The Electromagnetic Spectrum
What Are Electromagnetic Waves?
Electromagnetic (EM) waves are transverse waves that can travel through a vacuum — they do not need a medium. This makes them fundamentally different from mechanical waves like sound. In a vacuum, all EM waves travel at the same speed:
$$c = 3 \times 10^8 \text{ m/s}$$
EM waves are produced by changes in atoms and their nuclei, or by oscillating electrical charges. They transfer energy from a source to an absorber. Because the wave equation \(v = f\lambda\) still applies and the speed is fixed in a vacuum, a higher frequency always means a shorter wavelength — and vice versa.
The Electromagnetic Spectrum
The electromagnetic spectrum is a continuous range of waves ordered by wavelength and frequency. From longest to shortest wavelength:
Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays
A useful mnemonic: Raging Martians Invaded Venus Using X-ray Guns (Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma). There are no sharp boundaries — the spectrum is continuous and one region blends gradually into the next.
All EM waves travel at \(3 \times 10^8\) m/s in a vacuum. The spectrum runs from radio waves (long wavelength, low frequency, low energy) to gamma rays (short wavelength, high frequency, high energy). Visible light is a small section in the middle.
Radio waves (wavelength up to several km): TV and radio broadcasting, Wi-Fi, Bluetooth. Radio waves can induce oscillating currents in aerials, which is how receivers pick up signals.
Microwaves (wavelength cm): microwave ovens (water molecules in food absorb the energy and heat up), satellite communications, mobile phone networks.
Infrared (IR) (wavelength μm): thermal imaging cameras, remote controls for TV and other devices, grills and toasters, fibre optic communications, night-vision equipment. All warm objects emit IR radiation.
Visible light (roughly 400–700 nm): human vision, photography, fibre optic data transmission.
Ultraviolet (UV): fluorescent lamps, security marking (invisible ink visible under UV), sterilising drinking water, detecting forged banknotes.
X-rays: medical imaging of bones and teeth, airport security scanners, checking welds in pipelines for faults.
Gamma rays (shortest wavelength): radiotherapy for cancer treatment (targeted beams kill cancer cells), sterilising medical equipment and food, detecting leaks in industrial pipes using radioactive tracers.
Hazards of EM Radiation
Higher frequency EM waves carry more energy per photon and can be more hazardous to living tissue. Key hazards:
• Infrared: prolonged exposure can cause skin burns.
• Ultraviolet: causes sunburn, premature skin ageing and increases the risk of skin cancer. UV can also damage the eyes and may lead to cataracts. Sunscreen and UV-blocking lenses reduce exposure.
• X-rays: can damage cells and DNA, potentially causing cancer. Medical exposure is carefully controlled — lead aprons protect staff, exposure times are minimised, and the number of scans is limited.
• Gamma rays: the most penetrating and most energetic. Can cause cell mutation and cancer with prolonged exposure. In medical use, beams are tightly focused to protect healthy tissue.
UV, X-rays and gamma rays can damage human tissue and DNA. Higher frequency means higher energy and greater hazard. The benefits of medical uses must always be weighed against the risks.
Absorption, Transmission and Reflection
Different materials interact with EM waves in different ways:
• Absorption: the wave’s energy is taken in by the material. Dark surfaces absorb visible light; skin absorbs UV.
• Transmission: the wave passes through the material. Glass transmits visible light; soft body tissue transmits X-rays.
• Reflection: the wave bounces off the surface. Mirrors reflect visible light; metal surfaces reflect radio waves.
• Refraction: the wave changes speed (and may change direction) when passing into a different material — how lenses focus light.
A material may absorb some wavelengths, transmit others and reflect the rest. Glass transmits visible light but absorbs UV. The atmosphere transmits visible light and radio waves but absorbs most UV, X-rays and gamma rays — a fortunate protection for life on Earth.
Radio waves are produced when electrons oscillate (move back and forth) in a transmitter aerial at a particular frequency. They generate radio waves at that same frequency. When radio waves reach a receiver aerial, they induce oscillating currents (alternating current) in the aerial circuit. This is the basis of all radio communication — from AM and FM broadcasts to Wi-Fi and mobile networks. The receiver circuit must be tuned to the correct frequency to pick up a particular signal.
Since all EM waves travel at \(c = 3 \times 10^8\) m/s in a vacuum, you can calculate wavelength from frequency using \(\lambda = \frac{c}{f}\).
Example 1: A radio station broadcasts at 100 MHz. \(\lambda = \frac{3 \times 10^8}{100 \times 10^6} = 3\) m.
Example 2: Visible green light has a wavelength of about 550 nm = \(5.5 \times 10^{-7}\) m. Frequency \(= \frac{3 \times 10^8}{5.5 \times 10^{-7}} \approx 5.5 \times 10^{14}\) Hz. Always convert nm to m (multiply by \(10^{-9}\)) before calculating.