Travelling Wave Characteristics

Objectives

State what is meant by wave motion.Distinguish between longitudinal and transverse waves.Define amplitude, wavelength, period and frequency and

state the relationship between them, State what is meant by crest and trough and identify

these on a graph.Find amplitude & period from a displacement-time graphFind amplitude & wavelength from a displacement-

position graph.Understand the meaning of the terms wavefront and ray.Use .

What is a wave?

Waves are a ‘new’ way of transferring momentum and energy without the large-scale motion of a material body. For example, light (a kind of wave) from the sun arrives on Earth having travelled a large distance in a vacuum, and upon arrival warms up the Earth. A soprano singing can break a crystal glass because energy and momentum have been transferred through the air by a sound wave.

What is a wave?

Most waves need a medium (like air, water, or glass) to propagate. These are mechanical waves, but electromagnetic waves (like visible light) do not need a medium. A wave is always associated with a disturbance of some kind. A rope held tight is horizontal when no wave is travelling on it. By moving one end up and down, we create a disturbance and individual points on the rope are now higher or lower than their original undisturbed positions.

What is a wave?

In the case of sound, the density of air becomes successively higher or lower when a sound wave travels through the air compared with when there is no sound wave. Remember, this is local, small-scale motion; the medium does not travel large distances.

What is a wave?

Definition: A wave is a disturbance that travels in a medium (which can be a vacuum in the case of electromagnetic waves) transferring energy and momentum from one place to another. The direction of energy transfer is the direction of propagation of the wave.

Transverse & Longitudinal waves

In addition to the division between mechanical and electromagnetic, waves can further be divided into two classes: transverse and longitudinal. Transverse waves propagate (transfer energy) at a 90° angle to the direction of the disturbance (example: wave on string). Longitudinal waves propagate in the same direction as the disturbance (example: sound, slinky hung from the ceiling).

Wave Pulses

You tie one end of a rope to a loop on the wall and move the other end sharply up and then back down to its starting position. You have produced a wave pulse. The direction of motion is perpendicular to the direction of the disturbance. It takes a certain time for this disturbance to move along the rope. The wave pulse travels with a certain speed down the rope.

Wave Pulses

This speed is not determined by the way in which the pulse was created (big/small/wide/narrow pulse), nor by how fast or slow your hand moved the rope; rather it is determined by the tension T in the rope and the mass per unit length () of the rope. The speed of the pulse on the string is given by.

Wave Pulses

This comes with the important revelation that the speed of the wave is determined by the properties of the medium and not by how the wave is created. The statement that the speed is independent of the amplitude is true provided that the amplitude of the pulse is not too big compared to the length of the string.

Travelling waves

Travelling waves are a series of wave pulses. If, in addition, the agent forcing the rope up and down executes simple harmonic motion, then the wave will look like a sine wave (also called a harmonic wave). If the sequence of pulses produced are square pulses, then the wave generated is a travelling square wave. Harmonic waves are important because any periodic disturbance can be expressed as a sum (superposition) of a number of harmonic waves. This is a general theorem in mathematics known as Fourier’s theorem.

Harmonic waves

A simple way of producing harmonic waves is to attach one end of a rope to a tuning fork. If the tuning fork is then made to oscillate, one full wave will be produced on the rope after a time equal to the period of the tuning fork. The length of a full wave is called a wavelength, λ, and the time needed to produce on full wave (or the duration of a full wave) is the period, T.

Harmonic waves

It follows that, since the wave moves forward a distance equal to a wavelength in a time equal to one period, the speed of the wave is given by Since one full wave is produced in a time of T seconds, it follows that the number of full waves produced in 1 s is . This is the frequency. [definition: the number of ful waves produced in 1s is called the frequency of the wave, . The unit of frequency is the inverse second, which is given the special name hertz (Hz). In terms of frequency the wave speed is .

Graphing waves

Waves can be represented graphically. This is a bit complicated because a wave depends on distance (where along the wave are we looking?) as well as time (at what time are we looking at the wave?). First, we have to decide how we will quantify the ‘disturbance’ of the wave. For a wave on a string, the obvious choice is to measure the height of a point on the string above or below the undisturbed position of the string. The disturbance here is thus the displacement of a point on the string and is measured in units of length. We normally denote this displacement by y, which will be a function of distance (x) and time (t).

Graphing sound waves

In the case of sound, the disturbance may be associated with the density of the medium trough which sound propagates. We may then define the difference as the displacement of density (ρ) relative to the equilibrium density (ρ0) of the medium when no sound is present in it. The displacement here has units of density and is also a function of position and time. In the case of sound, we could equally well define displacement as the difference, which is the difference between the pressure of the medium when sound is present (p) and the equilibrium pressure (p0) when no sound is present. Displacement would then have units of pressure.

Graphing waves

This discussion can be generalized to all waves. All waves have a displacement that is the difference of some quantity (position, density, pressure, etc.) and the equilibrium value of that quantity when no wave is present. The displacement of any wave is a function of position and time. We may therefore represent waves in graphs of displacement v. position (distance) and graphs of displacement vs. time. Graphs showing the variation of displacement with position enable us to determine the wavelength of the wave as the distance from one peak to the next peak.

Graphing waves

Graphs showing the variation of displacement with time enable us to determine the period of the wave as the time from one peak to the next peak. The amplitude of a wave is defined to be the maximum displacement of the wave away from the position when no wave is present. Wavelength, frequency, and amplitude are characteristics of a wave. Crests are points on the wave with the maximum displacement, while troughs have minimum displacement.

Graphing waves

Example:

A radio station emits at a frequency of 90.8 MHz What is the wavelength of the waves emitted? (Hint: the speed of light is .

Example

A sound wave of frequency 450 Hz is emitted from A and travels towards B, a distance of 150 m. Take the speed of sound to be 341 m/s. How many wavelengths fit in the distance from A to B?

Example:

Thunder is heard 3s after the flash of lightning. How far away is the place where the lightning struck? (Take the speed of sound to be 340 m/s).

Wavefronts & Rays

A wavefront is a surface through crests and normal to the direction of propagation of the wave. Lines in the direction of propagation of the wave (and hence normal to the wavefronts) are called rays.