Waves Revision Guide
GCSE Physics | Foundation & Higher
Key Facts
- Waves transfer energy without transferring matter.
- Transverse waves: oscillations are perpendicular to the direction of travel (e.g. light, water waves).
- Longitudinal waves: oscillations are parallel to the direction of travel (e.g. sound).
- Wave speed = frequency × wavelength (v = f × λ)
- Frequency is measured in hertz (Hz). 1 Hz = 1 wave per second.
- Period = 1 / frequency (T = 1/f)
Transverse vs Longitudinal
In a transverse wave, the particles move up and down (or side to side) while the wave travels forward. Think of a Mexican wave in a stadium or ripples on water. All electromagnetic waves are transverse.
In a longitudinal wave, the particles move back and forth in the same direction the wave is travelling. Sound is the classic example: air molecules are compressed and then spread out, creating compressions (squashed regions) and rarefactions (stretched regions).
Key Wave Properties
Amplitude is the maximum displacement from the rest position. Larger amplitude means more energy. Wavelength (λ) is the distance between two identical points on consecutive waves (e.g. crest to crest). Frequency (f) is how many complete waves pass a point per second. Period (T) is the time for one complete wave: T = 1/f.
The Wave Equation
Wave speed = frequency × wavelength, or v = fλ. Wave speed is in metres per second (m/s), frequency in hertz (Hz) and wavelength in metres (m). This equation applies to all waves, whether transverse or longitudinal.
The Electromagnetic Spectrum
The electromagnetic (EM) spectrum is a continuous range of transverse waves that all travel at the speed of light (3 × 10⁸ m/s) in a vacuum. From longest wavelength to shortest:
- Radio waves: TV, radio, Bluetooth. Longest wavelength, lowest frequency.
- Microwaves: Cooking, satellite communication, mobile phones.
- Infrared: Heating, remote controls, thermal cameras.
- Visible light: The only part we can see (red to violet).
- Ultraviolet: Tanning, fluorescent lights. Can cause skin cancer.
- X-rays: Medical imaging, airport security. Can cause cell damage.
- Gamma rays: Cancer treatment, sterilisation. Shortest wavelength, highest frequency, most energy.
All EM waves travel at the same speed in a vacuum but have different wavelengths and frequencies. Higher frequency means higher energy and greater potential danger.
Reflection and Refraction
Reflection: When a wave bounces off a surface. The angle of incidence equals the angle of reflection. Both angles are measured from the normal (a line perpendicular to the surface).
Refraction: When a wave changes speed as it passes from one medium to another, causing it to change direction. Light slows down and bends towards the normal when entering a denser medium (e.g. air to glass). It speeds up and bends away from the normal when entering a less dense medium.
Sound and Light
Sound is a longitudinal wave that needs a medium (solid, liquid or gas) to travel through. It cannot travel through a vacuum. The speed of sound in air is about 340 m/s. Pitch depends on frequency (higher frequency = higher pitch). Volume depends on amplitude (larger amplitude = louder).
Light is a transverse electromagnetic wave. It can travel through a vacuum at 3 × 10⁸ m/s. White light can be split into the visible spectrum using a prism (dispersion).
Worked Examples
Example 1: A wave has a frequency of 50 Hz and a wavelength of 4 m. Find the wave speed.
v = f × λ = 50 × 4 = 200 m/s
Answer: 200 m/s
Example 2: A sound wave travels at 340 m/s and has a wavelength of 0.85 m. Find the frequency.
f = v / λ = 340 / 0.85 = 400 Hz
Answer: 400 Hz
Example 3: A wave has a period of 0.02 s. What is its frequency?
f = 1 / T = 1 / 0.02 = 50 Hz
Answer: 50 Hz
Example 4: A radio wave has a frequency of 100 MHz. Calculate its wavelength. (Speed of EM waves = 3 × 10⁸ m/s)
λ = v / f = (3 × 10⁸) / (100 × 10⁶) = 3 m
Answer: 3 m
Common Mistakes
- Saying sound can travel through a vacuum (it cannot; it needs a medium).
- Measuring wavelength from crest to trough instead of crest to crest.
- Confusing amplitude with wavelength.
- Forgetting to convert units (e.g. cm to m, kHz to Hz, MHz to Hz).
- Confusing the EM spectrum order. Remember: Running Men In Vests Usually X-ray Giraffes.
Exam Tips
- Learn the wave equation and be able to rearrange it for any variable.
- Know the EM spectrum in order, with uses and dangers for each type.
- For refraction questions, always draw the normal line first, then show the direction change.
- Remember: all EM waves travel at the same speed in a vacuum. Different wavelengths, same speed.
- Show all working in calculations. Even with a wrong answer, you can gain method marks.
Practice Questions
- A wave has a frequency of 200 Hz and a speed of 1500 m/s. Find the wavelength.
- Explain the difference between transverse and longitudinal waves with an example of each.
- A ripple tank produces 10 waves in 5 seconds. The wavelength is 3 cm. Find the wave speed.
- Why can you see lightning before you hear thunder?
- What happens to the wavelength if the frequency doubles but the wave speed stays the same?
- List the electromagnetic spectrum in order of increasing frequency. Give one use for each type.
Recommended Revision Guides
Top-rated guides for GCSE Physics:
- CGP GCSE Physics Complete Revision and Practice — Covers all topics with practice questions.
- Hodder GCSE AQA Physics Revision Guide — Exam-focused with clear explanations.
- Oxford Revise AQA GCSE Physics — Visual revision with exam practice.