The World Communicates

1. The World Communicates Module 8.2

2. mechanical • EM waves • Transverse • longitudinal Velocity Frequency & Pitch Amplitude & Loudness What is this topic all about??? • wavelength • amplitude • frequency • period Wave Equation v = ƒ Nature of Sound Waves General Types of Waves Principle of Superposition Wave Measurements Graphing Waves Properties of Sound Waves The Nature of Waves The Electromagnetic Spectrum The World Communicates Digital Communication & Data Storage Electromagnetic Waves Reflection & Refraction EMR in Communication Production & Detection of EMR Refraction Snell’s Law Law of Reflection Dangers of EMR Inverse Square Law Total internal reflection & critical angle Reflection in communication Light & Mirrors

3. 1. The wave model can be used to explain how current technologies transfer 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 carry energy from one place to another. All forms of wave motion transport energy, without transporting matter. This may occur in 1 dimension (pulses moving along a slinky spring), 2 dimensions (ripples spreading out on a surface of water) and 3 dimensions (light radiating out from a light bulb). • identify that mechanical waves require a medium for propagation while electromagnetic waves do not Mechanical waves are those which require a medium to travel through. Sound waves need a substance such as air to travel through due to vibration of particles. Sound will not travel through a vacuum. “In space no one can hear you scream”. Electromagnetic (EM) waves do not need a medium. They can travel through a vacuum and in fact travel fastest through empty space. EM waves are self propagating. (They will be looked at in more detail in later topics). EM waves include light, radio waves and X-rays. • 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 • describe the relationship between particle motion and the direction of energy propagation in transverse and longitudinal waves There are two main types of mechanical waves; transverse and longitudinal (or compression).

4. In a longitudinal wave, the vibration of the particles of the medium is along the same direction as the motion of the wave. • With a transverse wave, the particles vibrate up and down in a direction perpendicular to the motion of the wave itself. longitudinal wave transverse wave • Measuring Waves • A transverse wave consists of a crest above the rest axis, and a trough below it. • A longitudinal wave consists of compressions, where the particles are close together, and rarefactions, where the particles are spread apart. • The amplitude (A) for both is the maximum displacement from the rest position. (Measured in metres) • The wavelength () is the distance between any two consecutive points. (Measured in metres)

5. The frequency () of a wave is the number of waves that pass a particular point in one second. Units are Hertz (Hz). • The period (T) of a wave is the time it takes for one complete wave to pass a point. Units are seconds (s) • quantify the relationship between velocity, frequency and wavelength for a wave: • v = ƒ  • solve problems and analyse information by applying the mathematical model of v = ƒ to a range of situations The wave equation indicates the speed of a wave in terms of wavelength and frequency. v = ƒ  The units of velocity are ms-1 • perform a first-hand investigation to observe and gather information about the transmission of waves in: • – slinky springs • – water surfaces • – ropes • or use 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 wave travelling at a constant velocity

6. 2. 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 vibrations or oscillations of particles in a medium. Sound waves are longitudinal waves in air. The speed of sound is different in different materials. In air at 0oC, sound travels at 331 ms-1. • relate compressions and rarefactions of sound waves to the crests and troughs of transverse waves used to represent them Instead of crests and troughs, a series of ‘compressions’ and ‘rarefactions’ pass through the medium as a sound travels. The atoms and molecules are alternately ‘pushed together’ and then stretched apart as the energy flows through. In a compression the air pressure is high, and lower in a rarefaction. • explain qualitatively that pitch is related to frequency and volume to amplitude of sound waves Two aspects of any sound are immediately evident to a human listener. These are “loudness” and “pitch” but to each of these subjective sensations there corresponds a physically measurable quantity. Loudness is related to the energy in the sound wave. This is measured as the amplitude of the wave. The more energy or louder the wave, the greater the amplitude. The pitch is related to frequency. The lower the frequency, the lower the pitch, and the higher the frequency, the higher the pitch.

7. explain an echo as a reflection of a sound wave Like all waves, sound can strike another medium and bounce off. This is what we often hear as an echo. • describe the principle of superposition and compare the resulting waves to the original waves in sound When two wave pulses meet, they pass through each other. However, at the meeting point the wave amplitudes add: two crests produce a larger crest, and similarly, two troughs make a deeper trough. In the region where they overlap, the resultant displacement is the sum of their separate displacements (a crest is considered positive and a trough negative). This is called the principle of superposition. • perform a first-hand investigation and gather information to analyse sound waves from a variety of sources using the Cathode Ray 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

8. A cathode ray oscilloscope (CRO) has a screen that shows the variation of a voltage input to the CRO with time. The input to the cathode ray oscilloscope may be from a microphone, a device that converts variations in sound pressure into corresponding voltage changes. A cathode ray oscilloscope can be used to investigate and measure properties of sound waves including: • Frequency • Period • Amplitude The CRO can also be used to investigate complex sound waves with components of many different frequencies. The following website is an excellent CRO simulation http://www.talkingelectronics.com/CRO-Ebook-1/html/CRO-P8-CRO-Simulation.html

9. 3. 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 Scientists have known since the early part of the 19th century that electrical fields and magnetic fields are intimately related to each other. Moving electric charge (electric current) creates a magnetic field. James Clerk Maxwell (1831-1879) put these ideas together and proposed that if a changing magnetic field can make an electric field, then a changing electric field (from an oscillating electric charge, for example) should make a magnetic field. A consequence of this is that changing electric and magnetic fields should trigger each other and these changing fields should move at a speed equal to the speed of light. To conclude this line of reasoning, Maxwell said that light is an electromagnetic wave. Electromagnetic waves are self propagating, do not require a medium and travel at the speed of light, c, in a vacuum. Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles travelling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy. The only difference between the various types of em radiation is the amount of energy found in the photons. Radio waves have photons with low energies and gamma rays are the most energetic of all.

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

11. Electromagnetic radiation from space is unable to reach the surface of the Earth except at a very few wavelengths, such as the visible spectrum, radio frequencies, and some ultraviolet wavelengths. The gamma rays and X-rays in the solar spectrum do not penetrate below the thermosphere. They are absorbed by the oxygen and nitrogen atoms with the energy subsequently released as heat. In the upper stratosphere high energy ultraviolet radiation is absorbed by oxygen molecules (O2) causing them to form free atoms. These free oxygen atoms then combine with oxygen molecules to form ozone (O3) and heat energy. Consequently the ozone layer is warm. The solar ultraviolet radiation from 200–400 nm is considered as three bands. UVa, UVb and UVc. UVa is the least energetic and the least damaging band of UV radiation, it has wavelengths that vary from 320 to 400 nm. It is not absorbed by ozone and penetrates to the Earth’s surface. UVb radiation ranges in wavelength from 280 to 320 nm, it is more energetic than UVa and is largely absorbed by ozone. UVc radiation ranges in wavelength from 200 to 280 nm, and is the most energetic and most damaging but is totally absorbed by ozone and oxygen (O2) high in the atmosphere. In the lower atmosphere the molecules of nitrogen and oxygen as well as carbon dioxide and water absorb in the visible and infrared spectrum.

12. identify methods for the detection of various wavebands in the electromagnetic spectrum The waves at the high wavelength end (radio) of the electromagnetic spectrum are detected by metal conductors—aerials. The radio wave induces an oscillation of the electrons in the aerial. This oscillation is amplified by the radio unit to produce an audio signal. Infrared radiation causes an increase in temperature of an absorbing (metal) surface. A mercury thermometer heats up in sunlight well above the air temperature. Our skin also is a sensitive detector of infrared (heat) radiation. Our eyes detect visible light through chemical changes in the rods and cones of the retina. Photographic emulsions also respond to visible light in cameras. Ultraviolet detectors include film, photo cells and fluorescent chemicals. Infra red – special film, semiconductor devices and our skin The waves at the short wavelength, high energy end of the spectrum (X-rays and γ-rays) are detected by devices such as the Geiger Müller tube. The energy of the radiation ionises the gas in the detector, creating a pulse of current through the high voltage tube. • explain that the relationship between the intensity of electromagnetic radiation and distance from a • source is an example of the inverse square law: Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This applies to electromagnetic radiation, gravity, electric fields, and sound.

13. 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 Microwave apparatus, light meter and data logging equipment • outline how the modulation of amplitude or frequency of visible light, microwaves and/or radio waves can be used to transmit information Amplitude Modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. A basic AM radio transmitter works by first DC-shifting the modulating signal, then multiplying it with the carrier wave using a frequency mixer. The output of this process is a signal with the same frequency as the carrier but with peaks and troughs that vary in proportion to the strength of the modulating signal. This is amplified and fed to an antenna AM radio's main limitation is its susceptibility to atmospheric interference, which is heard as static from the receiver. The narrow bandwidth traditionally used for AM broadcasts, further limits the quality of sound that can be received. http://en.wikipedia.org/wiki/Amplitude_modulation

14. Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. In frequency modulation the sound signal changes or modulates the frequency of the carrier signal. When the sound signal is large amplitude, the carrier is changed to a slightly lower frequency. When the sound signal is small amplitude, the carrier is changed to a slightly higher frequency. FM or frequency modulated radio transmissions are less affected by interference. It is unlikely that any outside disturbance could mimic the varying frequency that represents the sound signal. No attention is paid to the amplitude of the received signal, so lightning or car ignition systems have little effect on the quality of the sound. • 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 • - radar • analyse information to identify the electromagnetic spectrum range utilised in modern communication technologies • See page 14 of ‘keep it simple science’.

15. 4. 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 The law of reflection states that when a wave strikes a boundary, the angle of incidence equals the angle of reflection. Plane mirrors are the most common optical devices. The ordinary household mirror is a sheet of flat glass, ‘silvered’ on the back with a layer of metal paint. Light passes through the glass and is reflected by the silvering.

16. describe ways in which applications of reflection of light, radio waves and microwaves have assisted in information transfer Many of the devices that we now use for communication use the property of reflection. Light inside an optical fibre is ‘totally internally reflected’ and satellite dishes, radar mirrors are angled to reflect the signals to a focus. • describe one application of reflection for each of the following: • - plane surfaces • - concave surfaces • - convex surfaces • - radio waves being reflected by the ionosphere Plane mirrors Concave mirrors focus parallel rays of light to a point. Concave mirrors are used as shaving and make-up mirrors, and are also used as the objectives in large astronomical telescopes. Convex mirrors cause parallel rays of light to diverge. They are used to give a wide field of view in car rear-view mirrors, as security mirrors in shops and on roads at sharp bends and concealed entrances. The distance and paths followed by radio waves on Earth are affected by the earth itself and on the state of the atmosphere. At frequencies between 2 MHz and about 20 MHz, radio waves are reflected off the ionosphere, a layer of ionised molecules that reaches from about 40 km to about 300 km above the Earth’s surface. Using this mode of propagation, radio waves can travel 4,000 km in one ‘hop’ and can easily travel round the Earth with several hops. When a radio wave travels like this, it ‘skips’ over large areas where the signal strength will be very weak.

17. 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 wavelength • present information using ray diagrams to show the path of waves reflected from: • - plane surfaces • - concave surface • - convex surface • - the ionosphere

18. 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 When light passes into another transparent medium (air, glass, perspex) it slows down. The more optically dense the material, the more the light slows. As the light wave goes into the block it slows down and bends towards the normalline, so angle A is always bigger than angle B. As the ray comes out of the block the light wave speeds up again and bends away from the normal line, so angle B is always smaller than angle C. The only time light waves do not bend when changing speed, is if they are travelling along the normal line, at right angles to the boundary. http://www.s-cool.co.uk/

19. Converging Lenses As long as the object is outside of the focal point the image is real and inverted. When the object is inside the focal point the image becomes virtual and upright. Diverging Lenses The image is always virtual and is located between the object and the lens. • define refractive index in terms of changes in the velocity of a wave in passing from one medium to another For light that is refracted as it travels from medium 1 into medium 2, we can calculate a quantity called the index of refraction, which indicates the ratio of the speeds of light in the two materials. As the absolute index of refraction increases, the speed of light in the material decreases. • define Snell’s Law: • solve problems and analyse information using Snell’s Law

20. Incident ray Refracted ray LAWS OF REFRACTION 1 = angle of incidence 2 = angle of refraction normal 1 2 1. The incident ray, the refracted ray and the normal all lie in the same plane. 2. For two given media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. SNELL’s LAW

21. 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 to calculate the refractive index of glass or perspex A graph of sin i versus sin r will produce a gradient which is equal to the refractive index of perspex. • identify the conditions necessary for total internal reflection with reference to the critical angle When light passes from one material into a second material where the index of refraction is less, the light bends away from the normal. At a particular incident angle, the angle of refraction will be 90o, and the refracted ray would skim the surface. The incident angle at which this occurs is called the critical angle c. For incident angles greater than c there is no refracted ray and all of the light is reflected. This is called total internal reflection. • outline how total internal reflection is used in optical fibres An optical fibre is made from a fine cylinder of glass about 5m in diameter. This is the core and is enclosed by cladding, which is made from a less dense glass. If light enters the core at a large enough angle, it will be trapped in the core because the beam is totally internally reflected at the boundary between the core and the cladding. The beam travels down the fibre after multiple reflections. Signals travelling down a fibre are not affected by electrical interference, and they are very secure because they do not create a magnetic field, which a current flowing down a wire would create.

22. 5. 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 • 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 Library research http://www.trimble.com/gps/index.html An awesome site on GPS