The Electromagnetic Spectrum: Order, Wavelengths, Uses & Hazards

1

Properties & Order of the EM Spectrum

All electromagnetic waves are transverse waves (oscillations are perpendicular to the direction of energy transfer) that can travel through a vacuum at the speed of light: c = 3.0 x 10⁸ m/s (300,000,000 m/s). Order from longest wavelength (lowest frequency/energy) to shortest wavelength (highest frequency/energy): Radio waves -> Microwaves -> Infrared (IR) -> Visible light -> Ultraviolet (UV) -> X-rays -> Gamma rays. Wave equation: Wave speed, v (m/s) = Frequency, f (Hz) x Wavelength, λ (m).

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2

Radio Waves & Microwaves

Radio waves: Used for television and radio broadcasting (long-wave radio can diffract around hills and curve around the Earth by reflecting off the ionosphere; Bluetooth uses short-wave radio). Generated and absorbed via alternating currents in aerials (Higher Tier). Microwaves: Used for satellite television/communications (pass through the Earth's atmosphere without being reflected by the ionosphere) and for cooking food (absorbed by water molecules in food, transferring energy to heat the food).

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3

Infrared (IR) & Visible Light

Infrared: Emitted by all hot objects. Used in electrical heaters, cooking food in ovens/toasters (surface absorption), thermal imaging / night-vision cameras, and short-range TV remote controls. Visible light: The only part of the EM spectrum detectable by the human eye (Red has the longest wavelength; Violet has the shortest). Used in photography and fibre-optic communications (pulses of light travel down glass fibres by total internal reflection with very little absorption).

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4

Ultraviolet (UV), X-rays & Gamma Rays

Ultraviolet: Used in energy-efficient fluorescent lamps (UV is absorbed by a phosphor coating inside the bulb, which re-emits visible light), security marking (fluoresces under UV), disinfecting water, and sunbeds. X-rays & Gamma rays: Used in medical imaging (X-rays are absorbed by dense bone/metal but pass through soft tissue to form shadow radiographs) and medical treatments (radiotherapy to kill cancer cells, and sterilising medical equipment/food).

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5

Ionising Radiation Hazards & Sieverts (Sv)

Ultraviolet, X-rays, and gamma rays are ionising radiation. Radiation dose (measured in sieverts, Sv, or millisieverts, mSv, where 1,000 mSv = 1 Sv) is a measure of the risk of harm resulting from an exposure of the body to the radiation (depending on radiation type, intensity, and body organ). UV ages skin prematurely and increases skin cancer risk; X-rays and gamma rays cause gene mutation and cancer.

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6

Atomic electron energy level changes

Changes in atoms and nuclei can generate or absorb EM waves over a wide frequency range: 1. Gamma rays originate from changes inside the nucleus of an unstable atom. 2. Visible light, UV, and X-rays can be absorbed or emitted when electrons move between different energy levels (shells): absorbing EM radiation moves an electron to a higher energy level (further from the nucleus); when an electron falls back to a lower energy level, it emits an EM wave.

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1. Q1

   Explain how an alternating current in a transmitting station aerial produces radio waves, and explain what happens when these radio waves reach a metal receiving aerial on a car. (Higher Tier) [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. An alternating current in the transmitting circuit consists of electrons oscillating back and forth at a specific frequency [1 mark]. 2. These oscillating charges generate oscillating electric and magnetic fields, which radiate away from the aerial as electromagnetic radio waves with the exact same frequency as the alternating current [1 mark]. 3. When the radio waves reach the metal receiving aerial on the car, the waves are absorbed by the conductor [1 mark]. 4. The absorbed radio waves cause the delocalised electrons in the receiving aerial to oscillate at the same frequency, inducing an alternating current of identical frequency in the car's receiver circuit [1 mark].

2. Q2

   Compare the properties, uses, and health hazards of Infrared radiation and X-rays. [6 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Shared properties: Both are transverse electromagnetic waves that travel through a vacuum at 3.0 x 10⁸ m/s [1 mark]. 2. Wavelength and frequency differences: X-rays have a much shorter wavelength and a much higher frequency (and higher energy) than infrared radiation [1 mark]. 3. Uses of Infrared: Infrared is used for electrical heaters, cooking food in ovens/grills, thermal imaging cameras, and TV remote controls [1 mark]. 4. Uses of X-rays: X-rays penetrate soft tissue but are absorbed by dense bone, so they are used for medical imaging of bone fractures and airport security scanners (and high-energy X-rays for cancer radiotherapy) [1 mark]. 5. Hazards of Infrared: Infrared is non-ionising; it is absorbed by the skin surface and increases the kinetic energy of particles, causing skin burns if too intense [1 mark]. 6. Hazards of X-rays: X-rays are ionising radiation; they penetrate deep into body cells and knock electrons off atoms, causing DNA mutations that can lead to cell death and cancer [1 mark].

3. Q3

   A hospital patient needs a CT scan of their chest (which uses X-rays and gives a radiation dose of 8.0 mSv). Average natural background radiation in the UK gives a dose of 2.0 mSv per year. Explain why a doctor will still recommend the CT scan despite the radiation risk, and state two precautions the radiographer takes to protect themselves. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. One chest CT scan (8.0 mSv) is equivalent to 4 years of natural background radiation (8.0 / 2.0 = 4), which slightly increases the patient's lifetime risk of developing cancer [1 mark]. 2. However, the doctor judges that the medical benefit of accurately diagnosing a potentially life-threatening chest condition far outweighs the small radiation risk to the patient [1 mark]. 3. Precaution 1: The radiographer stands behind a lead-lined screen (or leaves the room / wears a lead apron) while the X-ray machine is switched on, because lead absorbs X-rays [1 mark]. 4. Precaution 2: The radiographer wears a radiation monitoring badge (dosimeter) to track their cumulative radiation dose and ensure it stays within safe annual limits [1 mark].

4. Q4

   Ultraviolet radiation from the Sun has a wavelength of 3.0 x 10⁻⁷ m. Given that the speed of electromagnetic waves in a vacuum is 3.0 x 10⁸ m/s, calculate the frequency of this ultraviolet radiation and state two ways people can reduce the health risks of UV exposure. [4 marks]

   Check Mark Scheme

   Click “Check Mark Scheme” to compare your answer, 1. Rearrange the wave equation v = f x λ for frequency: f = v / λ = (3.0 x 10⁸ m/s) / (3.0 x 10⁻⁷ m) [1 mark]. 2. Frequency f = 1.0 x 10¹⁵ Hz [1 mark]. 3. Risk reduction 1: Apply sunscreen (containing UV-absorbing filters/nanoparticles) and wear protective clothing/hats to prevent UV reaching skin cells and causing skin cancer or premature aging [1 mark]. 4. Risk reduction 2: Wear UV-filtering sunglasses to prevent UV damaging the cornea/lens and causing cataracts [1 mark].

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