Waves and space · GCSE Physics
Waves
Transverse and longitudinal waves, v = fλ, the electromagnetic spectrum, reflection, refraction and sound.
Waves transfer energy, not the medium itself. Wave speed = frequency × wavelength. Learn the EM spectrum in order, then attach one use and one danger to each part you are asked about.
The important bits
What you need to know
- 1
In a transverse wave, oscillations are at right angles to the direction of energy transfer (water waves, EM waves). In a longitudinal wave, oscillations are parallel to energy transfer (sound): compressions and rarefactions.
- 2
Amplitude is the maximum displacement from the rest position. Wavelength λ is the distance from one peak to the next (or one compression to the next). Frequency f is the number of waves per second, in hertz. Period T = 1/f.
- 3
Wave speed v = fλ. It is also v = d / t for a wave travelling a measured distance. The speed of a wave is set by the medium, not by how hard you shake the source (that changes amplitude).
- 4
The electromagnetic spectrum in order of increasing frequency (decreasing wavelength): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All EM waves travel at 3 × 10⁸ m/s in vacuum.
- 5
Uses and hazards: radio — communications; microwave — cooking, satellites (heating of water in tissues); infrared — remote controls, thermal imaging (burns); UV — fluorescence, sunbeds (skin cancer); X-rays and gamma — imaging and cancer treatment (ionisation).
- 6
Reflection: angle of incidence equals angle of reflection, measured from the normal. Refraction: waves change speed and can change direction when they enter a new medium. Light slows down in glass and bends towards the normal.
- 7
Required practicals include measuring wave speed on a string or in a ripple tank, and investigating infrared radiation. Sound needs a medium; it is faster in solids than in liquids or gases because particles are closer.
Go deeper
v = fλ only works if the units match
Frequency in hertz is s⁻¹. Wavelength must be in metres, not centimetres. A radio station at 100 MHz is 1.00 × 10⁸ Hz. In air, v is 3.00 × 10⁸ m/s, so λ = v/f = 3.00 m. Students leave frequency in megahertz and get a wavelength of 3 million metres. The speed in a string depends on tension and the string, not on how fast you waggle your hand; waggling faster raises f and cuts λ so that v stays the same. That is a classic ripple-tank result. If a question gives time for a wave to travel a measured distance, use v = d/t first, then find f or λ.
Go deeper
The spectrum is an order, then a pair of facts
Memorise the order with a mnemonic you will actually write in an exam. Then, for each region the question names, give a use and a risk. Ionising radiation starts around ultraviolet and includes X-rays and gamma: they can damage DNA. Radio waves are not ionising. Higher frequency means higher energy per photon, which is why gamma is used to sterilise and also why it is dangerous. Visible light is the only part we see; infrared is the thermal radiation given off by warm objects, which links this topic to energy transfer by radiation. Do not say “radiation” as if it only meant nuclear. In refraction questions, draw the normal, mark i and r, and state whether the wave slowed down or sped up.
See the idea in action
A ripple tank produces waves of frequency 5.0 Hz. Ten wavelengths occupy 20 cm, so λ = 2.0 cm = 0.020 m. v = fλ = 5.0 × 0.020 = 0.10 m/s. If the frequency is raised to 10 Hz and the speed in the water stays 0.10 m/s, the new wavelength is 0.010 m. The waves look tighter; they do not travel twice as fast. That is the point of the practical: speed is a property of the medium here, not of the frequency you set on the dipper.
Exam technique
Turn knowledge into marks
Write v = fλ, convert MHz to Hz and cm to m, then calculate. For EM questions, state the order, then a use and a hazard. For refraction, measure from the normal, not from the surface. Sound is longitudinal; light is transverse.
Common mistakes
Do not give these marks away
- 01
Leaving frequency in kilohertz or megahertz, or wavelength in centimetres, inside v = fλ.
- 02
Saying waves transfer particles, or that sound travels through a vacuum.
- 03
Listing the EM spectrum in the wrong order, or mixing up ultraviolet hazards with microwave heating.
A sound wave has frequency 200 Hz and wavelength 1.7 m. What is its speed?
A0.0085 m/s
B118 m/s
C340 m/s
D200 m/s
Show the answer
340 m/s. v = fλ = 200 × 1.7 = 340 m/s, a typical speed of sound in air. Always multiply frequency by wavelength in metres.
Quick questions
If this is the bit you searched
What is the difference between a transverse and a longitudinal wave?
Transverse: oscillations at right angles to energy transfer (light, water). Longitudinal: oscillations parallel to energy transfer (sound), as compressions and rarefactions.
Do all electromagnetic waves travel at the same speed?
Yes, in a vacuum: 3 × 10⁸ m/s. In a medium they can slow down, which is why refraction happens.