Chapter 13: Waves


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We are interested in waves in this course not so much for their own sake, but because we want to understand the nature of light. Is light a stream of particles or is it waves? We want to know what light is so that we can understand the matter which emits light. If atoms emit particles of light, they are likely to be much different than atoms that emit waves!

A wave
  • is a disturbance,
  • transports ,
  • is initiated by a source,
  • usually (not always) travels through an elastic .

    There are different kinds of waves. Waves may be
  • surface waves , or, if they travel through a medium rather than on its surface, waves may be
  • compression waves (sometimes called "longitudinal" waves),
  • shear waves (sometimes called "transverse" waves),
  • torsion waves (sometimes called "twist" waves).

    Waves have certain properties:
  • The of a wave is the speed of the disturbance. The speed of a wave depends primarily on the characteristics of the . The speed of sound waves, for example, (does, does not?) depend on the pitch and (does, does not?) depend on the loudness.

  • The of a repeating wave is the number of disturbances which pass a point each second. Different frequencies in sound waves are perceived as different pitches.

  • The of a repeating wave is the distance from peak to peak or from valley to valley.

  • The of a wave is a measure of the actual maximum displacement (movement) of the medium by the disturbance. The amplitude of the wave is closely associated with the amount of carried by the wave.

    Sound waves have a typical frequency of 1000 cycles per second.
    Sound waves have a typical speed of 1000 feet per second.
    Sound waves have a typical wavelength of 1 foot.

    Important rule: wave speed = (frequency) x ().

    Note, however, that if you double the frequency of a sound wave, the speed of the sound wave (does, does not?) change. Wavelength and frequency change together so that their product is equal to the wave speed which is determined by the medium alone.

    What do waves do? We are interested particularly in identifying things that waves do that a stream of particles does not do so that we can decide if light is a wave (because it does wave-like things) or a stream of particles (because it does particle-like things).

    Waves reflect (such as an echo), but so does a stream of particles.
    Waves refract. Refraction is the change of that waves make when they move from one to another.
    Waves diffract. Diffraction is the change of when it encounters an object or a slit or opening. Important Rule of Thumb: diffraction is most evident (greatest degree) when the of the wave is about the same (or larger) measure (size) as the size of the object or the opening causing the diffraction. Therefore, sound waves diffract significantly around objects that are a few (miles, feet, millimeters?) (see above for a typical wavelength of sound) in size.
    Waves interfere. Two waves that move through the same space at the same time can reinforce or cancel each other.

    Diffraction and particularly interference are things which waves do that a stream of particles neither does nor mimics. Thus, if we want to know what light is, we might ask if it causes interference patterns. Knowing what light is, we might then be closer to making a model of the atoms which create light.





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