Waves

Waves

1. General Properties

   • Waves transfer energy and information without transferring matter
   • Transverse waves
     • oscillations are perpendicular to the direction of energy transfer
     • electromagnetic waves are transverse
     • mechanical waves are both
   • longitudinal waves
     • oscillations are parallel to the direction of energy transfer.
     • Show areas of compression and rarefaction
     • sound waves are longitudinal
   • Waves can be reflected, refracted or diffracted
     • for appreciable diffraction to take place, the wavelength of the wave must be of the same order of magnitude as the size of the obstacle or gap
   • when identical sets of waves overlap, they interfere with each other

2. Key Terminology

   • Frequency; how many waves passing a certain point at a certain time
     • determines the pitch of a sound wave
   • wavelength; distance of a wave from trough to trough
   • time period; time needed for one complete cycle of vibration to pass a given point
   • amplitude; maximum disturbance from undisturbed position
     • determines the loudness of a sound wave

3. Electromagnetic spectrum

   • electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same speed through a vacuum (space).
   • waves for communication are radio waves, microwaves, infrared and visible light
   • Radiowaves (television, radio, bluetooth), microwaves (mobile phones, satellite), infrared (remote controls), visible light (photography), ultraviolet (security marking), x rays (medical imaging), gamma rays (sterilising surgical instruments and killing harmful bacteria in food)
   • radio waves; lowest energy, lowest frequency, longest wavelength)
   • wavelengths range from 10 -15 to 10 4 m and beyond
   • hazards of exposure; microwaves heat body tissue, infrared burns skin, ultraviolet causes skin cancer and blindness, x rays in high doses kill cells and gamma rays cause genetic mutations

4. X rays

   • x rays are part of the electromagnetic spectrum, they have a short wavelength, high energy and cause ionisation
   • properties; affect photographic film in the same way as light, absorbed by metal and bone and are transmitted by healthy tissue.
   • x rays can diagnose medical conditions e.g. computed tomography scanning, bone fractures and dental problems.
   • They can also kill some cancer cells

5. sound and ultrasound

   • sound waves are longitudinal waves and cause vibrations in a medium, which are detected as sound
   • the range of human hearing is 20Hz to 20 000Hz
   • sound waves can be reflected (echoes) or diffracted
   • ultrasound is acoustic energy, in the form of waves with a frequency above the human hearing range
     • electronic systems can be used to produce ultrasound waves which have a higher frequency than the upper limit of hearing for humans
     • ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to detect how far away a boundary is
     • ultrasounds can be used in medicine e.g. pre-natal scanning and the removal of kidney stones.

6. Reflection

   • when waves are reflected the angle of incidence is equal to the angle of reflection
   • the normal is a construction line perpendicular to the reflecting surface at the point of incidence
   • the image produced in a plane mirror will be virtual, upright and laterally inverted