waves › Electromagnetic waves
Practise Visible light. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How colours relate to wavelength and frequency, the difference between specular and diffuse reflection, and how filters and the reflection or absorption of different wavelengths give objects their colour. GCSE Physics only, not in Combined Science.
Visible light is the part of the EM spectrum our eyes detect. White light is a mixture of all the colours.
Each colour has its own narrow band of wavelengths and frequencies.
In order: red, orange, yellow, green, blue, indigo, violet. Red has the longest wavelength and lowest frequency; violet has the shortest wavelength and highest frequency.
Specular and diffuse reflection
diagram
Specular reflection: reflection from a smooth surface in a single direction, e.g. a mirror. You see a clear image.
Diffuse reflection: reflection from a rough surface, which scatters the light in many directions, e.g. paper. No clear image forms.
The law of reflection still works at every point. On a rough surface the normals point in different directions, so the reflected rays spread out.
Each ray obeys i = r, but on a rough surface the normals point different ways.
Transparent, translucent and opaque
Transparent objects, such as clear glass, transmit light straight through, so you can see clearly through them.
Translucent objects, such as frosted glass, transmit light but scatter it, so you cannot see clearly through them.
Opaque objects do not transmit light: they reflect or absorb it.
Colour and filters
diagram
The colour of an opaque object depends on which wavelengths it reflects most strongly. Wavelengths that are not reflected are absorbed.
In white light, a red object reflects red and absorbs the other colours. A white object reflects all wavelengths equally; a black object absorbs all wavelengths.
A colour filter transmits certain wavelengths (its own colour) and absorbs the rest. A red filter lets only red light through.
A red filter transmits red and absorbs the other colours (only three colours drawn).
To work out the colour you see, follow the light: which colours reach the object, which it reflects, and which get through any filter to your eye. If no light reaches your eye, the object looks black.
How to answer each type of question
Explain why an object looks a certain colour
2 marksGrade 4
Say which colour (wavelengths) the object reflects.
Say that it absorbs the other colours.
Example. Explain why a green leaf looks green in white light.
Show the model answerHide the model answer
The leaf reflects green light (1). It absorbs the other colours (wavelengths) in white light (1).
Explain specular and diffuse reflection
2 to 3 marksGrade 5
Link each surface to smooth or rough.
Say how the light is reflected: in a single direction, or scattered.
Link this to whether a clear image forms.
Example. Explain why you can see your reflection in a mirror but not in a sheet of white paper.
Show the model answerHide the model answer
A mirror has a smooth surface, so light is reflected in a single direction (specular reflection) (1). Paper has a rough surface, so light is scattered in many directions (diffuse reflection) (1). So only the mirror forms a clear image (1).
Don’t lose marks
Saying a red object 'absorbs red'. It reflects red and absorbs the other colours.
Saying a filter 'adds colour' or 'turns light red'. A filter only absorbs colours; it cannot add any.
Mixing up translucent (scatters light) and transparent (clear view through).
Saying diffuse reflection does not obey the law of reflection. It does, at every point.
More tips
Memory tricks
ROY G BIV: red, orange, yellow, green, blue, indigo, violet, from longest to shortest wavelength.
Specular = Smooth = Single direction (three S's).
Filters are one-colour gates: only their own colour gets through.
Black means no light reaches your eye; white means all colours reach your eye.
Exam technique
Use the words reflect, absorb and transmit, and name the colours (or wavelengths) each time.
In filter questions, work step by step: light source → object → filter → eye.
When explaining a colour, say what happens to the colour you see and to all the other colours.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Order the colours of the visible spectrumRed, orange, yellow, green, blue, indigo, violet; red has the longest wavelength.
Grade 4
State why an object looks a colourAn opaque object reflects the wavelengths of its colour most strongly and absorbs the others.
Grade 4
Explain why objects look white or blackWhite objects reflect all wavelengths equally; black objects absorb all wavelengths.
Grade 5
Describe specular and diffuse reflectionSpecular: from a smooth surface in a single direction. Diffuse: from a rough surface, scattered in many directions.
Grade 5
Explain how a colour filter worksA filter transmits certain wavelengths (its own colour) and absorbs the others.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Explain why a piece of coal looks black.
It absorbs almost all the light and reflects hardly any.
Light of several different colours falls on an opaque object. The object reflects some wavelengths of the light. State what happens to the wavelengths of light that the object does not reflect.
They are absorbed.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Opaque objects look different colours in white light.
(a) Why does a red apple look red in white light? Tick (✓) one box.[1]
It absorbs red light and reflects all other colours.
It reflects red light and absorbs all other colours.
It emits red light.
It transmits red light only.
(b) Explain why a sheet of paper looks white in white light.[1]
(c) Explain why a piece of black card looks black.[1]
(d) What name is given to the reflection of light from a smooth surface in a single direction? Tick (✓) one box.[1]
Diffuse reflection
Refraction
Specular reflection
Transmission
Show the answer and mark scheme
(a)Answer: It reflects red light and absorbs all other colours.
(b)Answer: It reflects all colours of light equally.
it reflects all wavelengths / colours of light (equally)
(c)Answer: It absorbs all colours of light.
it absorbs all wavelengths / colours of light (and reflects none)
(d)Answer: Specular reflection
Question 2Medium6 marks
A student shines a beam of white light through coloured filters onto a white screen.
(a) Describe what happens to the white light as it passes through a red filter.[2]
(b) The student places a green filter in the beam after the red filter. Describe and explain what is seen on the screen.[2]
(c) The student removes the green filter. The red light shines onto a blue book. What colour does the book appear? Explain your answer.[2]
Show the answer and mark scheme
(a)Answer: Red light is transmitted; all other colours are absorbed.
the filter transmits red light
and absorbs all the other colours / wavelengths
(b)Answer: The screen is dark, because the green filter absorbs the red light that the red filter lets through.
no light reaches the screen / the screen looks dark
the green filter absorbs the red light (it only transmits green light, which was absorbed by the red filter)
(c)Answer: Black: the book absorbs the red light and there is no blue light for it to reflect.
black
the book absorbs red light / only reflects blue light, and there is no blue light shining on it