5A. Transverse WavesTransverse waves are the type of wave you usually think of when you imagine a wave. The motion of the material constituting the wave is up and down so that as the wave moves forward the material moves perpendicular (or transverse) to the direction the wave moves.
5B. Longitudinal WavesLongitudinal waves are waves where the motion of the material in the wave is back and forth in the same direction that the wave moves. Longitudinal waves are sometimes called compressional waves. Sound waves (in air and in solids) are examples of longitudinal waves.
5C. Torsional WavesIf you stretch a slinky out between two points and gently twist it at one end, the twist will travel down the slinky as a wave pulse. This is an example of a torsional wave.
5D. AntennaThis simulation shows an oscillating electron in a sending antenna on the left. Because electrons have an electric field, an accelerating electron will create a wave in the electric field around it. Magnetic fields are created by moving charges so a magnetic wave is also formed by an accelerating charge. Only the y-component of the change in the electric field is shown (so an oscillation frequency of zero will show nothing, because there is only a constant electric field).
6. Wave SpeedThere are two different speeds involved with describing a wave. In previous chapters we saw that the individual points on a wave oscillate (up and down for transverse waves, back and forth for longitudinal waves) with simple harmonic motion, just like masses on springs. But the up and down speed of a point on a transverse wave doesn't tell us how fast the wave moves from one place to the next. The wave speed, v, is how fast the wave travels and is determined by the properties of the medium.
7A. Mirrors and RelfectionIn many cases waves of all types will travel in a straight line, reflecting off of objects and surfaces at the same angle that they strike the surface. This is called the law of reflection and is true for sound waves as well as light as long as the surface is smooth relative to the wavelength.
7B. RefractionA wave that changes speed as it crosses the boundary of between two materials will also change direction if it crosses the boundary at an angle other than perpendicular. This is because the part of the wavefront that gets to the boundary first, slows down first. The bending of a wave due to changes in speed as it crosses a boundary is called refraction.
7C. LensesA wave that passes all the way through a piece of material with parallel sides leaves the material at the same angle that it entered. The wave un-bends when it exits the material by the same amount that it bent when entering but this is only true if the sides of the material are parallel. Convex and concave lenses have sides that are not parallel (except near the center). In this case parallel rays of light end up exiting in different directions.
7D. DispersionThe speed of a wave can depend on the frequency of the wave, a phenomenon known as dispersion. Although this effect is often small. it is easy to observe with a prism.
7E. Adding Wave PulsesWhen two waves of the same type come together it is usually the case that their amplitudes add. So two overlapping water waves have an amplitude that is twice as high as the amplitude of the individual waves. This is called constructive interference and it can occur for sinusoidal waves as well as pulses.
7F. Adding Sinusoidal WavesSinusoidal waves have the property, called superposition, that their amplitudes add linearly if they arrive at the same point at the same time. This gives rise to several interesting phenomena in nature which we will investigate in this and the next few simulations.
7G. Path DifferenceIf two sources of waves are in phase to start with, when they reach a distant location they may be in-phase (leading to constructive interference) or out-of-phase (leading to destructive interference) depending on slight differences in the distance traveled. This path difference gives rise to many interesting phenomena such as interference patterns (in the case of light) and dead spots in auditoriums (in the case of sound).
7H. InterferenceThis simulation shows a top view of waves interfering on the surface of a tank of water (imagine tapping the surface of a pond with the end of a stick at regular intervals). The white circles coming from the spot represents the wave crests with troughs in between. Two sources can be seen at the same time and the separation between them and the wavelength of both can be adjusted.
7I. DiffractionSometimes waves don't travel in a straight line, even if their speed does not change (as in the case of refraction). For example, you can hear the conversation in the next room even though you cannot see the source. This is because sound waves undergo diffraction, bending as they go through the doorway between the two rooms.
7J. Doppler ShiftIf the wave source or receiver is moving, the waves will appear to have a different frequency. For example if you are moving towards a sound source you catch up with the next peak in the wave sooner than you would expect because you are moving towards it. This effect is called the Doppler Shift and occurs for both light and sound.