Module 6: Waves

A wave is a repeated vibration of a substance from its rest or equilibrium position that carries energy.  The substance that is vibrating is called the medium. The substance that is vibrating and carrying the wave is called the medium. The medium does not move with the wave, but vibrates around a fixed location. When the medium is vibrating perpendicular to the direction of energy transport, the wave is transverse. When the medium is vibrating parallel to the direction of energy transport the wave is longitudinal. The maximum positive displacement from rest is called a crest and the maximum negative displacement from rest is called a trough.

All waves can be characterized by their frequency, wavelength, and amplitude. Frequency is measured in Hertz and is a measure of the rate of vibration of the source of the waves. The wavelength is a measure of the length of one complete wave, usually measured from crest to crest. The speed of a wave is calculated by multiplying the frequency by the wavelength. The amplitude is the maximum displacement of the medium from rest.  The energy carried by a wave is a function of its amplitude.

Waves interact in a variety of ways. 

When waves encounter a barrier, they will often reflect. The angle of reflection will equal the angle of incidence. Waves can also pass around the edge of a barrier or through an opening, resulting in the wave spreading out in a phenomenon known as diffraction.

When two waves overlap in a medium, they interfere. Interference can be either constructive, when two crests align, resulting in an increased amplitude, or destructive, when a crest and a trough align, resulting in a decreased amplitude. When two sources produce waves that spread out in two or more dimensions, point source interference occurs. A pattern of alternating high and low intensity will be produced.

When waves pass from one medium to another, they will often change speed. When this happens, the frequency remains the same, as it depends on the source of the disturbance; therefore, the wavelength will change, becoming shorter if the wave decreases in speed and longer if there is an increase in speed in the new medium. If the wave hits the new medium at an angle, the wave will change direction as well in a process called refraction.

Sound is a longitudinal wave.  A vibrating object, such as a tuning fork, creates variations in pressure that are transmitted through the air and perceived by our ears as a musical note.   The frequency of the wave is perceived as the pitch, with high frequencies being high notes and low frequencies low notes.  The amplitude of the wave is perceived as intensity or loudness.

The size and shape of musical instruments cause them to resonate at certain frequencies that are associated with certain standing wave patterns and produce specific notes.  These standing wave patterns are multiples of half or quarter wavelengths, and the combination of these various modes of vibration produce the specific timbre associated with individual instruments.

When the source producing a sound and the observer listening to the sound are moving with respect to each other, the Doppler effect occurs.  If the source and listener are approaching, the listener perceives a higher frequency and if they are receding from one another, a lower frequency is perceived.

Light has wave properties because it interferes, diffracts, and exhibits the Doppler effect, just like other types of waves. Light can be polarized, demonstrating that it is a transverse wave that oscillates in many directions.

Visible light is just a narrow part of the electromagnetic spectrum that ranges from very high frequency (gamma waves) to very low frequency (radio waves). Different portions of the electromagnetic spectrum have different uses and names. From highest to lowest frequency, these are gamma rays, x-rays, ultraviolet, visible, infrared, microwave and radio.