Electromagnetic WavesAQA GCSE Physics: Revision notes

Electromagnetic waves are transverse waves that consist of oscillating electric and magnetic fields perpendicular to the direction of travel. All electromagnetic waves travel at the same speed in a vacuum: 3 × 10⁸ m/s (this is the speed of light, c).

Because they are transverse waves, electromagnetic waves:

• Have oscillations perpendicular to their direction of travel
• Can be polarised
• Do not need a medium to travel (unlike sound waves)

The relationship between wavelength, frequency, and wave speed still applies: c = fλ, where c = 3 × 10⁸ m/s, f = frequency in Hz, and λ = wavelength in m.

The electromagnetic spectrum is the complete range of all electromagnetic waves, ordered by increasing frequency and decreasing wavelength:

Key relationships:

• As frequency increases, wavelength decreases
• All waves travel at c = 3 × 10⁸ m/s in a vacuum
• The spectrum is continuous—there are no gaps between types

Each type of electromagnetic radiation has distinctive properties and practical applications:

Radio waves:

• Used for broadcast communication (TV, radio), mobile phones, and Wi-Fi
• Long wavelengths allow them to diffract around obstacles and penetrate buildings
• Produced by oscillating electrons in transmitter circuits

Microwaves:

• Used in mobile phone communication and microwave ovens
• Penetrate the atmosphere and are absorbed by water molecules (causing heating in ovens)
• Wavelength small enough to be focused into narrow beams

Infrared (IR):

• Used in thermal imaging cameras, night vision goggles, and infrared heaters
• Absorbed by most materials, causing them to heat up
• Used in remote controls and fibre optic cables

Visible light:

• The only EM radiation human eyes can detect
• Used in photography, lighting, and optical fibres
• Different wavelengths perceived as different colours (red to violet)

Ultraviolet (UV):

• Used in sterilisation (kills bacteria), security marking, and tanning beds
• Mostly absorbed by the Earth's ozone layer (protection from solar UV)
• Can cause skin damage and cancer in humans

X-rays:

• Used in medical imaging to see inside the body (bones absorb more X-rays than soft tissue)
• Used in security scanners at airports
• Produced when fast electrons hit a metal target

Gamma rays:

• Used in cancer radiotherapy and sterilisation of medical equipment
• Produced by radioactive nuclei
• Extremely penetrating; dangerous to living tissue

Ultraviolet (UV) radiation hazards:

• UV radiation from the Sun damages skin cells and can cause skin cancer
• Exposure to UV can cause cataracts (clouding of the lens in the eye)
• UV is ionising radiation—it has enough energy to remove electrons from atoms, creating ions

X-ray hazards:

• X-rays are ionising radiation with high frequency and high energy
• Ionisation in living cells damages DNA and can cause cancer
• Repeated exposure increases cancer risk
• X-rays can cause damage to reproductive cells, leading to genetic mutations
• Used in medicine but exposure must be minimised through shielding with lead

Gamma ray hazards:

• Gamma rays are highly ionising—the most energetic EM radiation
• Ionisation of atoms in cells damages or kills the cells
• Can cause cancer, leukaemia, and cell death
• Extremely penetrating; dense shielding (lead or concrete) needed for protection
• Exposure to high doses causes radiation sickness and death

Why ionisation is hazardous: When UV, X-rays, or gamma rays knock electrons out of atoms, they create ions. In living cells, this ionisation damages important molecules like DNA. Cells may die, mutate, or become cancerous. Lower-energy EM radiation (radio, microwave, IR, visible) is non-ionising and does not have this effect.

Different materials interact with different types of electromagnetic radiation in different ways. The interaction depends on the wavelength of the radiation and the properties of the material.

Three main interactions:

1. Reflection: The EM radiation bounces off the surface of the material

   • Example: Visible light reflects off mirrors; radio waves reflect off metal surfaces
   • Materials with free electrons (metals) are good reflectors of radio waves
   • Shiny surfaces reflect visible light

2. Transmission: The EM radiation passes through the material

   • Example: Visible light passes through glass; radio waves pass through brick
   • The material is transparent to that wavelength
   • Different materials transmit different wavelengths

3. Absorption: The EM radiation is taken in by the material and converted to other forms of energy (usually heat)

   • Example: Infrared radiation is absorbed by skin, causing heating; X-rays are absorbed by bone more than soft tissue
   • Dark surfaces absorb more visible light; shiny surfaces reflect it
   • Water absorbs microwaves strongly (used in microwave ovens)

Why wavelength matters:

• Long-wavelength radiation (radio, microwave) passes through many materials because the wavelength is large compared to atomic structures
• Short-wavelength radiation (UV, X-rays, gamma) interacts strongly with atoms and is more easily absorbed or causes ionisation
• Visible light interacts with materials in various ways depending on their atomic structure and electron behaviour

Practical example: A mobile phone works because:

• Radio waves (long wavelength) transmit through air and pass through buildings
• Metal shielding reflects these waves (Faraday cage effect)
• Different materials absorb different amounts of the signal

How radio waves are produced:

Radio waves are produced when electrons in a transmitter circuit oscillate (accelerate back and forth) at a specific frequency. This process works as follows:

1. An alternating current (AC) voltage is applied to an antenna
2. This AC voltage causes electrons in the antenna to oscillate
3. Oscillating charged particles create oscillating electric and magnetic fields
4. These oscillating fields radiate outward as radio waves
5. The frequency of the radio wave equals the frequency of the oscillating electrons

Key point: The frequency of oscillation determines the frequency (and wavelength) of the emitted radio wave. Higher oscillation frequency produces higher-frequency radio waves with shorter wavelengths.

How radio waves are detected:

Radio waves are detected using a receiver antenna that works as the reverse of a transmitter:

1. An incoming radio wave has an oscillating electric field
2. This oscillating field exerts a force on free electrons in the receiver antenna
3. The electrons in the antenna are made to oscillate at the same frequency as the radio wave
4. This oscillating electron motion induces an AC voltage in the antenna
5. The induced AC voltage is then amplified and processed by the receiver circuit
6. The receiver is tuned to respond to a specific frequency (you select a radio station)

Tuning: Receiver circuits are designed to resonate at specific frequencies. A variable capacitor or inductor allows you to change the resonant frequency, letting you select different radio stations from the many signals arriving at the antenna.

Electromagnetic radiation is produced when electrons in atoms change energy levels or when nuclei undergo radioactive decay.

Radiation from electrons changing energy levels:

1. Atomic excitation: An electron in an atom absorbs energy and jumps to a higher energy level (excited state)
2. Instability: The electron in the higher energy level is unstable
3. Emission: The electron falls back down to a lower energy level
4. Photon emission: As the electron falls, it releases energy in the form of an electromagnetic wave (photon)
5. Energy determines wavelength: The energy released (ΔE) determines the frequency and wavelength of the emitted EM wave
   • High energy change = high-frequency radiation = short wavelength (e.g., UV, X-rays)
   • Low energy change = low-frequency radiation = long wavelength (e.g., infrared)

Relationship: ΔE = hf, where ΔE is energy change (J), h is Planck's constant, and f is frequency (Hz)

X-ray production example:

• Fast electrons from a cathode strike a metal target (anode)
• These electrons have high kinetic energy
• When they suddenly decelerate (or stop) in the metal, they lose energy very rapidly
• This energy is released as X-rays (high frequency, short wavelength)
• Alternatively, electrons knock inner-shell electrons out of metal atoms; outer electrons fall down to fill the gaps, releasing X-rays

Radiation from nuclear decay:

1. Radioactive nuclei: Unstable nuclei spontaneously decay
2. Three types of decay:
   • Alpha decay: Nucleus emits an alpha particle; usually produces no EM radiation
   • Beta decay: Nucleus emits a beta particle; often followed by gamma ray emission
   • Gamma decay: Nucleus emits a gamma ray photon (high-frequency EM radiation)
3. Gamma rays: Produced directly from the nucleus when it transitions to a lower energy state
   • Highest frequency EM radiation
   • Most penetrating and ionising
   • Result from changes in nuclear energy levels (not electron energy levels)

Key differences:

• Visible light, IR, UV: Produced by electron transitions in atoms
• X-rays: Produced by rapid deceleration of fast electrons or electron transitions in inner shells
• Gamma rays: Produced directly by nuclear decay; highest energy EM radiation