Core ideas and wave types

• A travelling wave is a disturbance that transfers energy from one place to another.

• In a travelling wave, there is no resultant transfer of matter: particles of the medium oscillate about equilibrium.

• Transverse waves: particle oscillation is perpendicular to the direction of wave travel.

• Longitudinal waves: particle oscillation is parallel to the direction of wave travel.

• Know the difference between particle motion and wave propagation.

• Mechanical waves require a medium.

• Electromagnetic waves do not require a medium and can travel through a vacuum.

• Exam trap: the wave moves on, but individual particles usually only vibrate locally.

This diagram compares a transverse wave on a string with the direction of energy transfer. It is useful for distinguishing particle motion from wave direction, a common exam skill. Use it to explain why particles do not travel with the wave. Source

Wave quantities and equations

• Wavelength $\lambda$: distance between two successive points in phase.

• Frequency $f$: number of oscillations per second.

• Time period $T$: time for one complete oscillation.

• Wave speed $v$: speed at which the disturbance travels.

• Fundamental relationship: $v = f\lambda = \frac{\lambda}{T}$.

• Also remember: $f = \frac{1}{T}$.

• If frequency stays constant and the medium changes, then speed and wavelength change together.

• For a given source, frequency is set by the source.

• Rearrangement skills required: $\lambda = \frac{v}{f}$, $f = \frac{v}{\lambda}$, $T = \frac{1}{f}$.

• Unit check: $v$ in m s$^{-1}$, $\lambda$ in m, $f$ in Hz, $T$ in s.

This diagram shows the standard labelled form of a transverse travelling wave. It is ideal for identifying wavelength as a distance in space, not time. It also helps with graph interpretation questions. Source

Particle motion in transverse and longitudinal waves

• In a transverse wave, particles move up and down while the wave travels horizontally.

• In a longitudinal wave, particles move back and forth parallel to the wave direction.

• Longitudinal waves have compressions and rarefactions instead of crests and troughs.

• One wavelength in a longitudinal wave is the distance between two adjacent compressions or two adjacent rarefactions.

• Be able to describe particle motion at one point over time and along the wave at one instant.

• A particle in the medium performs oscillatory motion; it does not travel with the whole wave.

• Questions may ask for displacement with respect to position or with respect to time.

This image shows how a longitudinal wave is represented in a medium such as a slinky. It is especially useful for identifying compression, rarefaction, and wavelength in non-transverse diagrams. This directly matches common IB interpretation questions. Source

Sound waves

• Sound waves are mechanical longitudinal waves.

• Sound requires a material medium such as a solid, liquid, or gas.

• Sound cannot travel through a vacuum.

• Sound travels as regions of compression and rarefaction in the medium.

• In sound, particles of the medium oscillate parallel to the direction of energy transfer.

• The wave carries energy, but the particles show only local oscillations.

• In exam questions, relate longitudinal motion to pressure/density variations in the medium.

Electromagnetic waves

• Electromagnetic waves are not mechanical: they do not require a medium.

• They can travel through vacuum.

• They are part of the electromagnetic spectrum.

• You should recognize the approximate order of the spectrum: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.

• All electromagnetic waves are the same type of wave but differ in wavelength and frequency.

• Shorter wavelength means higher frequency.

• For electromagnetic waves in vacuum, the wave speed is the speed of light.

• IB questions often test ordering by wavelength/frequency rather than detailed memorization of exact values.

This diagram shows the full electromagnetic spectrum arranged by wavelength and frequency. It is useful for comparing the major regions and locating visible light within the spectrum. This supports ordering and classification questions in IB Physics. Source

Mechanical vs electromagnetic waves

• Mechanical waves need a medium; electromagnetic waves do not.

• Sound is a mechanical longitudinal wave.

• Light and the rest of the electromagnetic spectrum are electromagnetic waves.

• Both mechanical and electromagnetic waves can transfer energy.

• Both can be described using $\lambda$, $f$, $T$, and $v$.

• A change in medium can change wave speed and therefore wavelength.

• For a wave crossing into a new medium, the frequency remains the same because it is fixed by the source.

• Be ready to explain why sound cannot travel in space but light can.

Common exam traps

• Do not confuse wave speed with particle speed.

• Do not say the medium is carried along with the wave.

• Do not confuse wavelength with amplitude.

• Do not forget that frequency is measured in Hz and time period in s.

• In a new medium, the frequency usually stays constant while speed and wavelength may change.

• Sound is longitudinal, not transverse.

• Electromagnetic waves can travel in vacuum; mechanical waves cannot.