Waves — GCSE Physics Revision
Everything you need to revise waves for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
What are Waves?
Waves are disturbances that transfer energy from one place to another without transferring matter. They can be categorised into two main types: transverse and longitudinal. Transverse waves oscillate perpendicular to the direction of energy transfer, like light or water waves. Longitudinal waves oscillate parallel to the direction of energy transfer, like sound waves. All waves have properties such as amplitude, wavelength, frequency, and wave speed, which help describe their behaviour and energy carrying capacity. Understanding these fundamental types is crucial for studying wave phenomena.
Properties of Waves
Key properties define a wave. Amplitude is the maximum displacement from the equilibrium position, related to the wave's energy. Wavelength (λ) is the distance between two consecutive identical points on a wave, such as two crests or two troughs. Frequency (f) is the number of waves passing a point per second, measured in Hertz (Hz). Period (T) is the time taken for one complete wave to pass a point (T = 1/f). Wave speed (v) is the distance a wave travels per second, calculated using the wave equation: v = fλ.
Electromagnetic Spectrum
The electromagnetic spectrum is a continuous range of transverse waves that travel at the speed of light in a vacuum (3 x 10^8 m/s). They don't require a medium for transmission. It includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. These waves are arranged by wavelength and frequency. All electromagnetic waves are generated by changes in electrons or other charged particles and can travel through space. Each type has distinct uses and potential hazards.
Wave Interactions: Reflection, Refraction, Diffraction
Waves interact with materials in various ways. Reflection occurs when a wave bounces off a surface; the angle of incidence equals the angle of reflection. Refraction is the change in direction of a wave as it passes from one medium to another, caused by a change in speed. Diffraction is the spreading out of waves as they pass through an aperture (gap) or around an obstacle. The amount of diffraction depends on the wavelength relative to the size of the gap or obstacle.
Key facts
- Waves transfer energy, not matter.|Transverse waves oscillate perpendicular to energy transfer.|Longitudinal waves oscillate parallel to energy transfer.|The wave equation: wave speed = frequency × wavelength (v = fλ).|All electromagnetic waves are transverse and travel at 3 x 10^8 m/s in a vacuum.|Reflection, refraction, and diffraction describe how waves interact with boundaries and obstacles.
Key terms
- Amplitude::The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position.|Wavelength (λ)::The distance between two consecutive identical points on a wave, typically from crest to crest or trough to trough.|Frequency (f)::The number of complete waves or oscillations passing a point per second, measured in Hertz (Hz).|Period (T)::The time taken for one complete wave or oscillation to pass a given point, measured in seconds (s).|Transverse Wave::A wave in which the oscillations (vibrations) are perpendicular to the direction of energy transfer.|Longitudinal Wave::A wave in which the oscillations (vibrations) are parallel to the direction of energy transfer.
Common mistakes
- Confusing the direction of oscillation with the direction of energy transfer for transverse vs. longitudinal waves.|Forgetting to convert units (e.g., cm to m, MHz to Hz) when using the wave equation.|Thinking that electromagnetic waves require a medium to travel.|Mixing up reflection (bouncing off) and refraction (changing direction due to speed change).
Exam tips
- Always state units in your answers, especially for calculations involving wave properties.|Be prepared to draw and label diagrams of transverse and longitudinal waves, showing wavelength and amplitude.|Memorise the order of the electromagnetic spectrum and at least one use/hazard for each type.|Understand the relationship between wave speed, frequency, and wavelength (v = fλ) and be able to rearrange it.
Quick quiz
1. Which of the following is a longitudinal wave?
- Light wave
- Water wave
- Sound wave
- Radio wave
Show answer
Sound wave — Sound waves are longitudinal waves where particles oscillate parallel to the direction of energy transfer. Light, water, and radio waves are transverse.
2. A wave has a frequency of 5 Hz and a wavelength of 2 meters. What is its wave speed?
- 0.4 m/s
- 2.5 m/s
- 10 m/s
- 7 m/s
Show answer
10 m/s — Using the wave equation v = fλ, wave speed = 5 Hz × 2 m = 10 m/s.
3. Which property of a wave is related to the energy it transfers?
- Wavelength
- Frequency
- Amplitude
- Period
Show answer
Amplitude — The amplitude of a wave is directly related to the amount of energy it carries. A larger amplitude means more energy.
4. Which of these electromagnetic waves has the longest wavelength?
- Gamma rays
- Ultraviolet
- Microwaves
- Visible light
Show answer
Microwaves — The electromagnetic spectrum in order of decreasing wavelength (increasing frequency/energy) is: Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays, Gamma rays. Microwaves have a longer wavelength than UV, visible light, and gamma rays.
5. What happens to a wave when it undergoes refraction?
- It bounces off a surface.
- Its oscillations become parallel to its direction of travel.
- It changes direction due to a change in speed as it enters a new medium.
- It spreads out as it passes through a gap.
Show answer
It changes direction due to a change in speed as it enters a new medium. — Refraction is the change in direction of a wave as it passes from one medium to another, caused by a change in its speed. Bouncing off is reflection, spreading out is diffraction.
Exam-style questions
Sound waves are longitudinal waves. Describe what this means in terms of particle oscillation and energy transfer. [2 marks]
Show mark scheme
- Particles oscillate (vibrate) parallel to the direction of energy transfer.
- This causes compressions and rarefactions to move through the medium.
A student measures the wavelength of a wave to be 0.5 m and its frequency to be 40 Hz. Calculate the speed of the wave. State the formula and show your working. [3 marks]
Show mark scheme
- Formula: v = fλ (or wave speed = frequency × wavelength)
- Substitution: v = 40 Hz × 0.5 m
- Answer with unit: v = 20 m/s
Explain why light refracts when it passes from air into a glass block. [3 marks]
Show mark scheme
- Light changes speed when it enters the glass block (it slows down).
- If it enters at an angle (not along the normal), one side of the wavefront slows down before the other.
- This causes the wave to change direction (bend).
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