AQA GCSE Physics Foundation (8463), Foundation tier · Waves › Electromagnetic waves
Practise EM waves: refraction and absorption. 9 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 to draw ray diagrams for refraction, and the required practical on the infrared radiation emitted and absorbed by different surfaces.
Key facts
Refraction = change of direction at a boundary
Slows down → bends towards the normal; speeds up → bends away from the normal
Along the normal: no change in direction
Angles are measured from the normal
Matt black: best emitter and absorber of infrared; shiny silver: worst
Notes
Refraction and ray diagrams
diagram
Refraction is the change in direction of a wave when it crosses a boundary between two media at an angle.
Light slows down when it passes from air into glass, water or Perspex, and bends towards the normal. When it speeds up (e.g. from glass into air) it bends away from the normal.
Light slows down in glass and bends towards the normal (r is smaller than i).
A ray that meets the boundary along the normal does not change direction.
In a ray diagram, draw the normal (dashed) where the ray meets the boundary, and measure the angles of incidence and refraction from the normal.
For a rectangular glass block, the ray that comes out is parallel to the ray that went in, but shifted sideways.
Required practical: infrared and different surfaces
diagram
Fill a Leslie cube (a metal cube whose faces are, for example, matt black, shiny black, matt white and shiny silver) with hot water, so every face is at the same temperature.
Hold an infrared detector the same distance from each face in turn and record the readings.
Leslie cube: all faces are at the same temperature; only the surface changes.
Result: the matt black face emits the most infrared and the shiny silver face the least.
Matt black surfaces are also the best absorbers of infrared, and shiny surfaces the worst, because shiny surfaces reflect most of it.
How to answer each type of question
Draw or describe a refraction ray diagram
2 to 3 marksGrade 5
Draw the normal where the ray meets the boundary.
Decide whether the wave speeds up or slows down.
Bend the ray towards the normal if it slows down, away if it speeds up.
Example. A ray of light passes from air into a rectangular glass block at an angle of incidence of 40°. Describe the path of the ray as it enters and then leaves the block.
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Towards the normal going in, away from the normal coming out; the ray leaves parallel to the one that went in. As it enters the glass the ray bends towards the normal, so the angle of refraction is less than 40° (1). As it leaves the block into air it bends away from the normal (1). The ray leaving the block is parallel to the ray that entered it (1).
Explain why a variable must be controlled
2 marksGrade 5
Say how the variable would affect the reading.
Say that controlling it means only the type of surface affects the result (a fair test).
Example. Suggest why the infrared detector must be the same distance from each face of the Leslie cube.
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The reading depends on the distance: more infrared reaches the detector when it is closer to the surface (1). Keeping the distance the same means only the type of surface affects the reading (1).
Don’t lose marks
Measuring angles from the boundary instead of from the normal.
Saying a ray along the normal bends. It does not change direction.
In the practical, not keeping the distance between the detector and each face the same.
Saying shiny surfaces are good absorbers. Shiny surfaces reflect most of the infrared.
More tips
Memory tricks
FAST: Faster → Away from the normal; Slower → Towards the normal.
Infrared practical in two words: matt black wins (best emitter and best absorber).
Exam technique
Use a ruler and a sharp pencil for ray diagrams, and draw the normal as a dashed line.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 5
Draw a ray diagram for refractionDraw the normal; a ray that slows down (e.g. air into glass) bends towards the normal.
Grade 5
State which surfaces emit and absorb infrared bestMatt black surfaces are the best emitters and absorbers of infrared; shiny silver surfaces are the worst.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
What happens to the frequency of the light as it enters the glass?
It stays the same.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
A ray of light travels from air into a glass block.
(a) What happens to the speed and the wavelength of the light as it enters the glass? Tick (✓) one box.[1]
Both decrease
Both increase
Speed decreases, wavelength increases
Speed increases, wavelength decreases
(b) What happens to the frequency of the light as it enters the glass?[1]
(c) The ray meets the glass surface along the normal (angle of incidence 0°). Does the ray change direction as it enters the glass?[1]
Show the answer and mark scheme
(a)Answer: Both decrease
(b)Answer: It stays the same.
it stays the same (does not change)
(c)Answer: No, it continues in the same direction.
no, it carries straight on
Question 2Medium3 marks
A student models the greenhouse effect using two identical open-topped clear plastic boxes placed in sunlight. Box A is empty apart from a thermometer. Box B is the same, but has a pane of glass fixed over the top. Most visible light passes through glass, but glass absorbs infrared radiation strongly.
(a) Predict which box reaches the higher temperature. Give a reason for your answer.[2]
(b) Give one variable that must be controlled for this to be a fair comparison.[1]
Show the answer and mark scheme
(a)Answer: Box B: infrared re-emitted inside is absorbed by the glass rather than escaping, trapping more energy.
box B
visible light passes through the glass and is absorbed inside, then re-emitted as infrared; this infrared is absorbed by the glass instead of escaping, so more energy stays inside box B
(b)Answer: The amount of direct sunlight each box receives (or the size/material of the boxes).
the size/material of the boxes / the position of the thermometer / the amount of direct sunlight each box receives / the starting temperature