Waves: Properties, Types & the Electromagnetic Spectrum
What Are Waves?
A wave is a disturbance that transfers energy and information from one place to another without transferring matter. When a wave passes through water, the water molecules oscillate up and down, but the water itself does not travel with the wave — it is the wave pattern that moves.
Evidence for this comes from watching a floating object on water: it bobs up and down as waves pass, but it does not move forward with the waves. Similarly, sound waves cause air particles to vibrate back and forth, but the air itself does not travel from the source to the listener.
Waves transfer energy and information without transferring matter. The particles of the medium oscillate about a fixed position.
Wave Properties
All waves can be described using these key terms:
Amplitude is the maximum displacement from the rest position. Wavelength (\(\lambda\)) is the distance from one point on a wave to the same point on the next wave (e.g. crest to crest). Frequency (\(f\)) is the number of complete waves passing a point per second, measured in hertz (Hz). Period is the time for one complete wave to pass a point.
Wave speed (\(v\)) is how fast the wave travels. A wavefront is an imaginary line joining all the points at the same phase of a wave.
Transverse and Longitudinal Waves
There are two types of wave:
In a transverse wave, the oscillations are perpendicular (at right angles) to the direction of energy transfer. Examples include all electromagnetic waves, water surface waves, and waves on a string.
In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. The wave creates alternating compressions (where particles are pushed together) and rarefactions (where particles are spread apart). Sound is a longitudinal wave. Seismic P-waves are also longitudinal.
Wave speed, frequency, and wavelength are linked by the wave equation:
$$v = f \times \lambda$$
where \(v\) is wave speed in m/s, \(f\) is frequency in Hz, and \(\lambda\) is wavelength in metres (m). This equation applies to all types of wave.
Wave speed can also be calculated using:
$$v = \frac{x}{t}$$
where \(x\) is the distance travelled in metres and \(t\) is the time taken in seconds.
You must recall two wave equations: \(v = f \times \lambda\) and \(v = \frac{x}{t}\). Both are used in the exam and must be recalled from memory.
The Electromagnetic Spectrum
The electromagnetic (EM) spectrum is a continuous range of waves. All EM waves are transverse, transfer energy from source to observer, and travel at the same speed in a vacuum — the speed of light, approximately \(3 \times 10^8\) m/s.
The spectrum is ordered from longest wavelength (lowest frequency) to shortest wavelength (highest frequency):
Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays
Our eyes can only detect a narrow range — the visible spectrum, which ranges from red (longest wavelength) through orange, yellow, green, blue, and indigo to violet (shortest wavelength).
Radio waves: broadcasting, communications, and satellite transmissions.
Microwaves: cooking, mobile phone communications, and satellite transmissions.
Infrared: cooking, thermal imaging, short-range communications, optical fibres, TV remote controls, and security systems.
Visible light: vision, photography, and illumination.
Ultraviolet: security marking, fluorescent lamps, detecting forged banknotes, and disinfecting water.
X-rays: medical imaging, airport security scanners, and observing internal structures.
Gamma rays: sterilising food and medical equipment, detecting cancer, and cancer treatment.
The danger of electromagnetic radiation generally increases with increasing frequency. Excessive exposure can cause harm:
Microwaves: internal heating of body cells. Infrared: skin burns. Ultraviolet: damage to surface cells and eyes, potentially leading to skin cancer and eye conditions. X-rays and gamma rays: can cause mutation or damage to cells inside the body.
Refraction of Waves
When a wave crosses the boundary between two different substances, its speed changes. If the wave hits the boundary at an angle (not along the normal), this change in speed causes a change in direction — this is called refraction.
Different substances may absorb, transmit, refract, or reflect waves in ways that vary with wavelength. This is why a glass prism can split white light into the visible spectrum — different colours (wavelengths) are refracted by different amounts.
All electromagnetic waves travel at the same speed in a vacuum (\(3 \times 10^8\) m/s), are transverse, and can be reflected, refracted, and absorbed. The danger increases with frequency.
Exam Tip: When describing the EM spectrum, state the correct order and mention that all EM waves travel at the same speed in a vacuum. For wave calculations, make sure you rearrange \(v = f \lambda\) correctly and convert units (e.g. kHz to Hz, nm to m) before substituting.
Waves transfer energy without transferring matter and come in two types: transverse (oscillations perpendicular to direction) and longitudinal (oscillations parallel to direction). The wave equation \(v = f \lambda\) links speed, frequency, and wavelength. The electromagnetic spectrum is a continuous range from radio waves to gamma rays, all travelling at the speed of light in a vacuum but differing in wavelength, frequency, uses, and dangers.