LEARNING OBJECTIVES
What you will be able to do
- distinguish transverse and longitudinal waves
- use wave speed, frequency, wavelength and period
- explain reflection, refraction and diffraction
- compare sound and electromagnetic waves and their uses
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Describe wave behaviour and apply it to sound and the electromagnetic spectrum.
A wave transfers energy and information without a net transfer of matter. The common language of amplitude, wavelength, frequency and speed lets us compare water waves, sound and electromagnetic radiation.
Wave behaviour is predictable at boundaries and gaps, making it possible to explain echoes, communication systems and medical imaging.
SECTION 01
Wave quantities and types
Transverse vibrations are perpendicular to energy transfer; longitudinal vibrations are parallel and form compressions and rarefactions. Amplitude measures maximum displacement and is linked to transferred energy.
Frequency is waves per second and is measured in hertz. Period is time for one complete wave. Wave speed depends on the medium, while frequency is set by the source.
Radio wavelength
- A radio wave has frequency 100 MHz = 1.00 × 10⁸ Hz.
- Use electromagnetic wave speed 3.00 × 10⁸ m/s.
- λ = v/f.
Answer: Wavelength = 3.0 m.
SECTION 02
Reflection, refraction and diffraction
Reflection obeys angle of incidence = angle of reflection, measured from the normal. Refraction happens when speed changes at a boundary; wavelength changes but frequency remains constant.
Diffraction is spreading around edges or through gaps. It is greatest when gap width is similar to wavelength; longer wavelengths diffract more around the same obstacle.
| Behaviour | What changes | Key condition |
|---|---|---|
| Reflection | direction | angles measured from normal |
| Refraction | speed, direction, wavelength | crosses into a different medium |
| Diffraction | wavefront spreads | gap or obstacle comparable to wavelength |
SECTION 03
Sound
Sound is a longitudinal mechanical wave and cannot travel through a vacuum. Greater amplitude produces louder sound; greater frequency produces higher pitch. Human hearing is approximately 20 Hz to 20 kHz.
Ultrasound exceeds 20 kHz. It is used for imaging, non-destructive testing and distance measurement because reflections reveal boundaries. A microphone converts sound to an electrical signal for display on an oscilloscope.
Ultrasound depth
- A pulse returns after 160 μs = 1.60 × 10⁻⁴ s.
- Speed in tissue is 1500 m/s.
- Depth = 1500 × 1.60 × 10⁻⁴ ÷ 2.
Answer: The boundary is 0.120 m deep.
SECTION 04
Electromagnetic spectrum
All electromagnetic waves are transverse, travel at 3.0 × 10⁸ m/s in a vacuum and form a continuous spectrum. From lowest frequency to highest: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.
Uses depend on frequency and interaction with matter: radio for communication, microwaves for satellite links and heating, infrared for thermal imaging, visible for vision and fibre optics, ultraviolet for fluorescent lamps, X-rays for imaging and gamma for sterilisation. Ionising ultraviolet, X-rays and gamma can damage cells.
| Toward gamma | Change |
|---|---|
| Frequency | increases |
| Wavelength | decreases |
| Photon energy and ionising risk | increase |
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Waves transfer energy without net matter transfer.
- Use v = fλ consistently with SI units.
- Refraction changes speed and wavelength, not frequency.
- Electromagnetic waves share a nature but differ in frequency, use and hazard.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- distinguish transverse and longitudinal waves
- use wave speed, frequency, wavelength and period
- explain reflection, refraction and diffraction
- compare sound and electromagnetic waves and their uses