5.4.1 Dangers of electromagnetic radiation
Electromagnetic radiation can damage tissue
Ionising radiation
Radiation with enough energy per photon to remove electrons from atoms or molecules, producing ions.
- The risk from electromagnetic radiation depends on frequency, intensity, exposure time and which tissue absorbs the energy.
- Higher-frequency ultraviolet, X-rays and gamma rays can damage cells because their photons carry more energy.
- Microwaves and infrared are non-ionising, but intense exposure can transfer enough energy to heat tissue.
Different regions cause different harm
- Microwaves penetrate into body tissue and can cause internal heating when their energy is absorbed by water molecules.
- Infrared radiation is absorbed mainly near the skin surface and can cause skin burns and eye damage at high intensity.
- Ultraviolet radiation can damage surface cells and DNA, causing sunburn, premature skin ageing, eye damage and an increased risk of skin cancer.
- X-rays and gamma rays can ionise molecules inside cells, damage DNA, cause mutations, kill cells or increase cancer risk.
- Gamma radiation is very penetrating, so an external source can expose tissue deep inside the body.
Protection reduces dose
Radiation dose
A measure of the energy deposited by radiation in body tissue, adjusted where necessary for the radiation's biological effect.
- Reduce exposure time, increase distance from a source and use suitable shielding.
- Use sunscreen, covering clothing and shade to reduce ultraviolet exposure.
- Use lead or thick concrete around X-ray and gamma sources, with remote handling and monitoring badges where appropriate.
- Keep microwave ovens and other sources properly shielded and never bypass safety interlocks.
Comparing two hazards
- Infrared from a heater is absorbed mainly at the skin and can cause surface burns.
- Microwaves can penetrate farther before absorption and may heat tissue below the surface.
- Neither is ionising, so the harm in these examples comes from energy transfer and heating rather than direct ionisation.
Do not call microwaves ionising radiation; explain microwave harm through internal heating and explain ultraviolet, X-ray and gamma harm through cell or DNA damage.
Name the radiation and mechanism
- State the region of the electromagnetic spectrum.
- Describe what the radiation does to tissue, such as heating or ionising molecules.
- Link the tissue change to the harm, such as burns, mutation or cancer.
- How can microwaves harm body tissue?
- Why can infrared cause burns?
- How can ultraviolet increase skin-cancer risk?
- Why are X-rays and gamma rays ionising?
- What three general methods reduce exposure?
5.4.2 Uses of electromagnetic radiation
Each waveband has useful properties
Electromagnetic spectrum
The continuous range of electromagnetic waves arranged according to wavelength or frequency.
- All electromagnetic waves travel at the same speed in a vacuum, but their wavelengths and frequencies make them interact differently with matter.
- A suitable use depends on transmission, absorption, diffraction, heating, ionisation and the information that the wave can carry.
Communication uses longer wavelengths
- Radio waves are used for broadcasting because they can travel long distances and some frequencies diffract around obstacles.
- Microwaves are used for satellites because they pass through the atmosphere, carry high data rates and can be directed in narrow beams.
- Microwaves heat water-rich food when their energy is absorbed.
- Infrared is used in remote controls, short-range links and thermal imaging because warm objects emit it.
- Visible light carries data through optical fibres and forms images for vision and photography.
Short wavelengths image and treat
- Ultraviolet is used in fluorescent lamps, security marking and sterilising surfaces because it causes fluorescence and can kill microorganisms.
- X-rays image bones because dense bone absorbs more X-rays than soft tissue, creating contrast at the detector.
- Gamma rays sterilise sealed medical equipment because they penetrate packaging and kill microorganisms.
- Focused gamma radiation is used in radiotherapy to damage cancer cells.
Selecting radiation for a satellite link
The signal must pass through the atmosphere, carry information and form a narrow beam.
Microwaves satisfy these requirements, so dishes can transmit and receive the signal accurately.
Lower-frequency radio waves diffract more and require larger aerials, making them less suitable for this link.
Justify the use
Name a property of the radiation.
Link the property directly to the task.
Include the relevant hazard or control when ionising radiation is used.
Do not say bones reflect X-rays; bones absorb more X-rays than soft tissue, creating contrast at the detector.
- Why are microwaves suitable for satellite communication?
- How does thermal imaging use infrared radiation?
- Why do X-ray images show bones clearly?
- Why can gamma rays sterilise sealed equipment?
5.4.3 Radio waves and oscillations in circuits
Radio waves come from oscillating circuits
Oscillation
A repeated variation about a central value.
- An alternating potential difference makes electrons in a transmitting circuit oscillate, so the current repeatedly changes size and direction.
- An accelerating charge produces changing electric and magnetic fields, which travel away from the circuit as an electromagnetic wave.
- The radio wave has the same frequency as the oscillation that produces it, so a faster electrical oscillation produces a higher-frequency radio wave.
- A transmitting aerial transfers energy from the oscillating circuit to the electromagnetic field.
Radio waves induce electrical oscillations
Induced oscillation
An oscillation produced in a circuit by the changing electric and magnetic fields of an incoming electromagnetic wave.
- When a radio wave reaches a receiving aerial, its changing electric field exerts forces on electrons in the conductor.
- The electrons oscillate at the frequency of the incoming wave, producing a small alternating potential difference and current in the receiving circuit.
- A tuned circuit responds most strongly when its natural frequency matches the radio-wave frequency, allowing one signal to be selected from many.
- The signal can then be amplified and decoded to reproduce the transmitted information.
Matching transmitter and receiver
- A transmitter circuit oscillates at 95.8 MHz95.8\,\text{MHz}95.8MHz, so it emits radio waves with frequency 95.8 MHz95.8\,\text{MHz}95.8MHz.
- A receiving aerial develops an alternating signal at the same frequency.
- Tuning the receiver to 95.8 MHz95.8\,\text{MHz}95.8MHz makes that station's signal produce the strongest response.
Explain the complete chain
- Link oscillating electrons in the transmitter to the production of a radio wave.
- State that the incoming wave makes electrons in the receiving circuit oscillate.
- Use the word frequency to connect the transmitter, electromagnetic wave and receiver.
Do not state that the radio wave carries electrons from the transmitter to the receiver; energy and information travel in the electromagnetic wave while electrons oscillate locally in each circuit.
Communication depends on modulation
- Information is carried by deliberately varying a property of a high-frequency carrier wave, a process called modulation.
- The receiver extracts these variations to recover sound, images or digital data.
- The detailed electronics of modulation are not required here, but the carrier still originates from electrical oscillations.
- How does an oscillating current produce a radio wave?
- What determines the frequency of the radio wave?
- What happens to electrons in a receiving aerial?
- Why can a tuned receiver select one station?
5.4.4 Changes in atoms and nuclei generating radiation
Atomic changes produce photons
Photon
A discrete packet of electromagnetic energy.
- Electrons in atoms occupy discrete energy levels.
- When an electron moves to a lower energy level, the atom emits a photon whose energy equals the difference between the two levels.
- An atom absorbs a photon only when its energy matches an allowed energy-level difference.
- Different atomic energy gaps can produce infrared, visible or ultraviolet radiation.
Nuclear changes produce gamma rays
Gamma radiation
Gamma radiation is electromagnetic radiation emitted from the nucleus of an unstable atom when that nucleus loses energy.
- After alpha or beta decay, a daughter nucleus may remain in an excited state.
- The nucleus can move to a lower-energy state by emitting a gamma photon.
- Gamma emission changes nuclear energy but does not change proton number or nucleon number.
- Nuclear energy gaps are usually much larger than atomic gaps, so nuclear transitions often produce high-frequency gamma radiation.
Energy determines frequency
- Photon energy and frequency are related by E=hfE=hfE=hf, where EEE is energy in joules, hhh is the Planck constant and fff is frequency in hertz.
- A larger energy change produces a higher-frequency photon.
- Changes in atoms and nuclei can therefore generate electromagnetic radiation across a wide frequency range.
Comparing two transitions
Transition A releases 3.0×10−19 J3.0\times10^{-19}\,\text{J}3.0×10−19J and transition B releases 6.0×10−14 J6.0\times10^{-14}\,\text{J}6.0×10−14J.
Since f=Ehf=\dfrac{E}{h}f=hE, transition B produces the higher-frequency photon.
Transition A is consistent with an atomic transition, while transition B is consistent with a nuclear transition.
Describe the change precisely
Identify whether an electron or an excited nucleus changes state.
State that moving to a lower energy state emits a photon.
Link a larger energy difference to a higher frequency using E=hfE=hfE=hf.
Do not state that gamma emission removes protons or neutrons; it removes excess nuclear energy without changing proton number or nucleon number.
- How can an atomic electron produce a photon?
- What determines photon energy?
- How is gamma radiation produced?
- What stays constant during gamma emission?
