AQA GCSE Physics Foundation (8463), Foundation tier · Waves › Waves in air, fluids and solids
Practise Reflection of waves. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What can happen when a wave meets a boundary between two materials: it can be reflected, absorbed or transmitted. You need to draw ray diagrams for reflection and know the required practical on the reflection and refraction of light. GCSE Physics only, not in Combined Science.
Key facts
At a boundary a wave can be reflected, absorbed or transmitted
Normal = line at 90° to the surface where the ray hits
Angle of incidence = angle of reflection
All angles are measured from the normal
Absorbed wave energy is transferred to the material (often heating it)
Refraction = change of direction of a transmitted wave at a boundary
Practical: ray box with slit, mirror, glass or Perspex block, protractor, ruler, sharp pencil, paper
Notes
Waves at a boundary
When a wave meets a boundary between two different materials it can be reflected (bounce back), absorbed (its energy is taken in by the material) or transmitted (it passes through and carries on).
Usually some of each happens. What happens depends on the wavelength of the wave and on the materials.
Absorbed energy is transferred to the material, often heating it.
A transmitted wave may change direction as it enters the new material. This is refraction.
Examples: a mirror reflects most of the light that hits it, clear glass transmits most light, and black card absorbs most light. Hard, smooth walls reflect sound (echoes), while soft materials such as curtains and foam absorb it.
The law of reflection
diagram
The normal is a line at 90° to the surface at the point where the ray hits it. Draw it as a dashed line.
Angle of incidence = angle of reflection. Both angles are measured between the ray and the normal, not the surface.
Both angles are measured from the normal, not from the mirror.
To draw a reflection: draw the normal, measure the angle of incidence with a protractor, then draw the reflected ray on the other side of the normal at the same angle. Put arrows on both rays.
A ray that hits a surface along the normal (angle of incidence 0°) reflects straight back along the same line.
Required practical: reflection and refraction of light
Use a ray box with a slit to make a narrow ray of light, and work in a darkened room so the ray is easy to see.
Reflection: stand a plane mirror (or another surface) on paper and draw the normal. Shine the ray at the point where the normal meets the mirror, mark the incident and reflected rays with crosses, join them with a ruler and measure both angles with a protractor. Repeat for several angles of incidence and for different surfaces.
Refraction: draw round a glass or Perspex block, shine the ray into it at an angle, mark the rays going in and coming out, join them up and measure the angle of refraction. Repeat with blocks of different materials.
Smooth, shiny surfaces give one clear reflected ray. Rough or dull surfaces give a weak reflected ray that is spread out.
How to answer each type of question
Draw or use a ray diagram for reflection
2 to 3 marksGrade 5
Draw the normal (dashed) at 90° to the surface where the ray hits.
Measure the angle of incidence from the normal. If you are given the angle to the surface, subtract it from 90°.
Draw the reflected ray at the same angle on the other side of the normal, with an arrow.
Example. A ray of light hits a plane mirror. The angle between the ray and the surface of the mirror is 35°. (a) Calculate the angle of incidence. (b) State the angle of reflection.
Show the model answerHide the model answer
The normal is at 90° to the mirror, so i = 90° − 35° = 55°, and r = i. (a) The normal is at 90° to the mirror, so angle of incidence = 90° − 35° (1) = 55° (1) (b) 55° (1)
Don’t lose marks
Measuring angles from the surface instead of from the normal.
Forgetting arrows on rays, or drawing the normal as a solid line that looks like another ray.
Saying absorbed waves 'disappear'. Their energy is transferred to the material.
Joining the crosses freehand or using a blunt pencil, which makes the measured angles inaccurate.
More tips
Memory tricks
'i = r': the angle of incidence equals the angle of reflection, measured from the normal, never from the surface.
Given the angle to the surface? Take it away from 90° to get the angle to the normal.
RAT: at a boundary a wave can be Reflected, Absorbed or Transmitted.
Exam technique
Use a ruler and a sharp pencil for ray diagrams: examiners check that your angles are accurate.
In method questions, say what you measure the angles from (the normal) and what with (a protractor).
Say 'most' of the light is reflected, transmitted or absorbed: at real boundaries some of each usually happens.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what can happen at a boundaryA wave can be reflected, absorbed or transmitted when it meets a boundary between two different materials.
Grade 4
State the law of reflectionThe angle of incidence equals the angle of reflection, both measured from the normal.
Grade 5
Draw a ray diagram for reflectionDraw the normal at 90° to the surface, then the reflected ray at an equal angle on the other side, with arrows on both rays.
Grade 5
State what happens to absorbed wave energyWhen a wave is absorbed, its energy is transferred to the material, usually heating it.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
State what happens to the frequency of a wave when it reflects off a surface.
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) Which law is true whenever any wave reflects at a plane surface? Tick (✓) one box.[1]
The angle of incidence equals the angle of reflection
The angle of incidence is always 90°
The angle of reflection is always less than the angle of incidence
The wave speeds up after reflecting
(b) State what happens to the frequency of a wave when it reflects off a surface.[1]
(c) A sound wave reflects off a hard, flat wall. What name is given to a reflected sound that is heard as a distinct repeat of the original sound?[1]
Show the answer and mark scheme
(a)Answer: The angle of incidence equals the angle of reflection
(b)Answer: It stays the same.
it stays the same (does not change)
(c)Answer: An echo
echo
Question 2Medium8 marks
A music teacher records students playing instruments in two rooms. Room A has bare concrete walls and a tiled floor. Room B has thick curtains, a carpet and soft foam panels on the walls.
(a) The recording made in room A has echoes. Explain why.[2]
(b) Explain why there are fewer echoes in the recording made in room B.[2]
(c) Some sound is transmitted through the walls of room B into the next room. Suggest one change that would reduce the amount of sound transmitted into the next room.[1]
(d) The speed of sound in air is 340 m/s. A student stands next to the microphone in room A and claps once. The recording shows that the echo from the far wall arrives 0.050 s after the clap. Calculate the distance between the student and the far wall.[3]
Show the answer and mark scheme
(a)Answer: Hard surfaces reflect sound, and the reflected sound arrives after the direct sound.
sound is reflected by the hard walls / floor
the reflected sound reaches the microphone after the sound that travels directly
(b)Answer: The soft materials absorb sound, so less is reflected.
the soft materials / curtains / carpet / foam absorb (most of the) sound
so less sound is reflected
(c)Answer: Add a layer of sound-absorbing material inside the wall.
make the walls thicker / add a layer of sound-absorbing material inside the wall / seal gaps around the doors and windows
(d)Answer: 8.5 m
distance travelled by the sound = 340 × 0.050 = 17 (m)
(the sound travels to the wall and back, so) divide by 2