Waves: Physics AS LevelPage 1 Waves, Photons and Medical Physics AS Module 2 2.1Waves 2.2 Refraction...

$Page 1: Waves: Physics AS LevelPage 1 Waves, Photons and Medical Physics AS Module 2 2.1Waves 2.2 Refraction 2.4Superposition and Interference 2.5 Diffraction.$
Waves: Physics AS LevelPage 1

Waves, Photons and Medical Physics AS Module 2

2.1 Waves

2.2 Refraction

2.4 Superposition and

Interference

2.5 Diffraction

2.6 Sound


2.1 WavesYou need to be able to:

• Demonstrate a knowledge and understanding of the terms ‘transverse wave’ and ‘longitudinal wave’

• Be able to categorise waves as transverse or longitudinal

• Understand polarisation as a phenomenon associated with transverse waves

• Recall and use v = fʎ• Recall radio waves, microwaves, infrared, visible,

ultraviolet, x-rays and gamma rays as regions of the electromagnetic spectrum

• State typical wavelengths for each of these regions• Analyse graphs to obtain data on amplitude, period,

frequency, wavelength and phase


How many different types of waves are you aware of?

Sound

Light

Microwaves

X-ray

Infrared

Radio Water


What is a Progressive wave?

The sound wave produced from a tuning fork is produced from vibrations in the air molecules. These vibrations travel through the air to your ear, which then detects these vibrations as sound.

Defn: A wave that transports energy by causing vibrations in the material or medium through which it moves is called a progressive wave.

1. Is a sound wave a progressive wave?2. What is the medium through which the sound wave moves?


Two types of wavesThere are two different types of wave: Transverse and Longitudinal. What type of wave is this below?

Longitudinal: The spring is being pushed in and out, causing compression (pushed together) and rarefaction (pulled apart).

Longitudinal wave


The Longitudinal Wave

1. What direction are the vibrations moving?The vibrations are moving back and forward – the spring is not moving anywhere.

2. What direction is the wave energy moving? (called ‘the direction of Propagation) The wave energy is moving from the left to the right.

Longitudinal wave

Defn: A longitudinal wave is a wave where the vibrations of the medium are parallel to the direction of propagation

A sound wave is a longitudinal wave because the vibrations of air molecules move parallel to the direction of propagation.


Transverse Waves

Transverse wave

1. What direction are the vibrations moving this time?The vibrations are moving up and down (look at the hand)

2. What direction is the wave energy moving? (called ‘the direction of Propagation) The wave energy is moving from the left to the right.


Examples of Transverse Waves

Electromagnetic waves, such as radio and light are all transverse. Unlike mechanical waves (sound or waves), electromagnetic waves do not need a medium to transfer their energy and can travel through evacuated space at 3 x 108ms-1.

Radio waves originate from electrons oscillating in an aerial. This produces a varying electric field that is transmitted to the next part of space.

Light waves originate from transitions of electrons in atoms.

Defn: A transverse wave is a wave where the vibration of the medium is perpendicular to the direction of propagation











What is Polarisation?

Where have you heard the term ‘polarisation’ or ‘Polaroid’ before?

Both Polaroid sunglasses and cameras will reduce the glare from light using Polaroid lenses. This is process is called polarisation and is a characteristic of only transverse waves.

Can radio waves be polarised? Why/why not?

Radio waves can be polarised because they are transverse


An unpolarised waveWhat is the definition of a transverse wave?

Electromagnetic waves are all transverse. These waves consist of both electric and magnetic vibrations (hence why they are called electromagnetic). For the purposes of polarisation we just need to look at the electric vibrations:

Defn: A transverse wave is a wave where the vibration of the medium is perpendicular to the direction of propagation

This transverse light wave is vibrating in every plane perpendicular to the direction of travel e.g. the wave is travelling into the page, but vibrations are occurring at every 90o angle.

This is called an Unpolarised light wave.

Defn: A wave which is vibrating in more than one plane is called an unpolarised light wave.


How are transverse light waves polarised?

Light waves are naturally not polarised, so there is light vibrating (or oscillating) in all directions.

Light can be polarised by absorbing all the planes of polarisation except one. For example, polaroid plastic used in sunglasses will only let the oscillations that are in one direction pass through. The intensity of light into your eye is therefore reduced:

Unpolarised lightPassing through polaroid plastic

Polaroidplastic

Polarised light, which only oscillates in one direction

Important Terms: This polaroid plastic filter is called the polariser. The light which only oscillates in one plane has been ‘Plane polarised’.


The Second Filter: The Analyser

One polaroid filter will reduce the glare by producing light which only oscillates in one plane (plane polarised light). What would happen if another polaroid filter (called the Analyser) is introduced?

1. Analyser in same orientation as polariser:

At this point the light intensity after the analyser is at a maximum. It hasn’t absorbed any of the oscillations.


The Second Filter: The Analyser

3. Analyser at 90o to polariser:

At this point the light intensity will be at a minimum (zero). This is because all of the oscillations in the plane have been absorbed by the filter.

2. Rotating the analyser from 0o to 90o

What do you think will happen to the light intensity as the analyser is rotated?

As the analyser is rotated, the light intensity will slowly reduce as the angle of rotation is increased.


The Intensity/Angle Graph

As the analyser is rotated, the light intensity is reduced until it is 90o to the first filter. At this stage no light shines through the filters. As the rotation of the analyser continues, the light intensity starts to increase again until it is once again at the same orientation as the polariser.

Angle of rotation of analyser0o 90o 180o

Intensity of transmitted light

Question: If you draw a graph of Intensity of Transmitted light (y-axis) against angle of rotation of the Analyser (x-axis), what shape of graph would you obtain?. Explain your answer











The Wave equationThe wave equation helps us find the speed of a wave from its frequency and wavelength:

Velocity, v = frequency, f x wavelength, λ or v = f λ ms-1 Hz m

Question: Station Radio 4 on ‘Long Wave’ radio has a frequency of 198 kHz. What is the wavelength of the waves that arrive at your radio? The velocity of radio waves is 3 x 108ms-1.

(i) In the wave equation frequency should always be in Hz, not kHz, so 198kHz = 198 x 103Hz

(ii) Then using the wave equation: v = f λ

3 x 108 = 198 x 103 x λ

Therefore λ = 3 x 108 = 1520m (3 s.f)198 x 103






• Recall and use v = fʎ • Recall radio waves, microwaves, infrared, visible,





The Electromagnetic SpectrumVisible light waves are one part of a very wide range of radiation all of the same type called electromagnetic radiation.The picture below shows the entire electromagnetic spectrum:

Do you see that the wavelengths do not go up in equal steps, but 10x? This is an example of a logarithmic scale, with every number 10 times bigger than the next.


The Characteristics of the EM SpectrumThere are several things that all waves in the EM Spectrum have in common:• Electromagnetic waves can travel through a vacuum

• Energy is transferred by a variation of electric and magnetic fields. These field oscillations are transverse and at right angles to each other – this is why all electromagnetic waves can be polarised.

• Electromagnetic waves are delivered by photons, or ‘packets’ of wave energy. The energy of the photon is proportional to the frequency of the wave, so the higher the freq (short wavelength), the more energy in the photons e.g. γ rays have a frequency of 1021Hz and are the most dangerous!

• They all travel at the same speed, c in a vacuum: 3.0 x 108ms-1. The speed in other media depends on the wavelength of the radiation (e.g. blue light travels slower than red light in glass). As with all waves, v = fλ, but for electromagnetic waves, v = c.


More about the EM Spectrum

This gives the full electromagnetic spectrum. A couple of thingsto note:

1. Many of the types of waves in the spectrum overlap: there is no distinct dividing line between many of the types of waves.

2. As a result, the waves are characterised by their origin e.g how they are formed.

3. The detection method for each type of wave obviously depends upon how much energy they are emitting e.g you can see visible light, but have to use a Geiger tube for gamma rays.

You need to learn typical wavelength of these waves for your exam








• State typical wavelengths for each of these regions • Analyse graphs to obtain data on amplitude, period,



Representing WavesThere are two methods of representing waves, so make sure you do not get confused and mix them up:

Distance along medium

Displacement

Time

Displacement T

T

(1) With distance on the x-axis, this graph shows a snap shot of how the wave has moved from its original position.

(2) With time on the x-axis, this graph shows how the wave is moving in time.


(i) Displacement against Distance graph

This wave has equally spaced crests, C and troughs, T. The distance between any point on a wave and the next identical point on that wave is called the wavelength. The symbol for wavelength is λ (lambda), units metres(m).

The amplitude of a wave is its maximum displacement from its rest position – this occurs at a crest or trough

C C

T T

A

A

A

Displacement

Distance along medium

The displacement against distance graph provides information for us to find the (i) wavelength and (ii) amplitude of the wave:


Amplitude and Energy

Which carries more energy? A big (amplitude) wave, or a small (amplitude) wave?

AB

Clearly a bigger amplitude wave has more energy. So amplitude has something to do with how much energy is transmitted in a wave.

To calculate the potential energy = mass x g x height. Consider two waves below: If the amplitude of B is twice that of A, then the

average height of B is twice that of A. Also B has twice the mass of A. These factors increase the PE of B by 4 times.

If we make the amplitude of B 3 times that of A, this will mean the average height and mass go up 3 times, increasing the energy to a factor of 9 times. Can you see a pattern in the Amplitudes and Energies?

The energy of a wave α(amplitude)2This is true for all waves and oscillations


Phase DifferenceWhen waves have the same wavelength, and the same amplitude but they are not in step with each other, we say that these waves are out of phase.

Which of the three pictures above show two waves which are:(a) With a phase difference of 90o (b) phase difference 0 – called “in phase” or (c)with a phase difference of 180o (in anti-phase)?

The phase difference between waves is referred to by an angle difference, rather than in terms of the time period or wavelength:

• one cycle is equivalent to a phase difference of 360o (or 2π radians)• 1/2 a cycle would be referred to a phase difference of 180o (π radians)


Some Definitions……….

Mysteries in movie (properties)


(ii) Displacement against time graph

This wave is repeating itself, moving up and down. The time it takes for the wave to repeat itself is called its Periodic Time, or Period. Its symbol is T, units seconds (s).

However some waves are so fast that the period is too small to measure easily. So instead of counting the number of seconds for one oscillation, we count the number of oscillations (or cycles) in one second. This is the frequency. Its symbol is f, units Hertz (Hz).

The displacement against time graph allows us to find the periodic time and frequency of a wave:

Time

Displacement T

T


Period and FrequencyAs you can see below, as the time period, T increases, the frequency decreases and vice versa:

The frequency and period of a wave are therefore related by the equation:

Frequency, f (Hz) = 1 or f = 1/T period, T (s)








• State typical wavelengths for each of these regions • Analyse graphs to obtain data on amplitude, period,



Intensity of transmitted Light/Wm-2

Angle of rotation of Analyser/o

0 90 180

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