Properties of electromagnetic waves 1 — AQA GCSE Combined Science
Test yourself on Properties of electromagnetic waves 1 with AQA GCSE practice questions.
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Properties of electromagnetic waves 1 explained
When electromagnetic waves meet a substance, absorption, transmission, refraction or reflection can occur.
Read the full explanation
The effect depends on the substance and the wavelength. For example, visible light passes through glass (transmission) but ultraviolet is strongly absorbed; radio waves pass through walls while visible light does not. Refraction happens when a wave changes speed at a boundary, and the amount of bending varies with wavelength. Reflection also varies: shiny metals reflect visible and infrared strongly, and bulk metals reflect radio waves. To describe these effects, name the substance, name the wave region, and state the effect and how it changes with wavelength.
(HT only) Some effects, for example refraction, are due to the difference in velocity of the waves in different substances.
Refraction happens because a wave changes speed when it crosses a boundary between substances. If the wave slows down, it bends towards the normal; if it speeds up, it bends away from the normal. The velocity depends on the substance, so the same wave can travel at different speeds in different materials. For example, light travels more slowly in glass than in air, so a ray entering glass at an angle bends towards the normal, and leaving glass it speeds up and bends away. When explaining refraction, identify the boundary, state which substance the wave speeds up or slows down in, and link that change to the direction of bending.
Students should be able to construct ray diagrams to illustrate the refraction of a wave at the boundary between two different media.
Refraction occurs when a wave crosses a boundary between two media and changes speed, so its direction changes unless it travels along the normal. To construct a ray diagram, draw the boundary as a straight line and a dashed normal at 90° to it at the point where the ray meets it. Draw the incident ray meeting that point, then draw the refracted ray on the other side. If the wave slows down, for example light entering glass from air, the ray bends towards the normal, so the angle of refraction is smaller than the angle of incidence. If it speeds up, for example light leaving glass into air, the ray bends away from the normal. Label the normal, incident ray, refracted ray and the two media, and mark the angles from the normal, not from the boundary.
(HT only) Students should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium.
A wave front is a line joining points on a wave that are in step, such as successive crests. When plane wave fronts meet a boundary at an angle, one end of each front reaches the new medium before the other. If the wave slows down in the new medium, that end travels more slowly while the other end is still moving faster in the first medium, so the whole front pivots and the direction of travel bends towards the normal. If the wave speeds up, the front pivots the other way and the wave bends away from the normal. The spacing between wave fronts also changes: closer spacing means shorter wavelength and slower speed in that medium. This explains refraction without needing to treat each ray separately.
Required practical activity 21: investigate how the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.
Infrared is a region of the electromagnetic spectrum beyond red visible light. All objects emit infrared; the amount radiated or absorbed depends on surface colour and texture. In this practical, you compare surfaces like black matt, white matt and shiny silver. For absorption, place identical boiling tubes painted differently, filled with equal volumes of cold water, at equal distances from an infrared heater. Record the temperature rise over time. For emission (radiation), fill a Leslie cube with hot water and use an infrared detector at equal distances from each face to measure emitted radiation. Control variables include starting temperature, volume, distance, and time. Black matt surfaces are the best absorbers and emitters; shiny silver surfaces are the poorest. This explains why solar panels are black and survival blankets are shiny.
Your focus
- State the four possible effects when electromagnetic waves meet a substance: absorption, transmission, refraction and reflection.
- Describe how the effect of a substance on electromagnetic waves can vary with wavelength, using a named example.
- Compare the behaviour of two different wavelengths in the same substance, or the same wavelength in two substances, using correct terminology.
Show all 15 objectives
- Explain that refraction is caused by a change in the velocity of a wave when it passes between substances.
- Predict the direction of bending from whether a wave speeds up or slows down at a boundary.
- Use wave-speed changes to explain refraction at a boundary without claiming that frequency changes.
- Draw a boundary, normal, incident ray and refracted ray for a wave crossing between two media.
- Determine whether a ray bends towards or away from the normal from the change in wave speed.
- Label a ray diagram accurately and measure angles from the normal.
- Describe how wave fronts behave when a wave crosses a boundary between two media.
- Explain refraction in terms of the change in wave speed and the pivoting of wave fronts.
- Relate the spacing of wave fronts to wavelength and wave speed in each medium.
- Plan a fair test to compare how much infrared different surfaces absorb or radiate.
- Record and present data to show the relationship between surface nature and infrared absorption or emission.
- Explain the results using the idea that good absorbers are also good emitters of infrared.
Properties of electromagnetic waves 1 exam tips
Marking Points
- Absorption: the wave's energy is transferred to the substance, often increasing its internal energy; the amount absorbed depends on the substance and wavelength.
- Transmission: the wave passes through the substance; a substance may transmit some wavelengths well and absorb others, as with coloured filters or glass.
- Refraction: the wave changes direction at a boundary because its speed changes; the change in speed, and so the bending, varies with wavelength.
- Reflection: the wave bounces off a surface; the proportion reflected depends on the substance and wavelength, for example bulk metals reflecting radio waves.
- A correct answer links a named substance to a named electromagnetic wave region and states how the effect varies with wavelength, rather than treating all wavelengths alike.
- Refraction is caused by a change in the velocity of the wave when it passes from one substance to another.
- If the wave slows down at a boundary, it bends towards the normal.
- If the wave speeds up at a boundary, it bends away from the normal.
- The velocity of an electromagnetic wave depends on the substance it is travelling through, so different substances give different speeds.
- Draw the boundary between the two media as a straight line and mark the point where the incident ray meets it.
- Draw a dashed normal at 90° to the boundary at the point of incidence, and measure or mark angles between each ray and the normal.
- Show the correct direction of bending: towards the normal when the wave slows down, away from the normal when it speeds up.
- Keep the incident and refracted rays on opposite sides of the normal, with the refracted ray continuing into the second medium.
- Label the normal, incident ray, refracted ray and the two media clearly so the diagram communicates the refraction event.
- For a ray travelling along the normal, show it continuing straight through without bending.
- Define a wave front as a line joining points on a wave that are in step, such as adjacent crests.
- Explain that when a wave front meets a boundary at an angle, one part of the front enters the new medium before the rest.
- Link the change in speed in the new medium to the pivoting of the wave front and therefore to the change in direction of the wave.
- State that a wave slowing down bends towards the normal, while a wave speeding up bends away from the normal.
- Relate the spacing of wave fronts to wavelength and speed: fronts closer together indicate a slower wave with shorter wavelength in that medium.
- Identifies independent variable as surface nature (colour and texture) and dependent variable as amount of infrared absorbed or radiated, measured by temperature change or detector reading.
- Describes a valid method for absorption: identical containers with different surface coatings, equal volumes of cold water, placed at equal distances from an infrared heater, measuring temperature rise over time.
- Describes a valid method for emission: a Leslie cube or differently coated containers filled with hot water, using an infrared detector or thermometer to compare emitted radiation or cooling rates.
- States control variables: starting temperature, volume of water, distance from heater/detector, and time interval.
- Explains that black matt surfaces are the best absorbers and emitters of infrared, while shiny silver surfaces are the poorest.
- Uses data to compare rates of temperature change or detector readings and links the pattern to the nature of the surface.
- Evaluates the method, identifying sources of error such as heat loss to surroundings or inconsistent coating thickness, and suggests improvements.
Examiner Tips
- 💡Name the substance and the electromagnetic wave region in every example, then state the effect and how it varies with wavelength.
- 💡Use the four key terms precisely: absorb, transmit, refract, reflect; avoid vague words such as 'goes through' or 'bounces'.
- 💡Always name the two substances and state whether the wave speeds up or slows down before describing the bending.
- 💡Link the change in velocity to the change in direction in one clear causal sentence, for example 'because it slows down in glass, it bends towards the normal'.
- 💡Use a sharp pencil and a ruler for the boundary, rays and normal so the geometry is clear and angles can be checked.
- 💡Annotate the diagram with 'towards the normal' or 'away from the normal' to make your reasoning explicit.
- 💡If the question gives refractive indices or wave speeds, use them to decide the direction of bending before drawing the ray.
- 💡Check that every ray is straight except at the boundary, where the change of direction occurs.
- 💡Sketch the boundary and draw at least three wave fronts before and after it, showing the change in spacing and direction.
- 💡Use the phrase 'one end of the wave front reaches the new medium first' to explain the pivoting clearly.
- 💡When describing the method, clearly distinguish between testing absorption (heating cold water with an IR lamp) and testing emission (cooling hot water or using a Leslie cube).
- 💡Name the independent, dependent and at least two control variables explicitly; this is often where marks are earned.
- 💡Use comparative language such as 'black matt surface heated fastest' or 'shiny surface gave the lowest detector reading' rather than vague statements.
Common Mistakes
- Treating all electromagnetic waves as behaving identically in a substance. Correction: state that absorption, transmission, refraction and reflection can each vary with wavelength, and give a specific example such as glass transmitting visible light but absorbing ultraviolet.
- Confusing refraction with reflection. Correction: refraction is a change of direction due to a change of speed inside a substance; reflection is bouncing off a surface.
- Saying a substance 'absorbs' a wave when it actually transmits it, or vice versa. Correction: check whether the wave passes through, is taken in, bends or bounces, and use the precise term.
- Saying refraction is caused by the wave 'hitting' the surface or by reflection. Correction: refraction is caused by a change in velocity as the wave enters a different substance.
- Mixing up the direction of bending. Correction: slower in the new substance means bending towards the normal; faster means bending away from the normal.
- Thinking the velocity of light is the same in all substances. Correction: the velocity depends on the substance, which is why refraction occurs.
- Measuring the angle of incidence from the boundary rather than from the normal; correction: always draw and use the normal, because angles in refraction are defined relative to it.
- Drawing the refracted ray bending the wrong way, for example bending away from the normal when light enters glass; correction: decide whether the wave slows down or speeds up, then bend towards the normal for slowing and away for speeding up.
- Drawing the refracted ray on the same side of the normal as the incident ray; correction: the refracted ray passes into the second medium on the opposite side of the normal.
- Forgetting the normal or drawing it along the boundary; correction: the normal is a construction line at 90° to the boundary at the point of incidence.
- Thinking that the frequency of the wave changes at the boundary. Correction: frequency stays the same, while speed and wavelength change, so the wave fronts pivot.
- Saying that a wave bends because it hits the boundary at an angle, without mentioning the change in speed. Correction: the change in speed in the new medium is the cause of the change in direction.
- Confusing wave fronts with rays. Correction: wave fronts are lines joining points in step, while rays show the direction of energy transfer and are perpendicular to wave fronts.
- Error: confusing the methods for absorption and emission. Correction: measuring the cooling of hot water tests emission; to test absorption, you must measure the heating of cold water using an external infrared source.
- Error: thinking shiny surfaces absorb infrared well because they reflect light. Correction: shiny silver surfaces reflect infrared too, so they absorb and emit less than black matt surfaces.
- Error: placing the infrared detector or heater at different distances from each surface. Correction: hold the detector or heater at the exact same distance, because infrared intensity decreases with distance.
- Error: recording only the final temperature rather than a series of readings. Correction: take regular readings so you can compare rates of cooling or heating, not just end points.