IB Physics/Oscillations and Waves

4.1 Kinematics of Simple Harmonic Motion

4.1.1

Describe examples of oscillations.

Simple harmonic motion is defined as...

1. When the body is displaced from equilibrium, there must exist a restoring force (a force that wants to put the body back in equilibrium)

2. This restoring force must be proportional to the displacement of the body

Examples of oscillations include:

• The motion of a mass at the end of a spring after the mass has been displaced away from its equilibrium position;
• The motion of an aeroplane wing;
• The motion of a tight guitar string that has been set in motion by plucking it.

Key IB Definitions.

Wave: Propagation of energy through a material substance.
Displacement: The distance of the oscillating object from equilibrium.
Amplitude: The maximum distance from equilibrium an oscillating particle reaches.
Frequency: The number of complete cycles of an oscillating particle per unit time.
Period: The amount of time it takes an oscillating particle to complete one cycle( an oscillation).
Phase Difference: The measure of how "in step" different particles are. If they are moving together they are said to be in phase. If not they are said to be out of phase.
Refraction: The bending of waves through materials due to a difference of speed (change in velocity).
Diffraction: The bending of waves through a small opening and around corners.
Waveray: The direction of energy propagation.
Wavefront: Series of particles in phase.
Wavelength: The distance between two successive wavefronts for travelling waves or The distance the wavefront moves in 1 cycle for travelling waves.

3 Types of waves.

Transverse Waves: The particle motion in this wave is perpendicular to the direction of energy propagation(transfer). The particles in this wave are oscillating up and down while the energy is propagated perpendicularly. It is important to note that a larger amplitude denotes greater energy. Electromagnetic waves and shallow water waves are both transverse waves.

Longitudinal Waves: The particle motion in this wave is parallel to the direction of energy transfer. It is key to note that the energy and particles move in the same direction. All sound waves are longitudinal waves. *It is important to note that sound waves graphs can look similar to transverse waves but they are always longitudinal*.

Standing Waves: Standing waves are formed when 2 waves travel towards each other(eg. incidental or reflected) with the same speed and similar amplitude as energy can be lost. The wavelength is the same and there is no net energy propagation. Standing waves have particles which remain stationary called nodes. The average speed of each particle is not the same at one cycle. The anti-node will be the fastest as it travels the farthest in one cycle. The distance between adjacent nodes or anti-nodes is half a wavelength. Microwaves are an example of standing wave. In a microwave, standing waves are established which is why a turntable is necessary.

Students must understand the equation: $f={\frac {1}{T}}$, where f is frequency and T is period.

$\lambda ={\frac {v}{f}}$
where λ is wavelength, v is the wave speed, and f is the frequency. The equation is the equivalent of that given in the data packet, which is:
$v=f\lambda$

Wave speed : The speed at which a given point on the wave is traveling through the medium (i.e. how far a particular crest travels in a second).

Crest : Relevant only for transverse waves, this is the point of highest positive displacement (i.e. upwards) from the mean position.

Trough : The point of largest negative displacement (i.e. downwards) from the mean position (in a transverse wave).

Compression : If a compression (longitudinal) wave is drawn like this || | | | | || | | | | || | | | | || , the compressions are where the bars are close together. Specifically, it is where the particles are most compressed in the wave.

Rarefaction : The opposite of a compression, i.e. where the bars (or particles) are most spread out.

Different graphs of waves.

Displacement vs time : This graph tracks the movement of a particle as a wave moves through it. With displacement on the vertical axis, and time on the horizontal. The particle will move up and down in a sine curve type pattern. The highest point is called a crest. The lowest point is called a trough.

An example of a displacement vs time graph for a wave

This type of graph allows us to find both frequency (which will be the number of crests in 1 sec) and period (which will be the time between crests). Note that the frequency and period have an inverse relationship. We can also find amplitude (the maximum distance that the wave travels) using this graph. This kind of graph tells us nothing about the wave speed or wavelength.

A demonstration of how to find the period and amplitude on a distance vs time graph

Displacement vs position : This is basically a 'snapshot' of the displacement of all the particles going through the medium at a given time. Displacement is on the vertical axis, and position (or ie distance from an arbitrary origin in the material) is on the x. The distance between peaks represents the wavelength. The wave speed can not be calculated directly from this graph, but can be found by combining the information from this and the displacement vs time graph (as described in the next section).

An example of a displacement vs position graph for a wave

4.1.5

v = f × λ (wave speed = frequency × wavelength)

This equation can be used to find the speed of a wave given it's wavelength and frequency. Deriving this is really rather obvious, but described below.

If the unit of frequency is ^cycles/_second and wavelength is ^meters/_cycle, then when the two are multiplied, cycles cancel out, and we're left with ^meters/_second, which is the unit of wave speed, and so the equation follows from the definitions of frequency and wavelength.

Note, the frequency for a given wave is constant (defined by the source) thus, if the wave speed changes (due to changing mediums) then the wavelength also changes, but frequency remains constant.

4.1.6

Electromagnetic waves are transverse waves, travelling at a wave speed of c (the speed of light = 3 × 10^{8 m}/_s) when in a vacuum (they can travel without a medium unlike all other waves).

There are different wavelengths that transverse waves travel at. The spectrum of different wavelengths have been divided into different sections. They are commonly given the following names (in order of increasing frequency and decreasing wavelength).

• Radio waves
• Microwaves
• Infra-red waves
• Visible light
• Ultraviolet waves
• X-rays
• Gamma waves

The easy way to remember this that our physics teacher taught us was "Red Monkeys In Vegas Usually X-ray Girls." This goes from lowest frequency to highest.

Going down the original list, frequency decreases and wavelength increases (because since v=λƒ and c remains constant). The amount of 'energy' in the waves decreases down the list, which is why X-rays are dangerous, and radio waves aren't.

Visible light is split into colours from violet to red, violet having the highest frequency and red having the lowest. Visible light ranges from 400nm (1 nano meter is 1 × 10^-9 meters) for violet to 700nm for red.

Electromagnetic waves are usually defined by their wavelength in a vacuum (which seems rather silly, since frequency never changes, frequency and is what defines the characteristics (i.e. color)) but who am I to argue.

However weird it may sound, a Microwave oven at Earth does not emit the same wavelength of wave as one in space. There is a difference between the two locations, albeit a small difference. This is because light travels more slowly when it travels through a medium such as air; all electromagnetic radiation is slowed to some extent by the medium it is passing through. That small difference may correspond to millions of light years in determining the distance of stars, so it is, in fact, very important to refer to vacuum values all the time just for setting a common ground for experiments.

Vacuum is chosen as the common reference point because all electromagnetic radiation, no matter what the frequency, travels at the same speed in vacuum. As mentioned, the speed of light is slower when it's traveling through something, and higher-energy radiation is slowed down less. It is only in vacuum that it all travels at the same speed no matter what the frequency or energy is.

4.2 The behavior of waves

4.2.1

During SHM, the pendulum swings from left to right and back the same way. When the pendulum reaches it's highest point or highest amplitude, it is momentarily at rest and has zero kinetic energy. The kinetic energy it previously possessed when it was moving has been converted to potential energy at this high point. When he swings back down, the potential energy is changed back to kinetic energy and this is highest at the equilibrium point. At this point, though, there is zero potential energy. This model also follows the law of conservation of energy. On a pendulum, the period is independent of mass. Period on a pendulum only changes by the length of the string and the force of gravity. By shortening the string, the time decreases and the frequency increases.

Longitudinal waves travel in one dimension, and so when they strike a boundary, they will be reflected back in the same direction, though the will experience a phase change (i.e. when a compression hits the boundary, a rarefaction is emitted back from it, and vice versa.

This also applies to standing waves travelling in a stretched string. If both ends are connected to a boundary, then nodes (points where the string doesn't move up and down) will occur at both ends, and a number of antinodes will occur through the string, separated by nodes.

In an air column, it is possible to have both open and closed boundaries. At an open boundary, and antinode will occur, while at a closed one a node will occur.

4.2.2

Whenever a wave is reflected from a boundary, the angle of reflection will equal the angle of incidence. Thus, if the wave strikes the boundary at 90°, then it will be reflected straight back, but other angles will reflect the waves away from the source.

Note that it is common for waves to travel in a full, or semicircle out from the source rather than in one line, which complicates reflection, because each wave is entering at a different angle.

Also, curved boundary's must be handled. The basic technique here is to draw in a few important lines representing different waves, see where they would reflect to, and then fill in the rest.

When waves in water strike a boundary, the crests will be reflected as troughs, the same goes for sound. Phase changes in light are a little more complex, but we'll come to those later.

4.2.3

Waves can be refracted when they move from one medium to another, and when they have different wave speeds in these two media. It is easiest to consider this as a series of wavefront lines entering a boundary at an angle. The frequency (the time between lines) must remain constant, but the speed slows (so they must become closer together). As they enter at an angle, the wavefront on one side slows down first, which effectively pulls the entire wave around towards that corner. In ray diagrams, light simply enters at one angle to the normal, and leaves the boundary at another (we get to how to find these angles next).

This phenomena can have some weird effects. When looking into a swimming pool, light from objects at the bottom is diverged (refracted away from the normal). this means that when the virtual rays are traced back, a virtual image is formed much closer to the surface than the actual object (apparent vs real depth).

4.2.4

The angles described above can be found with Snell's law.

n₁ × sin i = n₂ × sin R

n₁ is the refractive index of the initial medium (vacuum = 1, air = 1 (or close enough to 1 that it doesn't mater)), and n₂ is the index of the medium it's entering. i is the angle of incidence, and R the angle of Refraction, both of which are measured from the normal.

When light goes from a more to a less dense medium, then there comes a point where the angle of refraction will be 90°. The angle of incidence where this occurs is called the critical angle. If the angle of incidence is above the critical angle, then the light is totally internally reflected. Angle of incidence = angle of reflection applies, and the light is reflected from the boundary.

Unusual examples of this include water ripples travelling slower in shallow water, and sound travelling at different speeds through hot and cold air. Most of the time, though, the problems relate to light entering / leaving water or a glass prism.

4.2.5

When two waves are moving in the same medium, the displacements of the particles add together. It is therefore possible for two waves to produce one wave of larger amplitude, or to produce two waves where the total amplitude is zero. Note, the waves and energies are still there, it's just that the two waves are adding to zero.

Questions about this generally involve two waves traveling in opposite directions down a string (they're usually rather easy).

4.2.6

Principle of Superposition: The principle of superposition states that the resulting displacement of the interference between 2 waves is the algebraic sum of the displacement of each wave. Principle of superposition describes the combination of overlapping waves or wave interference.

Supercrest: When a crest overlaps with a crest

Supertrough: When a trough overlaps with a trough

Constructive Interference: If the result of 2 waves interfering is a greater displacement in the medium, constructive interference has occurred (eg. The formation of a supercrest).

Destructive Interference: If the result of 2 waves interfering is a smaller displacement in the medium, destructive interference has occurred (eg. an equal trough and crest interfering).

Resonance: Large amplitude vibration that is the result of a force that is applied at the same frequency as the object's natural frequency.

Natural Frequency: The frequency at which an object wants to vibrate at without external influence. The natural frequency of a human head is 7Hz.

Beats: The constructive or destructive interference of 2 waves. Beats is the change in amplitude in soundwaves. The beats can heard when the sound goes from loud to quiet to loud or when it goes from quiet to loud to quiet.

Beat Frequency: The number of beats per time or the absolute value of the change in frequencies. eg. If frequency 1 is 1012 Hz and frequency 2 is 1024 Hz, the beat frequency is 12 Hz.

Doppler Effect: The apparent change in a wave's frequency and the wavelength due to the relative motion between the source and the observer. If a wave is blue shifted, the electromagnetic waves are getting shorter. If a wave is red shifted, the electromagnetic waves are getting larger. As the source of the sound approaches me, the higher pitch frequency becomes a lower pitch frequency.

4.2.7

Young's double slit experiment is basically where two slits act as point sources, and form a diffraction pattern, thus demonstrating the wave nature of light. When light is shone onto the backs of the slits, they act as point sources which are initially exactly in phase. Light from each of these travels to a screen, striking it and producing light on it.

The issue is, however, that the light from each slit has to travel a different distance to reach the screen. When the difference between these distance is exactly N × wavelength + 1/2 wavelength of the light, the two waves will destructively interfere and produce a dark spot on the screen. When the difference is a multiple of the wavelength, the two waves arrive in phase, and produce a bright spot.

The resulting pattern is a series of bright and dark bands when monochromatic light is used. When white light is used, different colors will construct and destruct at different points, producing a series of spectra which will eventually overlap as we move away from the centre. In both cases, the center point will have a bright spot, of the original light colour.

The equation for this experiment is given in the optics section of the data book. m × λ = a sin Θ, or the order of the band × wavelength = the distance between the centres of the slits × sin of the angle of the bright band.

Since there are a series of bright bands, different values of m can be substituted. m=0 gives the central bright band, then m=1,2,3,4...give the subsequent band angles.

4.2.8

When two waves which have different frequencies interfere, beats will be heard. The beats are points where the amplitude (volume in the usual case of sound) reaches a peak. The frequency of the beats can be calculated by f_beats = |f₁-f₂|. Meaning the beat frequency will be the difference between the two frequencies.

This can be seen by drawing two sine curves, say sin x and sin 2x, then adding them. Both high and low points will be found, showing the beats (any graphing calculator will show you).

4.2.9

Diffraction of waves.

Water : When there is something blocking waves in the water, say, a log floating in it, immediately behind the log will be calm water, but eventually the waves wrap around it. This is due to diffraction, meaning as the waves go through, the motion of the particles affect those, not just in the direction of propagation, but also to the side, allowing the wave to spread to the side as well as forward. If waves were passed through a thin slit, they would form a semi circle point source, just like light in the double slit thing.

Sound : Just like water, sound can trace around obstacles, and join back up on the other side, or pass through a thin slit and form a sort of point source. This also shows that both longitudinal and transverse waves act in the same way with respect to diffraction.

Light : As was seen in the double slit experiment, light can be diffracted through a thin slit. Because it moves so fast, we tend not to notice light waves bending around corners, but it can.

4.2.10

Polarisation : Light naturally travels as a transverse wave in all planes, i.e. the 'particles' move left and right, up and down, and at every angle between. It is possible to cut out all but, say, the up and down motion. Light to which this has been done is called polarised.

(More info is in the Optics section if you think you need it.)

4.3 Standing waves

4.3.1

Standing waves occur when 2 waves travel towards each other (eg. incidental and reflected) with the same speed and similar amplitude (energy can be lost). There is no net energy propagation.

At a closed boundary, a node (particles which remain stationary) will always be formed, and a antinode will occur at an open boundary. This can occur in a stretched string, or in an air column. Standing waves in air columns can also be referred to as resonance in air columns.

In the first case where the column is opened at one end, the closed end will have a node and the open end will have an antinode. In the first harmonic of an open-end column, the distance is one-quarter wavelength. This is easy to remember when considering that the distance between a node and an antinode is one-quarter wavelength. To get the length of an tube open at one end, you continue to add one-quarter wavelength to the existing length.

In the second case where the column is open at both ends, both the open ends will have an antinode at each node. Between the two antinodes, there is a node. The length of the column will be half wavelength. This is easy to remember when considering that the distance between two antinodes is half wavelength. To get the length of a tube open at both ends, you continue to add half wavelength to the existing length.

To calculate the frequency of a tube , the following formulas can be used.

2 Ends Closed

Frequency = (Harmonic Number)(Fundamental Frequency)

1 End Closed

Frequency = (2n-1)(Fundamental Frequency)

n is the harmonic number

2 Ends Open

Length = [(2n-1)(Wavelength)] ÷ (4)

n is the harmonic number

In each case, the fundamental frequency is the wave with the longest wavelength which satisfies this (i.e. the lowest frequency). After this, harmonic frequencies can be found by adding half a cycle into the string/air column. With two closed boundaries, the fundamental wavelength will be 2 × the length, with one open, one closed, 4 × length, and with 2 open, 2 × length.

All objects have a natural frequency at which they will most naturally resonate. When a source produces such waves into this body (or medium) then it will vibrate 'in sympathy' with it. When this occurs, the amplitude of these vibrations will be at a maximum.

With two closed boundaries, the fundamental wavelength will be 2 × the length. with one open boundary and one closed, 4 × length, and with 2 open boundaries, 2 × length.

The first, second and so on harmonics can be found by adding 1/2 of a cycle in to the diagram, so with one open, one closed, we have 3/4 of a cycle in the pipe, so the wavelength is 4/3 × length. This applies to all other types (2 open and 2 closed), and allows the wavelength to be found. We can then calculate the frequency given speed or vice versa, etc.

References

• Diploma Programme Physics - Guide. Cardiff, Wales: International Baccalaureate Organization. 2007.
• Tsokos, K. A. (2008). Physics for the IB Diploma, Fifth Edition. Cardiff, Wales: Cambridge University Press. ISBN 978-0-521-70820-3 paperback. {{cite book}}: Check |isbn= value: invalid character (help)