Transverse and longitudinal waves: the differences

Move one end of a slinky side to side: a transverse wave travels along it. Push and pull the end back and forth along its length: you see bunched-up regions (compressions) and spread-out regions (rarefactions) travel along, a longitudinal wave.

• Amplitude: maximum displacement from the rest position.
• Wavelength (λ): distance from one point to the same point on the next wave (peak to peak, or compression to compression).
• Frequency (f): number of waves passing a point each second, in hertz (Hz).
• Period (T): time for one wave, $T = \tfrac{1}{f}$.

The wave equation links them: $v = f\lambda$. Try our wave speed calculator or read the wave equation guide.

Earthquakes produce both types. P-waves are longitudinal and can travel through solids and liquids. S-waves are transverse and can only travel through solids. Because S-waves don't reach the opposite side of the Earth, scientists deduced that part of the Earth's core is liquid.

Sound is a longitudinal wave of vibrating particles, so it needs a medium and can't travel through a vacuum; it moves at about 330 m/s in air. Light is a transverse electromagnetic wave, needs no medium, and travels at $3 \times 10^8$ m/s in a vacuum, which is why we see lightning before we hear thunder.

• Saying waves transfer matter (they transfer energy; particles oscillate in place).
• Calling sound transverse.
• Measuring wavelength from peak to trough (that's half a wavelength).

Practise with our wave speed questions.

Electromagnetic waves are all transverse and travel at the same speed in a vacuum. In order of increasing frequency (and decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Higher-frequency waves carry more energy, which is why ultraviolet, X-rays and gamma rays can be harmful to living cells.

1. A wave has a frequency of 50 Hz and a wavelength of 0.4 m. Find its speed.
2. Sound travels at 330 m/s in air. Find the wavelength of a 660 Hz note.
3. Find the period of a wave with frequency 50 Hz.

Answers: 1. $v = 50 \times 0.4 = 20$ m/s. 2. $\lambda = 330 \div 660 = 0.5$ m. 3. $T = 1 \div 50 = 0.02$ s.

All waves can be reflected (bounce off a surface, like an echo or a mirror image) and refracted (change direction when they enter a different material at an angle, because their speed changes). Refraction is why a straw looks bent in a glass of water. Waves can also be absorbed or transmitted, depending on the material and the wavelength.

When asked to describe a wave type, always say what oscillates and in which direction compared with the energy transfer. Label wavelength and amplitude carefully on diagrams: amplitude is measured from the rest position to a peak, not from peak to trough.