PHYSICS TUTORIAL NOTES

The World Communicates

SYLLABUS TOPIC 8.2

These notes are meant as a guide only and are designed to focus your thoughts on the dot points mentioned in the syllabus. They give a very brief overview of the topic and should be used in conjunction with your class notes, your textbooks and your research from other areas such as the library and the Internet.

Notes compiled by:

CARESA EDUCATION SERVICES

THE WORLD COMMUNICATES

1.      The wave model can be used to explain how current technologies transfer information.

Describe the energy transformations required in one of the following:

-          mobile telephone

-          fax/modem

http://www.vodafone.com.mt/page/phonesandmore_localcoverage_howdomobile.html This is a spiel for Vodafone but contains some useful information

Describe waves as a transfer of energy disturbance that may occur in one, two or three dimensions, depending on the nature of the wave and the medium.

WAVES:        A wave is a travelling vibration that transmits energy.

In longitudinal (compression) waves the vibrations are in one dimension, to and fro in the direction of propagation of the wave.

In transverse waves the vibrations are in two dimensions with the vibrations being at right angles to the direction of propagation of the wave.

Torsion waves are in three dimensions and represent a twisting and untwisting motion.

Identify that mechanical waves require a medium for propagation while electromagnetic waves do not.

Most waves are mechanical. In these cases the particles oscillate about fixed points in simple harmonic motion. While doing so they transmit energy to the particles next to them so that they in turn oscillate with simple harmonic motion. In this way the energy of the wave is transmitted but the matter is not.

Electromagnetic waves consist of electric and magnetic fields oscillating at right angles to each other. Since it is not particles that are vibrating, these waves do not require a material medium in which to be transmitted and hence can travel through a vacuum.

Define and apply the following terms to the wave model: medium, displacement, amplitude, period, compression, rarefaction, crest, trough, transverse waves, longitudinal waves, frequency, wavelength, velocity.

The MEDIUM is the type of substance that the wave travels through.

The DISPLACEMENT of a particle or field in a wave refers to its distance from the mean position with its sense being stated as either positive or negative,

The maximum displacement of a particle from its mean position is called the AMPLITUDE, (A) of the wave.

The PERIOD, (T) of a wave is the time taken for the particle or the electric and magnetic field to undergo one complete vibration. It is the time taken for one wavelength to pass a given point.

In LONGITUDINAL WAVES i.e. COMPRESSION WAVES the particles vibrate to and fro in the direction of transmission of the wave i.e. to and fro in the direction of energy flow. A longitudinal wave consists of a series of high pressure regions where the particles are close together (COMPRESSIONS) alternating with a series of low pressure regions where the particles are further apart (RAREFACTIONS). The wavelength of a longitudinal wave is the distance between two successive compressions or two successive rarefactions.

In TRANSVERSE WAVES the particles or electric and magnetic fields vibrate at right angles to the direction of transmission of the wave.

A CREST of a wave is a position of maximum positive displacement of the wave particle or field while a TROUGH is a position of maximum negative displacement of the wave particle or field.

The number of complete oscillations of a particle in unit time is called the FREQUENCY, (f) of the wave. If the second is taken as the unit of time then the unit of frequency is the HERTZ, (Hz) which is one vibration per second. The frequency may also be taken as the number of complete wavelengths that pass a given point in unit time.

The frequency is the reciprocal of the period: f = 1/T and T = 1/f

The WAVELENGTH, (l) of a wave is the distance between two successive crests or two successive troughs of the wave.

The VELOCITY, (v) of a wave refers to the displacement at which the wave is transmitted in a given time i.e. the velocity at which it travels. It is equal to the product of the length of the wave (wavelength) and the number of waves per unit time (frequency).

 v = f l

Describe the relationship between particle motion and the direction of energy propagation in transverse and longitudinal waves

In TRANSVERSE WAVES the particles (or electric and magnetic fields) vibrate at right angles to the direction of transmission of the wave i.e. at right angles to the direction of energy flow.

In LONGITUDINAL WAVES i.e. COMPRESSION WAVES the particles vibrate to and fro in the direction of transmission of the wave i.e. to and fro in the direction of energy flow. A longitudinal wave consists of a series of high pressure regions where the particles are close together (COMPRESSIONS) alternating with a series of low pressure regions where the particles are further apart (RAREFACTIONS). The wavelength of a longitudinal wave is the distance between two successive compressions or two successive rarefactions.

Quantify the relationship between velocity, frequency and wavelength for a wave: v = fl

The VELOCITY, (v) of a wave refers to the displacement at which the wave is transmitted in a given time i.e. the velocity at which it travels. It is equal to the product of the length of the wave (wavelength) and the number of waves per unit time (frequency).

 v = f l

Displacement (s)                                                                     Crest Trough

Perform a first-hand investigation to observe and gather information about the transmission of waves in:

-          water surfaces

-          ropes

or appropriate computer simulations.

Present diagrammatic information about transverse and longitudinal waves, direction of particle movement and the direction of propagation.

Perform a first-hand investigation to gather information about the frequency and amplitude of waves using an oscilloscope or electronic data-logging equipment

Present and analyse information from displacement-time graphs for transverse wave motion

Plan, choose equipment for and perform a first-hand investigation to gather information to identify the relationship between the frequency and wavelength of a sound traveling at a constant velocity

Solve problems and analyse information by applying the mathematical model of v = fl to a range of situations

Q.1.     A wave travels at 500 ms-1 at a frequency of 50 Hz. What is its wavelength?

(10 m)

Q.2.     The wavelength of the note emitted by a 512 Hz tuning fork was found to be   64.0 cm. What was the velocity of sound under these conditions.

(328 ms-1)

Q.3.     What is the velocity of the radio signal sent out by station Triple M?

If the frequency of its signal is 104.9 MHz, what is the wavelength of its signal?

(3 x 108 ms-1, 2.86m)

Q.4.     Radio station 2KA broadcasts at two different frequencies; (i) 783 kHz and (ii) 1476 kHz. What are the respective wavelengths of these signals?

( (i) 383m, (ii) 203m) )

Q.5.     What is the wavelength of light of the following colours, given their frequencies.

(i) Blue, f = 7.0 x 1014 Hz.      (4.3 x 10-7m)

(ii) Green, f = 5.5 x 1014 Hz    (5.5 x 10-7m)

(iii) Yellow, f = 5.2 x 1014 Hz (5.8 x 10-7m)

(iv) Red, f = 4.0 x 1014 Hz.     (7.5 x 10-7m)

Q.6.     The wavelength of a beam of gamma rays was found to be 6.0 x 10-12 m. What was the frequency of these rays?

(5.0 x 1019 Hz)

1. Features of a wave model can be used to account for the properties of sound.

Identify that sound waves are vibrations or oscillations of particles in a medium.

Sound waves are compression waves and require a medium to be transmitted. Their speed increases as the density of the medium increases. They travel fastest through solid, slower through liquids and slowest through gases. In gases they travel faster through gases with light molecules (e.g. Hydrogen) than gases with heavy molecules (e.g. Carbon Dioxide) but for the same gas their speed will increase as the temperature of the gas increases. This is because the molecules of a hot gas move faster than the molecules of a cold gas. Sound will not travel through a vacuum.

Relate compressions and rarefactions of sound waves to the crests and troughs of transverse waves used to represent them.

LONGITUDINAL WAVES

The particles in a longitudinal (compression) wave oscillate to and fro in the direction of propagation. This results in the particles being alternately bunched up (compression) and spread apart (rarefaction).

The first diagram below represents 17 particles at their equilibrium positions. The particles, which are evenly spaced, are numbered 1 to 17.

The second diagram below represents the same particles as a disturbance passes through them forming a progressive wave. It can be seen that some particles have moved to the right of their equilibrium positions while others have moved to the left.

Diagram 1. Diagram 2.

Take the displacements to the right of the equilibrium position as positive and those to the left as negative.  In the table below, list the displacement of particles 1 to 17 as shown in diagram 2.

 Particle 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Displacement

Plot the displacements on the axes below taking positive displacements above the axis and negative displacements below the axis. Draw a line through the points to represent the wave. Mark the wavelength and amplitude on the wave you have just plotted.

Mark the wavelength and amplitude on diagram 2.

Comment on the similarities between the two.

Explain qualitatively that pitch is related to frequency and volume to amplitude of sound waves

PITCH is a subjective quality of a sound by which it is described as high or low depending on its position on a musical scale. However pitch is related to frequency and a note with a high pitch has a high frequency while a note with a low pitch has a low frequency.

A loud sound carries more energy than a soft sound i.e. it has a higher intensity. This means that the higher the VOLUME of the sound the greater is the amplitude of the wave.

Explain an echo as a reflection of a sound wave

When sound hits a surface it can either be reflected or absorbed. A reflected sound is an ECHO. The distance between the reflecting surface and the person has to be at least 20 metres otherwise the time interval between the original and reflected sound is too small for the ear to distinguish them as separate. Sound tends to be reflected from hard surfaces and absorbed by soft surfaces.

Describe the principle of superposition and compare the resulting waves to the original waves in sound

SUPERPOSITION:

If two or more waves reach the same point at the same time a compound wave is formed that is the sum of the individual waves.

If two waves travel towards each other they form a compound wave as they pass through each other and resume their original form after they have passed through. Two waves approach each other from opposite directions. The two crests are at the same point at the same time. The waves have passed through each other.

Perform a first-hand investigation and gather information to analyse sound waves from a variety of sources using the Cathode Rat Oscilloscope (CRO) or an alternate computer technology

Perform a first-hand investigation, gather, process and present information using a CRO or computer to demonstrate the principle of superposition for two waves traveling in the same medium.

Present graphical information, solve problems and analyse information involving superposition of sound waves.

Exercise:

Each of the four diagrams below show two pulses moving towards each other at 1.0 cms-1.

Show the wave pattern formed by each pair of pulses 0.5s, 1.0s, 1.5s and 2.0s after the instant shown. SUPERPOSITION SOLUTIONS 1. Recent technological developments have allowed greater use of the electromagnetic spectrum.

Describe electromagnetic waves in terms of their speed in space and their lack of requirement of a medium for propagation

Electromagnetic waves consist of electric and magnetic fields oscillating at right angles to each other. They consist of gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radio waves. All travel at the same speed ( 3.0 x 108 ms-1) through a vacuum. Since it is electric and magnetic fields rather than particles that are vibrating, they do not need a medium to travel through.

Identify the electromagnetic wavebands filtered out by the atmosphere, especially UV, X-rays and gamma rays.

All electromagnetic waves are filtered out by the atmosphere to a certain extent however virtually none of the waves with wavelengths shorter than about 300 nm will penetrate sufficiently to reach the Earth’s surface. These wavelengths include gamma rays, X-rays and most of the ultraviolet rays. Medium to low frequency ultraviolet radiation is absorbed by the ozone (O3, an allotrope of oxygen) layer in the upper atmosphere. If the ozone layer is destroyed (e.g. by releasing too many chlorofluorcarbons into the atmosphere) then an increase in the concentration of UV rays reaching the Earth’s surface will lead to an increase in the occurrence of skin cancers and other serious skin diseases. Electromagnetic radiation with wavelengths from about 300 to 800 nm reach the Earth’s surface. This includes visible light as well as long wavelength UV waves and short wavelength IR waves. Red light penetrates the atmosphere more readily than blue light as it is scattered less.

Infrared rays with wavelengths longer than about 800nm are absorbed by the atmosphere as are microwaves with wavelengths shorter than about 1cm. Long wavelength microwaves and radio waves which includes TV, FM and AM waves, can penetrate the atmosphere although AM waves are reflected by the ionosphere.

Identify methods for the detection of various wavebands in the electromagnetic spectrum

Gamma rays can be detected by a Geiger-Muller tube.

X-rays can be detected by photographic film or by the fluorescence they can cause in some substances.

Ultraviolet radiation can be detected by a photoelectric cell or by the fluorescence it causes in some substances.

Visible light can be detected by the eye or by photographic film.

Infrared radiation can be detected with a thermometer as the heat absorbed by a surface.

Microwaves can be detected by an antenna and diode detector.

Explain that the relationship between the intensity of electromagnetic radiation and distance from a source is an example of the inverse square law: Ia 1/d2

The intensity of electromagnetic radiation decreases as its distance from the source increases according to the inverse square law: Ia 1/d2

e.g. The intensity of light from a floodlight at a distance “d” is I. What will be the intensity of light at a distance of (i) 2d and (ii) 3d from the floodlight?

Ans: (i) Inew = ( )2 x I = (ii) Inew = ( )2 x I = Outline how the modulation of amplitude or frequency of visible light, microwaves and/or radio waves can be used to transmit information

Modulation is the process of superimposing a signal wave onto a carrier wave so that the information contained in the signal wave is transmitted to the carrier wave. This is the way that radio and TV stations around the world transmit their information.

The two types of modulation used by Australian radio stations are amplitude modulation (AM) and frequency modulation (FM). TV stations use an FM wave.

The frequency of the carrier wave is allocated to the radio station. A typical AM frequency would be around 106 Hz i.e. 1000 kHz while a typical FM frequency would be around 108 Hz i.e. 100 MHz.

The wavelengths can be determined by applying the formula: c = fl

where c = speed of light and hence radio waves, f = frequency and l = wavelength.

The wavelength corresponding to a frequency of 1000 kHz would be 300m while the wavelength corresponding to a frequency of 100 MHz would be 3 m. The broadcasting aerial is half the wavelength of the carrier signal so FM stations have much shorter aerials than AM stations.

The diagram below represents a carrier wave. The frequency and hence the wavelength has been allocated to the station and the carrier wave is a constant signal at that frequency and wavelength. Carrier wave

The signal wave carries the voice or music produced at the radio station. It is transformed to an electromagnetic wave and this wave, the signal wave, is superimposed on the carrier wave. While a voice or music wave is quite complex, a square signal wave will be used here to demonstrate the principle. Signal wave

Amplitude modulation is the simple superposition of the two waves. Amplitude Modulated wave

In a frequency modulated signal the frequency of the carrier wave is increased or decreased as the amplitude of the signal wave increases or decreases. The amplitude of the carrier wave remains constant at the frequency of the signal wave. Frequency Modulated wave

Discuss problems produced by the limited range of the electromagnetic spectrum available for communication purposes

The atmosphere is opaque to radio waves and microwaves of wavelengths less than about 0.5 cm i.e. frequencies above 6x1010 Hz. (except for the window in and around the visible region). The main problem concerning communication with electromagnetic waves is the huge demand for a limited range of frequencies. Applications of communication with electromagnetic waves include AM radio, FM radio, TV, short wave radio, CB (citizen’s band) radio, emergency services, taxi and courier radios, radar, satellite TV and mobile phones among others. Each user does not use an exact frequency but a small range of frequencies called the bandwidth. The bandwidth decreases as the wavelengths increase, so AM radio stations have a narrower bandwidth than FM stations. As technology improves, users will be able to operate at narrower bandwidths and so increase the number of users. It is expected that demand will also increase to outstrip the increase in supply.

Plan, choose equipment or resources for and perform a first-hand investigation and gather information to model the inverse square law for light intensity and distance from the source

To perform this experiment you will need a bright light, a light meter and a metre rule.

Darken the room as much as possible by closing the blinds and blocking any sources of stray light. Turn on the bright light light and hold the light meter 10 cm away from the light. Record the reading on the light meter and gradually move it away from the light source, taking readings at 10 cm intervals. Repeat the procedure twice so that you have three intensity readings for each distance. Take the average intensity at each distance and plot your results on a graph showing intensity (vertical axis) against the square of the distance from the source.

 Units of Intensity The light meter is calibrated in lux and the Macmillan Dictionary of Physics gives the following definitions: “lux. Symbol lx. The SI unit of ILLUMINANCE, equal to the illumination produced by a luminous flux of 1 lumen uniformly spread over an area of 1 square metre.” “lumen. Symbol lm. The SI unit of LUMINOUS FLUX, equal to the luminous flux emitted by a uniform point source of intensity one CANDELA in a cone of unit solid angle.” Sample Results

Table of intensity and distance from source.

 Distance from source (cm) Square of distance from source (cm2) Intensity (lx) Reading 1 Intensity (lx) Reading 2 Intensity (lx) Reading 3 Average Intensity 10 100 1.71 1.67 1.73 1.70 20 400 0.41 0.43 0.43 0.42 30 900 0.19 0.18 0.21 0.19 40 1600 0.10 0.11 0.10 0.10 50 2500 0.06 0.07 0.06 0.06 60 3600 0.05 0.05 0.06 0.05 70 4900 0.05 0.04 0.04 0.04 80 6400 0.03 0.03 0.02 0.03 90 8100 0.02 0.02 0.02 0.02 100 10000 0.02 0.02 0.01 0.02

Analyse information to identify the waves involved in the transfer of energy that occurs during the use of one of the following:

-          mobile phone

-          television

analyse information to identify the electromagnetic spectrum range utilized in modern communication technologies

1. Many communication technologies use applications of reflection and refraction of electromagnetic waves

Describe and apply the law of reflection and explain the effect of reflection from a plane surface on waves

REFLECTION OF ELECTROMAGNETIC  WAVES

Just as sound waves are reflected as echoes, so too do we get the reflection of electromagnetic waves. The most obvious example is the reflection of light rays by a mirror but all objects reflect light and the only reason that we can see an object is that light is reflected from it to our eyes.

LAWS OF REFLECTION:

The reflection of light and other electromagnetic waves can be summarised by the “Laws of reflection”.

1.                  The incident ray, the normal and the reflected ray all lie in the same plane.

2.                  The angle of incidence is equal to the angle of reflection. This means that a wave will bounce off a surface at the same angle as it hit it (similar to a billiard ball hitting the side of the table and bouncing off at the same angle).

Describe ways in which applications of reflection of light, radio waves and microwaves have assisted in information transfer.

Once we realise that we can only see because light is reflected off other objects we begin to appreciate the importance of the reflection of light in information transfer. Radar is a particular application of the reflection of microwaves, which is vital in communicating information about the location of aircraft or shipping when visual means are not suitable because of distance or weather conditions. Radio waves and television waves are reflected by communication satellites and enable us to receive direct broadcasts or telecasts of events on the other side of the world. Prior to the development of communication satellites radio waves were reflected off the ionosphere, an upper region of the atmosphere containing ionised air and free electrons. However, due to their higher frequency and shorter wavelength, television waves would be transmitted through the ionosphere rather than reflected off it so it was not possible to transmit television over large distances on Earth.

Describe one application of reflection for each of the following:

-          plane surfaces

-          concave surfaces

-          convex surfaces

-          radio waves being reflected by the ionosphere

Plane surfaces: The most obvious example of reflection from a plane surface is a plane mirror such as the bathroom mirror that you look into each morning. Another example would be the reflection of light from a surface such as a book. Remember that we can only see because light transmitted from or reflected by an object enters our eyes. Since most of the objects we see reflect light rather than transmit it we rely on reflection of light from surfaces to get information about the things around us.

Concave surfaces: Concave mirrors are used when an enlarged, upright image is required. Cosmetic and shaving mirrors and dental mirrors are examples of concave mirrors where the object is placed inside the focal length to give an enlarged image.

Convex surfaces: Convex mirrors are used when a wide field of view is required. They are used as security mirrors in shops and libraries, safety mirrors to improve the visibility of blind corners and driveways on the road and in some rear vision mirrors in cars.

Radio waves being reflected by the ionosphere: Prior to the development of communication satellites radio waves were reflected off the ionosphere to get them to travel around the curvature of the Earth. News broadcasts from overseas used this method and most ships had a radio operator who could broadcast distress signals in an emergency.

explain that refraction is related to the velocities of a wave in different media and outline how this may result in the bending of a wavefront

REFRACTION: Refraction of a wave refers to the bending of its path as it goes from one medium to another.  It occurs because the wave changes velocity as the medium changes.

define refractive index in terms of changes in the velocity of a wave in passing from one medium to another

LAWS OF REFRACTION:

The refraction of light and other electromagnetic waves can be summarised by the “Laws of refraction”

1. The incident ray and the refracted ray are in the same plane as the normal and are on opposite sides of the surface boundary.
2. When a wave passes from one medium to another, the sine of the angle of incidence bears a constant ratio to the sine of the angle of refraction. This ratio is known as the refractive index and is equal to the ratio of the velocity in each medium.

This law is known as Snell’s Law.

 sin i  =  v1 = m             sin r      v2 When a wave travels from a less dense medium to a more dense medium (e.g. from air to water) the refractive index is greater than 1.0 and the wave bends towards the normal.

When a wave travels from a more dense medium to a less dense medium (e.g. from water to air) the refractive index is less than 1.0 and the wave bends away from the normal.

define Snell’s Law: v1 /v2 = sin i /sin r

The syllabus says it all for this dot point:

Snell’s Law: = = m

Identify the conditions necessary for total internal reflection with reference to the critical angle

When travelling from a more dense to a less dense medium a stage is reached where the angle of refraction reaches 90o i.e. the refracted wave just skims across the surface. A slight increase in the angle of incidence will see the wave refracted back into its original medium i.e. reflected. At this point the wave is reflected and the laws of reflection rather than the laws of refraction apply. Reflection of this type is known as TOTAL INTERNAL REFLECTION and the minimum angle of incidence where it occurs is known as the CRITICAL ANGLE.

Outline how total internal reflection is used in optical fibres.

An optical fibre consists of a transparent medium of high refractive index surrounded by a sheath of low refractive index. A thin beam of light is fed into the central fibre. As it travels along the fibre it strikes the boundary at an angle greater than the critical angle and undergoes total internal reflection. Consequently the beam travels along the optical fibre with very little energy loss. The beam can be a normal beam of light, a laser beam or an infrared beam. It can be digitally modulated to transmit information to the other end.

Optical fibres are used in telecommunications where they experience less interference and power loss than electrical signals. They are also used in medical instruments such as endoscopes that enable a doctor to look inside a patient’s body to examine such organs as the lungs or stomach.

Perform first-hand investigations and gather information to observe the path of light rays and construct diagrams indicating both the direction of travel of the light rays and a wave front

Present information using ray diagrams to show the path of waves reflected from:

-          plane surfaces

-          concave surfaces

-          convex surfaces

-          the ionosphere

IMAGES FORMED BY SPHERICAL MIRRORS

A spherical mirror as the name suggests is composed of a reflecting surface on part of a sphere.The centre of the sphere is known as the CENTRE OF CURVATURE and is represented in the following diagrams as C. If the reflecting surface faces the centre of curvature the mirror is CONCAVE (like a cave) and if the reflecting surface faces away from the centre of curvature the mirror is CONVEX.   The centre of the reflecting surface of the mirror is the POLE and a line passing through the pole and the centre of curvature is the PRINCIPAL AXIS.

For a concave mirror rays that are parallel and close to the principal axis will be reflected through one point. This point is known as the PRINCIPAL FOCUS. This is represented as F in the following diagrams and is located midway between the centre of curvature and the pole. For a convex mirror, all rays seem to come from the principal focus. The distance between the principal focus and the pole is known as the FOCAL LENGTH.

Parallel rays at different distances from the principal axis of spherical mirrors do not reflect to intersect at exactly the one point. This is known as SPHERICAL ABERRATION and can be overcome by using parabolic mirrors.

Ray diagrams can be drawn to find the size and nature of the image formed by a spherical mirror. Two of the following three rays are drawn from the top of the object. The point of intersection forms the top of the image. The bottom of both the object and the image lie on the principal axis.

Ray 1: Drawn parallel to the principal axis then reflected through the principal focus (if concave) or produced to pass through the principal focus (if convex).

Ray 2:  Drawn through the principal focus then reflectedparallel to the principal axis.

Ray 3:  Drawn through the centre of curvature and reflecting back on itself.

If the rays intersect in front of the mirror a REAL image is formed and the image can be projected onto a screen. If the rays when produced intersect behind the mirror a VIRTUAL image is formed and this cannot be projected onto a screen. OBJECT BETWEEN PRINCIPAL FOCUS AND POLE. Image: Magnified, Upright, Virtual.

OBJECT AT THE PRINCIPAL FOCUS. Image: Formed at infinity i.e. no image.

OBJECT BETWEEN PRINCIPAL FOCUS AND CENTRE OF CURVATURE. Image: Magnified, Inverted, Real.

OBJECT AT CENTRE OF CURVATURE. Image: Same size, Inverted, Real.

OBJECT OUTSIDE CENTRE OF CURVATURE. Image: Diminished, Inverted, Real.

CONVEX MIRRORS. Image: Diminished, Upright, Virtual.

perform an investigation and gather information to graph the angle of incidence and refraction for light encountering a medium change showing the relationship between these angles

perform a first-hand investigation and gather information to calculate the refractive index of glass or Perspex

Aim: To determine the refractive index of Perspex

Apparatus: Rectangular Perspex prism, Flat polystyrene sheet or sheet of thick cardboard, pins, ruler, protractor, paper.

Method: Place a sheet of paper on top of the polystyrene sheet. Place the perspex prism in the centre of the paper and trace its outline. Insert two pins, A & B, Through the paper and into the polystyrene on the incident side of the Perspex prism so that a straight line drawn through the pins will meet the perspex prism at an angle other than 90o.

Look through the perspex prism and insert two pins, C & D, on the opposite side of the prism to the first two so that when viewed through the block, all four pins appear to be in a straight line.

Remove the pins and the block from the paper and draw a line to show the path of the ray of light through the prism.

Draw a normal to the prism at the point where the ray enters the prism and at the point where the ray leaves the prism.

Use your protractor to measure the angles of incidence and refraction from the normal as the ray travels from air to perspex and then from perspex to air. Label these angles clearly on your diagram and write down their value.

Calculate the refractive index by using Snell’s Law: = m

i = angle of incidence, r = angle of refraction, m = refractive index.

solve problems and analyse information using Snell’s Law.

REFRACTIVE INDEX QUESTIONS:

Q.1.     A ray of light enters a rectangular glass slab of refractive index 1.5 at an angle of incidence of 30o.  Calculate the

(i)                 angle of refraction

(ii)               angle of incidence to the opposite surface as it leaves the slab.

(iii)             angle of refraction as it leaves the slab.

(iv)             speed of light in the glass.

Q.2.     A ray of light enters a pool of water of refractive index 1 1/3 at an angle of 40o to the surface.  Calculate the angle of refraction and the speed of light in water.

Q.3.     A ray of light travels from glass (refractive index = 1.5)  to air.

(i)                 What is the refractive index from glass to air?

(ii)               What is the angle of refraction if the angle of incidence is 30o?

(iii)             What is the angle of refraction if the angle of incidence is 40o?

(iv)             What is the angle of refraction if the angle of incidence is 50o?

Q.4.     A ray of light strikes a mirror at an angle of 20o to the surface.

What is the angle of reflection?

Q.5.     A ray of light enters a transparent plastic rectangular prism 10 cm long and 5 cm wide at an angle of incidence of 40o.

(i) If the refractive index of the plastic is 1.40 what is the angle of refraction?

(ii) What distance is the ray displaced sideways as it leaves the prism?

Q.6.     A light on the bottom of a swimming pool 1.0m deep acts as a point source. The refractive index of water is 1.33. An observer watching the pool sees a circle of light at the surface.

(i)                 Explain why the observer sees a circle of light.

(ii)               What is the critical angle for a ray travelling from water to air?

(iii)             What is the radius of the circle?

(iv)

Q.1. (i) 19o28’  (ii) 19o28’  (iii) 30o  (iv) 2 x 108 ms-1     Q.2. 28o49’  & 2.25 x 108 ms-1

Q.3. (i) 2/3  (ii) 48o35’  (iii) 74o37’  (iv) N/A (reflection)       Q.4. 70o

Q.5. (i) 27o20’ (ii) 1.6 cm

Q.6. (i) Light only emerges when the angle of incidence is less than the critical angle i.e. it does not undergo total internal reflection. Light at the critical angle traces out a cone with the perpendicular and this is seen as a circle at the surface.

(ii) 48o35’ (iii) 1.14m.

1. Electromagnetic waves have potential for future communication technologies and data storage technologies.

Identify types of communication data that are stored or transmitted in digital form

In analogue systems, continuous variations in such signals as light or sound are converted to continuous variations in electrical signals. For instance in sound and video tapes the strength of the magnetic field on the tape changes in an identical way to the original sound or light wave.

Digital systems convert the signal to a binary form. The amplitude of the signal is sampled thousands of times per second and converted to digital form. This is in the form of a “signal on” or “signal off” system, similar in many ways to the dots and dashes of Morse code. Some analogue signals are converted to digital signals by an A-D converter (e.g. the converter box for digital T.V. sets). Digital signals are used directly for such things as computer data, mobile phones, cable T.V. as well as CDs and DVDs.

Identify data sources, gather, process and present information from secondary sources to identify areas of current research and use the available evidence to discuss some of the underlying physical principles used in one application of physics related to waves, such as:

-          Global Positioning System

-          CD technology

-          The internet (digital process)

-          DVD technology

Global Positioning System:

The global positioning system (GPS) is a system using navigation satellites that enables the position of the receiver to be determined anywhere in the world. There are three components of the GPS: the satellites, the receiver and the ground control.

The global positioning system relies on 24 navigation satellites that circle the Earth. A GPS receiver will detect signals from these satellites in a direct line of sight, usually between 5 and 8. Each satellite carries a radio transmitter/receiver and four atomic clocks. The radios transmit and receive at 1575.42 MHz for civilian GPS systems and at 1227.60 MHz for military (PPS – Precise Positioning System) tracking devices. These frequencies are considered to be in the microwave region of the electromagnetic spectrum.

The GPS receiver relies on signals from three satellites (4 if height is also important such as in aircraft). The first satellite sends out a continuous radio signal and this is used to accurately determine the time. The receiver sends out a similar signal but the waves are out of phase. The receiver delays its signal to bring it into phase with the one emitted by the satellite.

By measuring the time it takes the signals from three satellites to reach the GPS receiver and knowing the speed of light, the instrument is able to compute the distance from each of the three satellites whose positions are precisely known. This will enable the position of the GPS receiver to be determined to an accuracy of about 100m for civilian systems or within a couple of metres for military systems.

Civilian uses of GPS systems include tracking devices for fleet vehicles such as couriers and taxis so that the vehicle nearest a job can be sent to it, tracking devices for bushwalkers, outback travelers and yachtsmen to prevent them becoming lost, and also as security additions to expensive cars and computers to track them if they are stolen.

CD & DVD technology:

CDs and DVDs use similar technology. A laser beam burns the information onto a master disc in digital form, namely a series of pits and flat sections in a tight spiral.

When playing CDs or DVDs a laser passes over the pits and flat areas and these reflect by different amounts. In the diagrams below, the laser beam will focus on a pit and it will reflect strongly. Because it does not focus on a flat area the reflection is very low. The laser beam will read this in digital form as a series of “reflection” and “non reflection” bits of information and will process it accordingly. The laser beam focuses on a pit and will reflect strongly. The laser beam does not focus on a flat area and reflects little light.

Although a CD and DVD are the same size, the DVD contains a lot more information. This is achieved by packing the information more densely in a DVD. A laser with a shorter wavelength is used and this allows smaller pits to be used and the tracks to be more closely spaced. Many DVDs are also dual layered which almost doubles the amount of information that they can store.

Comparison of CD and DVD

 CD DVD Diameter 120 mm 120 mm Thickness 1.2 mm 1.2 mm Layers Single layer Single or double layer Laser Wavelength 780 nm (infrared) 640 nm (red) Track pitch 1.6 mm 0.74 mm Pit/flat length 0.83 mm 0.40 mm Data capacity 680 Mb Single layer 4.7 Gb Double layer 8.5 Gb

Laser beam                                        Laser beam

Lens                                                    Lens

Protective coating of plastic              Protective coating of plastic

Reflective metal surface (Al)            Reflective surface (Al)

Plastic disc                                          Plastic disc