Terms and Vocabulary Words

Wave

Transverse wave

Longitudinal wave

Hertz

Crest

Trough

Oscillation

Frequency

Wavelength

Wavespeed

Propagation

Amplitude

Standing wave

Node

Anti-node

Fundamental frequency

Interference

Superposition principle

Constructive interference

Destructive interference

Beats

Natural frequency

Forcing frequency

Resonance

Reflected wave

Refraction/dispersion

Diffraction

Doppler Effect

Polarization

Background knowledge

  • Sound is a longitudinal wave. The substance that is “waving” is the pressure of the air or the pressure of the liquid that carries the sound energy.
  • Sound energy MUST be carried by a medium. The properties of that medium determine the speed of sound.
  • For sound in air at pressure and temperature typical of California, the speed of sound is approximately 340 m/s.
  • For sound in liquid, such as water, the speed of sound is much faster than 340 m/s. This is because a liquid medium is more elastic than a gas medium.
  • Light is electromagnetic wave (EM wave). It is a transverse wave.
  • For EM waves, no “waving” medium is required. We believe that EM waves consist of “waving” electric fields. As such, this energy can exist in the vacuum of free space. The waving electric fields create waving magnetic fields that are perpendicular. And these magnetic fields’ waviness prompts further waviness in the electric field. And on it goes…
  • When the EM wave travels through the vacuum, its speed is 3.00 x 108 m/s.
  • But an EM wave CAN ALSO travel through substances that are not vacuum: air, water, glass for example.
  • When the EM wave does travel through these other media, its speed is altered. The speed gets slower than 3.00 x 108 m/s.
  • It is easy to look in a table and find the speed at which light travels inside any medium.
  • For example, the speed of light in glass is approximately 2 x 108 m/s.

Wave behaviors

Anything that can be called a WAVE does all of the following behaviors described below no matter what type of wave it is. This means that all of the behaviors below happen with light AND with sound AND with water waves AND anything else that is called a wave.

Waves Carry Energy

  • Waves carry energy and information over great distances
  • The energy travels through space while the medium of the wave does not travel anywhere.
  • Every type of wave has a velocity. The wave’s energy travels at this velocity.

v = λf and the constancy of v

  • Sounds of radically different frequencies (e.g. 100 Hz, 1000 Hz, 19,000 Hz) all have the same speed if they are in the same medium
  • Light waves of radically different frequencies (microwaves, the color green, and X-rays for example) all have the same speed if they are in the same medium.
  • Conclusion: If the frequency for a type of wave changes, then wavelength must also change but in an inverse fashion. (Exercise: prove this mathematically.)

Interference

  • ALL waves can have their pattern altered by occupying the same space with a similar wave. The addition of the two is called interference.
  • The ability to add the waves together is called the Principle of Superposition
  • If identical waves add together such that they double, this is called complete constructive interference.
  • If the waves cancel each other out, this is called complete destructive interference.
  • Complex patterns of addition can make destruction in some places and construction in other places. Examples of these complex patterns explain the following phenomena
  • Beats (two simultaneous sound waves with slightly diff. freq’s)
  • Standing waves with nodes and anti-nodes (Caused by a wave interfering with its own reflection)
  • Diffraction
  • Nodes in a concert hall (why are there places where you hear little?)
  • Dark fringes when light passes through a diffraction grating
  • Bright fringes when light passes through a diffraction grating
  • Rainbow patterns on soap bubbles, oil slicks, or certain bird feathers

Refraction

  • Very simple – when a wave passes from one medium to another, it changes speed.
  • If it crosses the boundary at an angle, the change of speed causes the wave’s direction to change
  • Different colors bend different amounts – this dispersion explains prisms

Doppler Effect

  • Explains why a siren’s frequency sounds higher as it moves toward you, lower as it moves away from you.
  • Also happens with light. A star in the galaxy moving toward you would have a color that is more ______than it normally is. If the star is moving away from you, its color would appear more ______than it normally is.

Polarization

  • Defined by the axis of the E field in an EM wave.
  • Unpolarized light is made of light that is an equal mixture of axes for the E field.
  • When unpolarized light goes through a polarizing lens, the light that makes it through the lens is only polarized along one axis. All the rest is blocked.