Properties of electromagnetic waves 2 — AQA GCSE Physics
Test yourself on Properties of electromagnetic waves 2 with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Properties of electromagnetic waves 2 explained
This is Higher tier. Radio waves are made by making charges oscillate.
Read the full explanation
An alternating current in an aerial means electrons move backwards and forwards along the metal many times a second, and those accelerating charges produce a changing electric and magnetic field that travels away as an electromagnetic wave. The frequency of the radio wave produced is the same as the frequency of the alternating current driving the aerial, so a circuit oscillating at 100 MHz radiates a 100 MHz wave. This is the link between electricity and the electromagnetic spectrum: an oscillating circuit at one end of an aerial and a wave leaving it at 3 × 10⁸ m/s. Longer aerials are generally used to transmit the lower frequencies efficiently.
(HT only) When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.
This is Higher tier, and it is the reverse of the production statement. A radio wave arriving at a receiving aerial carries an oscillating electric field, and that field pushes the free electrons in the metal backwards and forwards. The result is an alternating current in the aerial with exactly the same frequency as the radio wave that produced it, so the information carried by the wave survives the transfer. The receiver circuit is tuned so that it responds strongly to one particular frequency, which is how one station is picked out from the many arriving at once. Transmission and reception are therefore a matched pair: oscillating charges make the wave, and the wave then makes charges oscillate again in the receiver.
Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range.
Electromagnetic waves come out of atoms in two different ways. Changes in the electrons of an atom, where an electron moves between energy levels, produce waves in the infrared, visible, ultraviolet and X-ray parts of the spectrum; this is why a hot filament glows and why a fluorescent coating gives out visible light after absorbing ultraviolet. Changes inside the nucleus produce gamma rays, which sit at the highest frequency end. The process runs both ways: an atom can absorb a wave of the right frequency and move to a higher energy level, which is exactly what ozone does with ultraviolet. Because the energy changes available range from very small to very large, the frequencies generated and absorbed cover a very wide range.
Gamma rays originate from changes in the nucleus of an atom.
Gamma rays are emitted when an unstable nucleus rearranges itself and loses energy, usually immediately after an alpha or beta decay has left it in an excited state. Nothing but energy leaves the nucleus, so the mass number and the atomic number do not change, there is no new element, and a nuclear equation shows the same nuclide before and after. Gamma rays sit at the highest frequency and shortest wavelength end of the electromagnetic spectrum. They are the most penetrating of the nuclear radiations, needing thick lead or metres of concrete to reduce them appreciably, and they are the least ionising per interaction, which is exactly why they travel so far. Nuclear fission also releases gamma rays along with neutrons.
Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue.
Ultraviolet waves, X-rays and gamma rays carry enough energy to have hazardous effects on human body tissue. The specific hazard depends on the type of radiation. Ultraviolet waves are absorbed by the skin, leading to premature skin ageing and an increased risk of skin cancer. In contrast, X-rays and gamma rays are highly penetrating and are classified as ionising radiation. This means they carry enough energy to knock electrons off atoms, which can cause the mutation of genes and lead to cancer. Lower frequency waves, like radio waves and visible light, do not carry enough energy to cause these specific hazardous effects.
The effects depend on the type of radiation and the size of the dose.
The risk of harm from electromagnetic radiation depends on two main factors: the type of radiation and the size of the dose. Different types cause different effects. For example, ultraviolet waves are absorbed by the skin, causing premature ageing and skin cancer. X-rays and gamma rays are ionising and can penetrate deep into the body to cause gene mutations and cancer. The size of the dose determines the probability of these harmful effects occurring. Radiation dose is a measure of the risk of harm resulting from an exposure of the body to radiation. Students do not need to recall the unit of radiation dose (sieverts), but should be able to interpret data given in sieverts (Sv) or millisieverts (mSv).
Radiation dose (in sieverts) is a measure of the risk of harm resulting from an exposure of the body to the radiation.
The sievert does not measure how much radiation is present; it measures the risk of harm to the body, which is why it combines the energy absorbed with how damaging that particular radiation is to living tissue. The same energy delivered as alpha therefore counts for far more sieverts than the same energy delivered as gamma. Dose depends on the type of radiation, on the time of exposure and on the activity or strength of the source, which is why standing further away or spending less time near a source lowers it. Typical exam figures are a background dose of about 2 to 3 mSv per year, a chest X-ray of about 0.02 mSv and a CT scan of several mSv; put any two on the same basis, per scan against per year, before comparing them. In passing: irradiation is exposure to a source, contamination is radioactive material on or inside you.
1000 millisieverts (mSv) = 1 sievert (Sv) Students will not be required to recall the unit of radiation dose.
You will never be asked to name the unit of radiation dose from memory, but you will be given doses in sieverts or millisieverts and expected to use them. The conversion is the ordinary milli prefix: 1000 mSv = 1 Sv, so to go from mSv to Sv divide by 1000, and to go the other way multiply by 1000. A dose of 6 mSv is 0.006 Sv, and a dose of 0.05 Sv is 50 mSv. Most exam data appear in millisieverts because everyday doses are small, so always convert both figures to the same unit before you compare or divide them. The same prefix logic you already use for millimetres and milliamps applies here unchanged.
Students should be able to draw conclusions from given data about the risks and consequences of exposure to radiation.
When evaluating data on radiation exposure, you must draw evidence-based conclusions about risks and consequences. Exam questions often provide a table or graph, such as radiation doses from medical scans versus background sources. Read the values, compare like with like, and state a conclusion supported by the data. For example, if a table shows a dental X-ray gives 0.005 mSv and a CT scan gives 10.0 mSv, conclude the CT scan carries a higher risk of cellular damage by calculating the ratio. Always quote specific figures from the provided data to justify your reasoning. Remember that correlation does not automatically prove causation; data might show a link, but further evidence is needed to confirm the exact mechanism causing the harm.
Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer.
Ultraviolet (UV) radiation has a shorter wavelength and higher frequency than visible light, carrying more energy. When UV waves reach humans, they are absorbed by the outer layers of the skin. This absorption has severe consequences. For example, repeated exposure damages the structural proteins keeping skin elastic, causing the skin to age prematurely. Furthermore, UV radiation damages the DNA inside skin cells. This genetic damage can cause mutations that significantly increase the risk of developing skin cancer. While UV causes short-term effects like sunburn, the long-term risks are severe. Sunscreen and protective clothing absorb or block ultraviolet waves before they can reach and damage the skin, reducing exposure.
X-rays and gamma rays are ionising radiation that can cause the mutation of genes and cancer.
X-rays and gamma rays have the highest frequencies in the spectrum, so each one carries enough energy to knock an electron off an atom it passes; that is what ionising means. Inside a cell, the ions and reactive fragments produced can break or alter a strand of DNA. If a gene is changed rather than repaired, the mutation is passed on when the cell divides and can lead to a tumour, which is cancer. Because both radiations penetrate deeply, the damage is not limited to the skin. That is why radiographers stand behind lead screens and why medical doses are kept as low as possible, and it is also why the same penetrating, ionising property is used deliberately in radiotherapy to kill cancer cells.
Your focus
- Describe how an alternating current in an aerial produces a radio wave.
- State the relationship between the frequency of the circuit oscillation and the frequency of the wave emitted.
- Explain why a steady direct current in an aerial does not produce a radio wave.
Show all 33 objectives
- Explain how a radio wave absorbed by an aerial produces an alternating current.
- State the frequency of the induced current when the frequency of the incoming wave is given.
- Compare the production of a radio wave with its detection, in terms of oscillating charges.
- Distinguish electromagnetic waves produced by electron changes from those produced by nuclear changes.
- Explain why an atom can absorb as well as emit electromagnetic radiation.
- Identify which part of the spectrum is produced by a change inside the nucleus.
- State where in the atom a gamma ray originates and what happens to the mass and atomic numbers.
- Explain how an absorbing material can be used to show whether a source emits beta or gamma radiation.
- Complete a nuclear equation for a decay that is followed by gamma emission.
- Describe the hazardous effects of ultraviolet waves on human body tissue.
- Explain how exposure to X-rays and gamma rays can lead to cancer.
- Distinguish between the hazards of ultraviolet waves and the hazards of ionising EM radiation.
- Explain how the risk of harm depends on the type of radiation and the size of the dose.
- Define radiation dose as a measure of the risk of harm resulting from an exposure of the body to radiation.
- Interpret given data regarding radiation dose in sieverts or millisieverts and the associated risk of harm.
- Calculate how many chest X-rays at 0.02 mSv deliver the same dose as one year of background radiation at 2.5 mSv.
- Explain what a dose in sieverts measures and what it depends on.
- Compare doses given in data, such as a chest X-ray and a year of background radiation.
- Convert a radiation dose between millisieverts and sieverts in both directions.
- Compare two doses given in different units by converting them to a common unit first.
- Calculate how many times larger one dose is than another using data from a table.
- Draw a conclusion about relative risk from a table of radiation doses, quoting figures from it.
- Interpret a graph of radiation exposure and relate it to the potential risk to human health.
- Explain why a correlation in the data does not by itself prove that the radiation caused the observed harm.
- Explain how ultraviolet radiation absorbed by the skin damages the DNA of skin cells and raises the risk of skin cancer.
- Describe how repeated exposure to ultraviolet radiation can cause the skin to age prematurely.
- Suggest practical precautions that reduce a person's exposure to ultraviolet radiation and explain how each one works.
- Explain the sequence by which ionising radiation can lead to cancer in a cell.
- State two precautions taken by hospital staff who work with X-ray equipment and say why each works.
- Evaluate the benefit and the risk of using gamma radiation to treat a tumour.
Properties of electromagnetic waves 2 exam tips
Marking Points
- one mark for stating that an alternating current in an aerial makes charges oscillate
- one mark for saying that the oscillating charges produce a radio wave
- one mark for stating that the frequency of the radio wave equals the frequency of the oscillation in the circuit
- one mark for saying that the wave travels away from the aerial at the speed of electromagnetic waves
- one mark for saying that the radio wave is absorbed by the aerial
- one mark for stating that this produces an alternating current in the aerial or circuit
- one mark for stating that the current has the same frequency as the radio wave
- one mark for saying that the circuit can be tuned to respond to one particular frequency
- one mark for saying that changes in the electron arrangement of an atom generate or absorb electromagnetic waves
- one mark for saying that changes in the nucleus generate gamma rays
- one mark for stating that absorption as well as emission can occur
- one mark for linking a larger energy change to a higher frequency of the wave produced
- one mark for stating that a gamma ray comes from a change in the nucleus, not from the electrons
- one mark for saying that the mass number and atomic number are unchanged by gamma emission
- one mark for identifying gamma as the most penetrating of the nuclear radiations
- one mark for choosing an absorbing material, such as aluminium, that stops beta while gamma still passes through
- one mark for stating that ultraviolet waves can cause premature skin ageing and skin cancer
- one mark for identifying X-rays and gamma rays as ionising radiation
- one mark for linking ionising radiation to the mutation of genes and cancer
- one mark for stating that X-rays and gamma rays penetrate the body while ultraviolet is absorbed by the skin
- State that the effect of radiation depends on both the type of radiation and the size of the dose.
- Define radiation dose as a measure of the risk of harm resulting from an exposure of the body to radiation.
- Identify that ultraviolet radiation is absorbed by the skin, leading to premature ageing or skin cancer.
- Explain that X-rays and gamma rays are ionising and can cause gene mutations or cancer.
- one mark for stating that dose measures the risk of harm rather than the amount of radiation present
- one mark for saying that the dose depends on the type of radiation and on the time of exposure
- one mark for saying that the dose also depends on the activity or strength of the source
- one mark for putting two supplied doses on the same basis, per scan against per year, before comparing them
- one mark for a correct conversion between mSv and Sv using the factor of 1000
- one mark for expressing two given doses in the same unit before comparing them
- one mark for a correct ratio or difference calculated from the converted values
- one mark for the answer written with the unit the question asked for
- quoting relevant figures from the data provided to support a conclusion
- calculating a comparison, such as a ratio or a difference between two radiation doses
- stating a valid conclusion about risk that logically follows from the provided figures
- linking a greater radiation dose to a greater risk of harm, such as cellular damage or cancer
- stating that ultraviolet radiation is absorbed by the skin
- naming premature ageing of the skin as a long-term effect of UV exposure
- linking damage to DNA or genes in skin cells to an increased risk of skin cancer
- identifying that sunscreen or protective clothing can absorb or block ultraviolet to reduce exposure
- one mark for stating that X-rays and gamma rays are ionising
- one mark for explaining that ionisation can damage or alter the DNA or genes in a cell
- one mark for linking a mutation to uncontrolled cell division and cancer
- one mark for stating that both penetrate the body, so internal organs are affected
Examiner Tips
- 💡Use the word oscillation for both the circuit and the wave, and state that the two frequencies match.
- 💡This is Higher tier only and usually appears with the absorption statement, so learn the pair together.
- 💡If a frequency is given in MHz, convert it to Hz before any calculation.
- 💡Answer transmission and reception as mirror images of each other; the same sentence works both ways.
- 💡The phrase same frequency as the radio wave is almost always a marking point, so include it.
- 💡Watch the word induce here: it means cause an oscillation, not electromagnetic induction in a generator.
- 💡Keep the two sources clearly separate: electrons for most of the spectrum, the nucleus for gamma rays.
- 💡If the question mentions a nucleus, the answer will involve gamma rays.
- 💡Use the word absorbed as well as emitted when the statement covers both.
- 💡In a nuclear equation, gamma emission leaves both numbers unchanged; check that before you write anything.
- 💡For absorber questions remember the ladder: paper stops alpha, aluminium stops beta, thick lead reduces gamma.
- 💡Say where the radiation comes from when the question uses the word originate.
- 💡Clearly distinguish between the effects of UV (skin ageing and cancer) and X-rays or gamma rays (ionising, gene mutation, cancer).
- 💡Remember that X-rays and gamma rays are the only EM waves classified as ionising radiation in this specification.
- 💡When discussing risk, always mention both the type of radiation and the size of the dose to secure full marks.
- 💡You will be given the units (Sv or mSv) if needed, but you must understand that dose measures the risk of harm.
- 💡Name all three of source strength, exposure time and type of radiation when asked what the dose depends on.
- 💡You are not required to recall the unit, but you must be able to use sieverts and millisieverts from data.
- 💡When given a table of doses, compare like with like, per scan or per year, before drawing a conclusion.
- 💡Convert both values to millisieverts first, since the numbers are usually easier to handle that way.
- 💡Say in your working which unit you have converted to, so the examiner can follow the comparison.
- 💡You are not expected to recall the unit, so spend no time worrying about its name.
- 💡Quote at least one specific number or calculated difference from the data in every conclusion you write.
- 💡State clearly which comparison you are making before you make it, for example comparing annual background dose against a single medical scan.
- 💡When asked about the risks of UV, give both a short-term effect, such as sunburn, and a long-term one, such as skin cancer.
- 💡Remember to specify that ultraviolet is absorbed by the skin, leading to potential DNA damage, rather than just saying it hits the body.
- 💡Spell out the chain: ionisation, damage to DNA, mutation, cancer; each step can carry a mark.
- 💡Mention the precaution as well as the hazard when the question is set in a hospital.
- 💡Keep X-rays and gamma rays together for hazard questions, but remember they are produced differently.
Common Mistakes
- saying the current itself travels out of the aerial as the wave
- claiming that a direct current in an aerial produces radio waves
- confusing the speed of the wave with the frequency of the circuit
- saying the aerial emits sound waves which are then converted into radio waves
- saying a direct current is produced in the aerial
- saying the frequency of the current is higher or lower than that of the wave
- describing the aerial as reflecting the wave rather than absorbing it
- explaining reception without mentioning the oscillation of charges in the metal
- saying gamma rays come from the electrons of an atom
- treating emission and absorption as separate processes that happen in separate materials
- saying X-rays come from the nucleus, when they come from electron transitions or decelerating electrons
- claiming that only radioactive atoms emit electromagnetic waves
- changing the mass number or atomic number in a nuclear equation for gamma emission
- saying gamma rays are particles emitted from the nucleus rather than electromagnetic waves
- claiming gamma is stopped completely by a thin aluminium sheet, which is what stops beta
- confusing gamma rays with X-rays by saying both come from the nucleus
- classifying ultraviolet as ionising radiation, which is incorrect at GCSE level
- stating that microwaves and radio waves cause cancer because they are radiation
- describing the damage as burning, with no mention of cells, genes or mutations for X-rays and gamma rays
- Confusing the type of radiation with the size of the dose; correct this by treating them as two distinct factors that both influence the risk of harm.
- Assuming ultraviolet is highly penetrating ionising radiation; correct this by noting it is primarily absorbed by the skin.
- Treating dose as just the physical amount of radiation energy absorbed; correct this by defining it specifically as a measure of the risk of harm.
- swapping the definitions of irradiation and contamination
- saying that an irradiated object becomes radioactive itself
- treating the sievert as a unit of energy or of activity
- comparing two doses without also comparing the exposure times
- multiplying when you should divide, so turning 5 mSv into 5000 Sv
- comparing a dose in mSv directly with a dose in Sv
- using a factor of 100 or of a million instead of 1000 for the milli prefix
- giving a bare number with no unit at the end of a dose calculation
- writing a conclusion from memory without using any of the numbers supplied, corrected by always quoting data from the source
- describing the trend but never answering the specific question asked, corrected by explicitly addressing the prompt's focus on risk
- claiming a cause from a correlation with no further evidence, corrected by stating the data only shows a relationship or link
- saying ultraviolet causes cancer because it is hot, corrected by distinguishing its effects from infrared radiation
- claiming sunscreen reflects all the ultraviolet so that no risk remains, corrected by noting it absorbs UV but some risk may persist
- describing sunburn as the only effect, corrected by including the longer-term risks of premature ageing and skin cancer
- saying an X-ray scan is not dangerous because it is quick, with no mention of dose
- describing ionisation as heating the cell
- saying gamma rays cure cancer without noting that they can also cause it
- claiming X-rays come from the nucleus in the way gamma rays do