Practise Properties of electromagnetic waves 1. 20 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Different substances can absorb, transmit, refract or reflect electromagnetic waves, and what happens depends on the wavelength. You need to draw ray diagrams for refraction, explain refraction using wave fronts (Higher tier), and know the required practical on the infrared radiation emitted and absorbed by different surfaces.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name what substances can do to EM wavesA substance can absorb, transmit, refract or reflect EM waves.
4
State what causes refractionRefraction happens because the wave changes speed when it enters a different substance.
5
Draw a ray diagram for refractionDraw the normal; a ray that slows down (e.g. air into glass) bends towards the normal.
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.
6
Describe the infrared radiation practicalFill a Leslie cube with hot water and measure the infrared from each face with a detector at the same distance.
7
Explain refraction using wave fronts (HT)The part of a wave front that reaches the slower medium first slows down first, so the wave front changes direction.
Notes
Absorb, transmit, refract or reflect
When EM waves meet a substance they may be absorbed, transmitted, refracted or reflected. What happens depends on the substance and on the wavelength of the waves.
Examples: glass transmits visible light; soft tissue in the body transmits X-rays but bone absorbs them; water in food absorbs microwaves; the ozone layer absorbs much of the Sun's ultraviolet.
Some effects, such as refraction, happen because EM waves travel at different speeds in different substances.
Refraction and ray diagrams
Refraction is the change in direction of a wave when it crosses a boundary between two media at an angle, because its speed changes.
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.
A ray that meets the boundary along the normal does not change direction, but its speed still changes.
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.
Wave front diagrams (Higher tier only) grade 7+
Wave fronts are lines joining matching points on a wave, such as the crests. The gap between neighbouring wave fronts is one wavelength.
When wave fronts meet a boundary at an angle, one end reaches the new medium first. If the wave is slower there, that end slows down while the rest of the wave front is still moving faster, so the wave front changes direction.
In the slower medium the wave fronts are closer together (a shorter wavelength), because the frequency does not change.
Required practical: infrared and different surfaces
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.
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.
Cheatsheet
EM waves can be absorbed, transmitted, refracted or reflected; this depends on the wavelength
Refraction = change of direction at a boundary, caused by a change in speed
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
Wave front diagrams: wave fronts are closer together in the slower medium (shorter wavelength, same frequency) grade 7+
Matt black: best emitter and absorber of infrared; shiny silver: worst
How to answer each type of question
Draw or describe a refraction ray diagram
2 to 3 marks5
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.
Show the model answer
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 refraction using wave fronts (Higher tier only)
3 marks7
Say which part of the wave front reaches the boundary first.
Say that part slows down while the rest is still moving faster.
Conclude that the wave front changes direction (bends towards the normal).
Example. Light waves travel from air into glass, where they travel more slowly. The wave fronts meet the boundary at an angle. Explain why the light changes direction.
Show the model answer
One end of each wave front enters the glass first (1). That end slows down while the rest of the wave front is still in air, travelling faster (1). So the wave front changes direction, bending towards the normal (1).
Describe the method for the infrared practical
4 marks6
Say how every surface is kept at the same temperature (hot water in a Leslie cube).
Say what you use to measure the infrared (a detector) and what you keep the same (the distance).
Say how you compare the surfaces.
Example. Describe how a student could use a Leslie cube to find which type of surface emits the most infrared radiation.
Show the model answer
Fill the Leslie cube with hot water (1). Place an infrared detector a fixed distance from one face and record the reading (1). Repeat for each of the other faces, keeping the distance the same (1). The face with the highest reading emits the most infrared (1).
Explain why a variable must be controlled
2 marks5
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.
Show the model answer
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).
Shortcuts and memory tricks
FAST: Faster → Away from the normal; Slower → Towards the normal.
Picture a line of marchers walking at an angle from a road onto mud: the first ones onto the mud slow down, so the whole line swings round.
Infrared practical in two words: matt black wins (best emitter and best absorber).
Where marks are lost
Measuring angles from the boundary instead of from the normal.
Saying light bends 'because the glass is denser' without mentioning the change in speed. The reason is the change in speed.
Saying a ray along the normal bends. It does not change direction, although its speed changes.
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.
Exam technique
Use a ruler and a sharp pencil for ray diagrams, and draw the normal as a dashed line.
In any explanation of refraction, mention the change in speed.
In wave front answers, say which part of the wave front slows down first and what the rest of the wave front is doing.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The student then shines a ray of light at the block along the normal (angle of incidence 0°). Describe what happens to the direction of the ray as it enters the glass.
It continues in the same direction.
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 2Medium6 marks
A student shines a ray of red light from air into a glass block. The ray meets the surface of the block at an angle of incidence of 40°.
(a) Explain why the ray changes direction as it enters the glass.[2]
(b) The frequency of the light does not change when it enters the glass. What happens to the wavelength of the light? Give a reason for your answer.[2]
(c) The student then shines a ray of light at the block along the normal (angle of incidence 0°). Describe what happens to the direction of the ray as it enters the glass.[1]
(d) The student repeats the experiment with blue light at the same angle of incidence. The blue light is refracted by a slightly different amount from the red light. Suggest why.[1]
Show the answer and mark scheme
(a)Answer: The light slows down as it enters the glass; because it arrives at an angle, it is refracted.
light travels more slowly in glass than in air / its speed changes at the boundary
so it is refracted (towards the normal) because it meets the boundary at an angle
(b)Answer: It decreases, because the speed decreases while the frequency stays the same (v = f λ).
the wavelength decreases
wave speed = frequency × wavelength, so if the speed decreases at constant frequency, the wavelength must decrease
(c)Answer: It continues in the same direction.
it does not change direction / carries straight on
(d)Answer: Blue and red light travel at slightly different speeds in glass.
blue light travels at a (slightly) different speed in glass from red light
Question 3Hard6 marks
A student has two identical empty metal cans. One can is painted matt black and the other is covered with shiny silver foil. The student also has a radiant heater, two thermometers, a measuring cylinder, a stopwatch and a supply of cold water.
Plan an investigation to compare how much infrared radiation is absorbed by the matt black surface and by the shiny silver surface.[6]
Show the answer and mark scheme
Answer: Level-of-response answer: equal volumes of water at the same starting temperature in each can, cans equidistant from the heater, measure the temperature rise of each over the same time; the larger rise shows the better absorber.
use the measuring cylinder to put the same volume of water in each can
the water in both cans starts at the same temperature
place the cans the same distance from the radiant heater (e.g. one on each side)
measure the starting temperature of the water in each can with a thermometer
switch on the heater and measure the temperature in each can at regular intervals (e.g. every minute for 10 minutes)
compare the temperature rises: the larger temperature rise shows the better absorber
use lids and stand the cans on an insulating mat to reduce other energy transfers
repeat the investigation and calculate a mean temperature rise
safety: the heater becomes very hot; do not touch it and let it cool before moving it
Marked with levels of response: the full level descriptors are in the app.