Waves
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Transverse and Longitudinal Waves
Waves transfer energy without transferring matter. There are two types based on the direction of oscillation.
Identifying the wave type from a description or diagram is a common exam starter.
Transverse and Longitudinal Waves — Key Knowledge
- Transverse waves oscillations perpendicular to direction of travel — e.g. light, water waves
- Longitudinal waves oscillations parallel to direction of travel — e.g. sound
- Compressions regions where particles are pushed together in a longitudinal wave
- Rarefactions regions where particles are spread apart in a longitudinal wave
Wave Properties
All waves can be described using four key properties that relate to their shape and timing.
These four terms appear in almost every waves question — students must define each precisely.
Wave Properties — Key Knowledge
- Amplitude maximum displacement from the rest position, metres
- Wavelength distance from one point on a wave to the same point on the next wave, metres
- Frequency number of waves passing a point per second, hertz, Hz
- Period time for one complete wave to pass a point, seconds, T = 1/f
Wave Speed Equation
Wave speed links frequency and wavelength in a single equation.
v = f x lambda
wave speed = frequency x wavelength
Rearranging this equation for f or lambda is a standard calculation question.
Wave Speed Equation — Key Knowledge
- Wave speed distance travelled by a wave per second, m/s
Sound Waves
Sound is a longitudinal wave that requires a medium to travel through.
The classic example: sound cannot travel in space because there are no particles to vibrate.
Sound Waves — Key Knowledge
- Sound waves longitudinal, produced by vibrating objects
- Medium sound travels through solids, liquids and gases but not through a vacuum
- Human hearing range approximately 20 Hz to 20 000 Hz
Reflection, Refraction and Diffraction
Waves change behaviour when they meet boundaries or pass through gaps.
Students must explain each behaviour in terms of what happens to the wave's speed and direction.
Reflection, Refraction and Diffraction — Key Knowledge
- Reflection wave bounces off a surface — angle of incidence equals angle of reflection
- Refraction wave changes direction when it crosses a boundary between two media due to a change in speed
- Diffraction wave spreads out when it passes through a gap or around an obstacle — most noticeable when the gap is similar in size to the wavelength
The Electromagnetic Spectrum
The EM spectrum is a continuous range of transverse waves, all travelling at the same speed in a vacuum.
Memorising the order and knowing that all EM waves share the same speed in a vacuum are essential.
The Electromagnetic Spectrum — Key Knowledge
- EM spectrum order radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays — from longest wavelength/lowest frequency to shortest wavelength/highest frequency
- Speed in a vacuum 3 x 10 to the 8 m/s for all EM waves
- All EM waves are transverse
Uses of EM Waves
Each part of the EM spectrum has characteristic uses linked to its properties.
Exam questions often ask students to match a use to the correct part of the spectrum and explain why.
Uses of EM Waves — Key Knowledge
- Radio waves TV and radio communication
- Microwaves satellite communication, cooking
- Infrared thermal imaging, remote controls, fibre optics
- Visible light seeing, photography, fibre optics
- Ultraviolet fluorescent lamps, detecting forged banknotes
- X-rays medical imaging, airport security
- Gamma rays cancer treatment, sterilising medical equipment
Dangers of EM Waves
Higher frequency EM waves carry more energy and pose greater risks to living tissue.
Risk increases with frequency — UV, X-rays and gamma rays can damage or destroy cells.
Dangers of EM Waves — Key Knowledge
- Microwaves can heat body tissue internally
- Infrared skin burns
- Ultraviolet sunburn, skin cancer, eye damage
- X-rays cell damage, cancer with prolonged exposure
- Gamma rays cell damage, cancer, mutation
Refraction of Light
Light changes speed when it enters a different medium, which can cause it to change direction.
Refraction explains everyday effects like objects looking bent in water.
Refraction of Light — Key Knowledge
- Light slows down in a denser medium e.g. air to glass — bends towards the normal
- Light speeds up in a less dense medium e.g. glass to air — bends away from the normal
- Normal line perpendicular to the boundary surface at the point of incidence
Black Body Radiation
All objects emit and absorb infrared radiation. The hotter the object, the more radiation it emits.
Black body radiation explains why hot objects glow — first red, then white as temperature increases.
Black Body Radiation — Key Knowledge
- Black body a theoretical perfect absorber and emitter of all radiation
- Emission and temperature hotter objects emit more infrared radiation and at higher peak frequencies
- Earth's temperature determined by the balance between radiation absorbed from the Sun and radiation emitted into space