C.2 Wave modelIB Physics HL: Revision notes
Section 1
Transverse and longitudinal waves
A travelling wave transfers energy (and information) without transferring matter.
- In a transverse wave the oscillations are perpendicular to the direction of energy transfer (waves on a string, S-waves, all electromagnetic waves).
- In a longitudinal wave the oscillations are parallel to the direction of energy transfer, producing compressions and rarefactions (sound, P-waves, push–pull waves on a slinky).
Section 2
Describing waves: v = fλ
- Wavelength λ: distance between two adjacent points in phase (crest to crest, compression to compression).
- Frequency f: number of oscillations per second.
- Time period T = 1/f.
- Wave speed v: speed at which energy travels.
v = fλ = λ/T
The speed is set by the medium; the frequency is set by the source. When frequency changes in the same medium, wavelength changes so that v stays constant.
Saying a higher frequency makes a wave travel faster. In a given medium, v is fixed; a higher f just gives a shorter λ.
Section 3
The nature of sound
Sound is a longitudinal mechanical wave: particles of the medium oscillate back and forth along the direction of travel, creating regions of higher pressure (compressions) and lower pressure (rarefactions). It needs a medium and cannot travel through a vacuum. It travels faster in solids and liquids (closely packed, strongly bonded particles) than in gases, typically about 340 m s⁻¹ in air, 1500 m s⁻¹ in water and 5000 m s⁻¹ in steel. Humans hear roughly 20 Hz to 20 kHz.
Section 4
The nature of electromagnetic waves
Electromagnetic (EM) waves are transverse oscillations of electric and magnetic fields, perpendicular to each other and to the direction of travel. They need no medium and all travel at c = 3.00 × 10⁸ m s⁻¹ in a vacuum. The EM spectrum, in order of increasing frequency: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays. Wavelength decreases in the same order.
Section 5
Mechanical versus electromagnetic waves
- Medium: mechanical waves need one; EM waves do not.
- What oscillates: particles of the medium; electric and magnetic fields.
- Type: mechanical waves can be transverse or longitudinal; EM waves are transverse only.
- Speed: mechanical waves depend on the medium (m s⁻¹ to km s⁻¹); EM waves travel at c in a vacuum, slightly less in matter.
Both transfer energy, and both show reflection, refraction and diffraction.
When a question compares, give both sides of each difference: 'sound needs a medium, whereas light does not'.
That's the notes covered.
Carry on to the next subtopic.