Kimberly Mangray 5c

Wave Motion and Light Waves

Covers 1.1 – 1.4 and 5.1 – 5.4 of Section D in the syllabus

Pulse & Wavetrain

A pulse is a single, short-lived wave motion.

A wave train is a continuous group of waves with features that repeat themselves regularly.

Distinguishing Between Pulses and Wavetrains

Consider this : Picture a slinky on a table and attach one end to the table and you are holding the next end.

To produce a pulse, flick your hand once very quickly.

To produce a wave train, flick your hand several times in the same manner.

What is a wave?

A pulse is a single disturbance; when it becomes repetitive, it forms a wave.

A wave is a means of transferring energy from one point to another; these points can be in one or more materials, even within a vaccum (a region of empty space).

There are two types of waves : progressive waves and stationary waves.

Stationary/ Standing waves

This type of wave localizes or keeps the energy contained within a fixed region of space.

To illustrate : Think of plucking a guitar string. What happens to the string?

Progressive Waves

These waves continually transfer energy away from the source and do not bring it back.

To illustrate : What happens to the heat from Bar-B-Que coals?

There are 2 types : tranverse and longitudinal waves.

Transverse waves

These waves oscillate the particles of a medium perpendicular to the direction of movement by the wave.

The particles of the medium move up (the crest) or down (the trough) from their rest position in order to transmit the wave.

Example : Electromagnetic waves (wave goes through vaccum)

Longitudinal Waves

These are waves in which the particles of the medium oscillate parallel to the direction in which the wave is travelling.

The particles physically transmit the energy through contact with each other.

When the particles bunch together (the crest), compression occurs. When the particles are spaced out (the tough), rarefaction occurs.

Example : Sound Wave

Distinguishing Between Both Progressive and Longitudinal Waves

To illustrate the 2 types of waves, let’s go back to the slinky on the desk.

If you move the slinky left to right continuously, you would produce a transverse wave.

If you pulse the slinky by pushing it forward and pulling it back several times, you would produce a longitudinal wave.

Distinguishing Between Longitudinal and Transverse Waves

Graphs

Displacement-Position Graph

Graphs

Displacement-Time Graphs

Things to notice about the Graphs

Either the time or the position of a particle must be kept constant for the graph to be drawn.

From the previous graphs, a transverse and a longitudinal wave cannot be differentiated from each other as they look the same.

Wavelength

The length of a wave is the distance of one wave, which is the distance between 2 simultaneous crests or 2 simultaneously troughs.

Crest – the highest part of a wave.Trough – the lowest part of a wave.Wavelength is measured in metres and

is represented by the greek symbol λ.

What is the amplitude of a wave?

The amplitude of a wave is the maximum displacement of the particles along the wave from their equilibrium position. Its SI unit is the metre (m).

What is the frequency of a wave?

The frequency of a wave is the number of completed oscillations made in one second. Its SI unit is the hertz (Hz) or per second (s-1).

Period/ Periodic Time

This is the time it takes to generate one wave/oscillation. It takes the SI unit of time, which is the second (s) and is represented by the symbol T.

Period and frequency

From the definitions, it can be seen that the periodic time (T) of a wave is inversely proportional to its frequency (f). Therefore, the following equation can be derived :

T = 1 f = 1

f TOR

Wave velocity, wavelength and frequency

Wave velocity is the displacement travelled by the wave in a given time. The following is the derivation of its formula :

From Speed = Distance Time => v = λ (remember : f = 1 T T) => v = λ x 1 1 T => v = f λ

Phase

This is the displacement of a particle from its equilibrium or rest position.

2 points along a wave are in phase when they are displaced equally from the rest position in the same direction and move at the same velocity.

Now let’s talk about light!

Is Light a wave or a particle?

Since the seventeenth century, there has been a lively debate as to whether light is a wave or a particle. There has been several experiments to support both theories. The following slides will outline a few.

Light as a Wave

Thomas Young devised an experiment in the early nineteenth century to prove that light was a wave. The following slide show a modern day version of the experiment.

Notice that in the previous slide, this experiment could also by explained using the particle theory; but for the sake of the syllabus, it will be explained using the wave theory.

Young’s Double Slit Experiment

Light as a Wave

In this experiment, the light from a sodium bulb or the sun is diffracted through a single slit. The light is further diffracted through 2 narrow parallel slits.

The interference pattern of fringes is produced on the screen. A scale can be printed on the screen for easy measurement.

Light as a Wave

The fringes are a series of light and dark bands. The intensity of the bands decrease from the centre outwards.

The bright bands represent constructive interference, where the waves from the 2 slits arrive in phase to reinforce each other.

The dark bands represent destructive interference where the waves cancel out each other.

Light as a Wave

The fact that an interference pattern appeared on the screen was evidence enough to refer to light as a wave then.

From the experiment, the wave length of light could be calculated using the formula :

Fringe separation x slit separation source-to-screen distance So, λ = yd L

Explanations of the Experiment

Tiny slits had to be used since the pattern would hardly be noticeable if they were more than one hundredth of a millimetre in size.

Also, a single slit had to be used to diffract the light at first because unlike with water, it is near impossible to get 2 coherent bulbs (waves that are constantly in phase with each other).

Light as a Particle

However, in the twentieth century, experiments were conducted by Einstein, Davisson and Germer that showed electrons being diffracted.

This supported the idea that light was actually a particle, like an atom, that acted like a wave under certain circumstances.

Wave-Particle Duality of Light

The wave theory explained the way that light travels while the particles theory explained how light interacts with matter.

Since both theories satisfactorily explained the major features of light, scientists decided to accept both of them. They called the it the Wave-Particle Duality of Light.

Properties of Light

No matter what light is, always remember that it travels in a straight line. The following examples will prove this.

The lines represent beams of light.

Examples : Lunar Eclipse

Example : Solar Eclipse

Example : Pinhole Camera