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4 Waves G482 Electricity, Waves & Photons 2.4.1 Wave Motion 2.4.2. EM Waves 2.4.3 Interference 2.4.4 Stationary Waves Mr Powell 2012 Index 2.4.2. Electromagnetic Waves Assessable learning outcomes..... a) state typical values for the wavelengths of the different regions of the electromagnetic spectrum from radio waves to X-rays; b) state that all electromagnetic waves travel at the same speed in a vacuum; c) describe differences and similarities between different regions of the electromagnetic spectrum; d) describe some of the practical uses of electromagnetic waves; e) describe the characteristics and dangers of UV-A, UV-B and UV-C radiations and explain the role of sunscreen (HSW 6a); f) explain what is meant by plane polarised waves and understand the polarisation of electromagnetic waves; g) explain that polarisation is a phenomenon associated with transverse waves only; h) state that light is partially polarised on reflection; i) recall and apply Malus’s law for transmitted intensity of light from a polarising filter. Book pages 146-149 Mr Powell 2012 Index EM Spectrum Can you remember any parts of it. Write out 1-8 in your books and test yourself? 1 2 3 4 5 6 7 Mr Powell 2012 8 Index Visible Light Wavelength in nanometres (nm) 1x10-9 or x 0.000000001m Copy text & diagram Red 750 - 609 Orange White light is dispersed by a prism to form a spectrum (not to scale) 610 - 589 Yellow 590 - 569 Green 570 - 599 Blue 500 - 465 Indigo 466 - 432 Violet 433 - 400 Visible light is detected by the human eye. White light consists of ROY-G-BIV (as shown above). Each colour is a range of wavelengths and is absorbed differently by the cells in the eye. Visible light is the middle part of the EM Spectrum sandwiched between Ultraviolet (more than violet) & Infra Red (less than red) Visible Light Copy & Complete Shorter Wavelength O Y V Infra Red >750 Ultraviolet 750 -609 610-589 Higher Frequency Wavelength in nanometres (nm) 1x10-9 or x 0.000000001m 500465 466-432 433-400 Mr Powell 2012 <400 Index Visible Light Answers Shorter Wavelength IR R O Y G B I V UV Infra Red Red Orange Yellow green Blue Indigo Violet Ultraviolet >750 750 -609 610-589 590-569 570-500 500465 466-432 433-400 <400 Higher Frequency Wavelength in nanometres (nm) 1x10-9 or x 0.000000001m Mr Powell 2012 Index a) state typical values for the wavelengths of the different regions of the electromagnetic spectrum from radio waves to X-rays; The types of electromagnetic radiation are broadly classified into the following classes: Gamma radiation X-ray radiation Ultraviolet radiation Visible radiation Infrared radiation Terahertz radiation Microwave radiation Radio waves http://en.wikipedia.org/wiki/Electromagn etic_spectrum Mr Powell 2012 Index a) state typical values for the wavelengths of the different regions of the electromagnetic spectrum from radio waves to X-rays; γ= Gamma rays MIR= Mid infrared HF= High freq. HX= Hard Xrays FIR= Far infrared MF= Medium freq. SX= Soft X-rays Radio waves LF= Low freq. EUV= Extremeultravi olet EHF= Extremely VLF= Very low high freq. freq. NUV= Near ultraviolet SHF= Super high freq. Visible light UHF= Ultra high SLF= Super low freq. freq. NIR= Near Infrared VHF= Very high freq. VF/ULF= Voice freq. ELF= Extremely low freq. Freq=Frequenc y http://en.wikipedia.org/wiki/Electromagnetic_spectrum Mr Powell 2012 Index b) state that all electromagnetic waves travel at the same speed in a vacuum; The wave theory of light developed into another theory about the propagation of electromagnetic waves through space with or with an medium. This was as a result of theoretical work by James Clark Maxwell who showed mathematically in 1865 that a changing current in a wire creates waves of changing electric and magnetic fields that radiate from the wire. Maxwell showed that the waves are transverse in nature and that the electric waves are in phase with and perpendicular to the magnetic waves as shown ....... Mr Powell 2012 Index b) state that all electromagnetic waves travel at the same speed in a vacuum; When an AC current is applied to a wire the alternating current in a wire creates an alternating magnetic field which generates an alternating electric field which generates an alternating magnetic field further from the wire which generates an alternating electric field which generates an alternating magnetic field further yet further from the wire and so on. Maxwell knew that the strength of the electric field depends on the permittivity of free space, 0. He also knew that the magnetic field strength depends on the equivalent magnetic constant, the permeability of free space, 0. He showed mathematically that the speed of electromagnetic waves in free space, c, is given by........ (3 x 108ms-1) 0 = 8.85 × 10−12 F m−1 0 = 4 × 10−7 T m A−1 Mr Powell 2012 Index c) describe differences and similarities between different regions of the electromagnetic spectrum; Wave Wavelength Use Long Wave Radio Medium Wave Radio Short Wave Radio 1500 m 300 m 25 m Broadcasting Broadcasting Broadcasting FM Radio 3m Broadcasting and communication UHF Radio 30 cm Microwaves 3 cm TV transmissions Communication Radar Heating up food Communication in optical fibres Remote Controllers Heating Infra red 3 mm Light 200 - 600 nm Ultra violet 100 nm X-ray 5 nm Shadow pictures of bones Gamma rays <0.01 nm Scientific research Seeing Communicating Sterilising Sun tanning Mr Powell 2012 Index c) describe differences and similarities between different regions of the electromagnetic spectrum; Wave Wavelength Hazard Prevention Long Wave Radio 1500 m No hazard Medium Wave Radio 300 m No hazard Short Wave Radio 25 m No hazard FM Radio UHF Radio 3m 30 cm Microwaves 3 cm Infra red 3 mm No hazard No hazard Heating of water in the body Heating effect Light 200 - 600 nm No hazard Ultra violet 100 nm Can cause cancer Sun cream (or cover up) X-ray 5 nm Causes cell damage Lead screens Gamma rays <0.01 nm Causes cell damage Thick lead screens or concrete Metal grid Reflective surface Mr Powell 2012 Index c) describe differences and similarities between different regions of the electromagnetic spectrum; • The Earth and all life on it has developed a tolerance & use for some parts of the EM Spectrum due to how it behaves as it passes through air. • Some parts are absorbed fully, partly or not at all. Mr Powell 2012 Index c) describe differences and similarities between different regions of the electromagnetic spectrum; Mr Powell 2012 Index Which word links all of these images... Mr Powell 2012 Index e) describe the characteristics and dangers of UV-A, UV-B and UV-C radiations and explain the role of sunscreen (HSW 6a); Ozone molecules are vitally important to life because they absorb the biologically harmful ultraviolet radiation from the Sun. (even in small amounts) There are three different types of ultraviolet (UV) radiation, UV-A (320-400nm), UV-B (280220nm), and UV-C (100-280nm). UV-C is entirely screened out by ozone around 35 km altitude. On the other hand, UV-a reaches the surface, but it is not as genetically damaging, so we don't worry about it too much. It is the UV-B radiation that can cause sunburn and that can also cause genetic damage, resulting in things like skin cancer, if exposure to it is prolonged. Ozone screens out most UV-b, but some reaches the surface. Were the ozone layer to decrease, more UV-b radiation would reach the surface, causing increased Index genetic damage to living things Mr Powell 2012 Skin Cancer For the two diagrams show on the right can you answer the following questions; 1. What part of the body is a man most at risk for? 2. What part of the body is a woman most at risk from? 3. Why is there a difference between men and women in term of the head and neck? 4. Why are men's legs only 15% when women's are 42% Mr Powell 2012 Index Yearly Trends Can you clearly explain the trend seen on this graph? Can you explain why the trend might be occurring? Mr Powell 2012 Index Age Trends Can you clearly explain the trend seen on this graph? Can you explain why the trend might be occurring? Mr Powell 2012 Index UV Index UV index depends on: where you are in the world the time of year the weather the time of day how high up you are (the altitude) Met Office UV index forecasts include the effects of: the position of the Sun in the sky forecast cloud cover amount of ozone in the stratosphere Mr Powell 2012 Index Types of Skin Try and work out your own skin...... Mr Powell 2012 Index What is your risk You can work out from the UV index and your skin type when you are in danger? Try it out........... Mr Powell 2012 Index Incidence of melanoma 1. Can you clearly explain any trend or pattern seen on this graph i.e. Highs and lows? 2. Can you explain why the trend might be occurring? Europe (2002 estimates) Mr Powell 2012 Index Incidence of melanoma 1. Can you clearly explain any trend or pattern seen on this graph i.e. Highs and lows? 2. Can you explain why the trend might be occurring? World (2002 estimates) Mr Powell 2012 Index Sun Creams If you were buying sun protection lotion, what factors would be important to you? Factors to be considered: 1. 2. 3. 4. 5. 6. volume sun protection factor (SPF) brand price target (adult/child) waterproof properties SPF means sun protection factor. SPF = 100 ÷ % of UV radiation transmitted If 10% is transmitted, SPF = 100/10 = 10 If 3% is transmitted, SPF = 100/3 = 33 What is the SPF of a sunscreen that transmits 5% UV radiation? Mr Powell 2012 Index Example Exam Question (Basic) The electromagnetic spectrum covers a very wide range of wavelengths, frequencies and photon energies. (i) State the names and wavelengths for the shortest and longest electromagnetic waves. shortest: name .................................. wavelength ......................................m longest: name .................................. wavelength .......................................m (4 marks) (ii) Calculate the ratio ..... Longest wavelength / shortest wavelength ratio = ............................... ( 1 mark) Mr Powell 2012 Index How does the intensity spread out Mr Powell 2012 Index Example Exam Question (Basic) The electromagnetic spectrum covers a very wide range of wavelengths, frequencies and photon energies. (i) State the names and wavelengths for the shortest and longest electromagnetic waves. shortest: name .................................. wavelength ......................................m longest: name .................................. wavelength .......................................m (4 marks) (ii) Calculate the ratio ..... Longest wavelength / shortest wavelength ratio = ............................... ( 1 mark) (i) shortest: gamma (1) allow any wavelength between 10–12 and 10–16 (m) (1) longest: radio (1) allow any wavelength between 102 and 105 (m) (1) (ii) candidates ratio e.g. 104 / 10–14 = 1018 (1) 4 1 Mr Powell 2012 Index Polarisation..... Mr Powell 2012 Index f,g,i) Polarisation Create your own diagram to show this concept clearly. Then explain it to another student. Electric field vector As you rotate 90 or /2 the light gradually fades Try it with a polariser! Mr Powell 2012 Index f,g,i) Polarisation Create your own diagram to show this concept clearly. Then explain it to another student. Electric field vector Mr Powell 2012 Index i) recall and apply Malus’s law for transmitted intensity of light from a polarising filter. Malus' law, which is named after Étienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the intensity, I, of the light that passes through is given by…………. I I max cos 2 Imax = is the initial intensity i = is the angle between the light's initial polarization direction and the axis of the polarizer. Examples.... Angle 0 Angle 45 Angle 90 Angle 180 http://en.wikipedia.org/wiki/Polarizer Mr Powell 2012 Index i) recall and apply Malus’s law for transmitted intensity of light from a polarising filter. Malus' law, which is named after Étienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the intensity, I, of the light that passes through is given by…………. I I max cos 2 Imax = is the initial intensity i = is the angle between the light's initial polarization direction and the axis of the polarizer. Examples.... Angle 0 ........ I = Imax * cos (0) * cos (0) = Imax Angle 45 or /4 ........ I = Imax * cos (45) * cos (45) = 0.52 * 0.5 2 = 2 = 0.5 Imax Angle 90 or /2........... I = Imax * cos (90) * cos (90) = 0 Angle 180 or ........... I = Imax * cos (180) * cos (180) = Imax http://en.wikipedia.org/wiki/Polarizer Mr Powell 2012 Index f,g,i) Applications of Transverse Polarisation… Can you research each one of these ideas and see how Polarisation has an impact. Draw out a mind map and write out the key points for each one. You only need the basic idea for the exam not the details…. Radio? (GCSE) Calculator? (Extension) Concentration Transverse Polarisation Sun Glasses G&T Sheet 12_1 Stress Testing Mr Powell 2012 Index f,g,i) Uses of Polarisation Mr Powell 2012 Index f,g,i) Fishing? Mr Powell 2012 Index f,g,i) Using polarisation to measure concentration 1. Some liquids are ‘optically active’ and rotate the electric vector. 2. The liquid’s concentration is proportional to the electric vector rotation. laser Sugar solution polariser © John Parkinson analyser Mr Powell 2012 Index 37 f,g,i) Stress Analysis The structure of certain plastics will show polarisation. When viewed under stress the structure polarises the light differently. The place where stress is greatest shows a more rapid colour change. Models can be made of complex components which are viewed with a polarising filter so engineers can design out the stresses. © John Parkinson Mr Powell 2012 Index 38 Mr Powell 2012 Index Mr Powell 2012 Index Polarisation Exam Question…. (Basic Level) (b) Daylight passes horizontally through a fixed polarising filter P. An observer views the light emerging through a second polarising filter Q, which may be rotated in a vertical plane about point X as shown in the diagram. Describe what the observer would see as Q is rotated slowly through 360°. (1 mark) Mr Powell 2012 Index Polarisation Exam Question…. (Basic Level) (b) Daylight passes horizontally through a fixed polarising filter P. An observer views the light emerging through a second polarising filter Q, which may be rotated in a vertical plane about point X as shown in the diagram. Describe what the observer would see as Q is rotated slowly through 360°. (1 mark) Answer (b) variation in intensity between max and min (or light and dark) (1) or two maxima (or two minima) in 360° rotation (1) Mr Powell 2012 Index Activity Ideas... Students can discuss the purpose of using sunscreen. (HSW 6a) The teacher can demonstrate polarisation using a metal grill for microwave and polarising filter for light. Students can observe light reflected from a glass surface through a polarising sheet. Students can discuss the use of polarising filters in photography and in sun glasses to reduce glare. (HSW 6a) Mr Powell 2012 Index f,g,i) Calculator LCD Displays Extension Work 1. Polariser filter film with a vertical axis to polarize light as it enters. 2. Glass with electrodes to show patterns when the LCD is turned ON. 3. Twisted nematic liquid crystal. Rotates light 90 or /2 when turned on. 4. Glass substrate with electrode film 5. Polarising filter film with a horizontal axis to block/pass light. 6. Reflective surface to send light back to viewer. KEY Point. System allows on/off change of transmission by use of Twisted nematic liquid crystal & crossed polarisers General Polarisation Nematic Crystals Mr Powell 2012 Index Quick Questions (From Real Exams)... Jan 2012 6 (a) X-rays and radio waves are two examples of electromagnetic waves. (i) Name two other examples of electromagnetic waves. [1] (ii) State one similarity and one difference between X-rays and radio waves. [2] (iii) Explain why X-rays are easily diffracted by layers of atoms, about 2 × 10–10 m apart, but radio waves are not. [2] (b) On the Earth, we are all exposed to ultraviolet radiation coming from the Sun. State one advantage and one disadvantage of UV-B radiation. [2] (c) (i) Circle a typical value for the wavelength of an X-ray from the list below.... 2 × 10–4 m 2 × 10–7 m 2 × 10–10 m 2 × 10–13 m [1] Mr Powell 2012 Index Markscheme Mr Powell 2012 Index Practical Skills are assessed using OCR set tasks. The practical work suggested below may be carried out as part of skill development. Centres are not required to carry out all of these experiments. Students should gain a qualitative understanding of superposition effects together with confidence in handling experimental data. Students should be able to discuss superposition effects and perform experiments leading to measurements of wavelength and wave velocity. Use an oscilloscope to determine the frequency of sound. Observe polarising effects using microwaves and light. Investigate polarised light when reflected from glass or light from LCD displays. Study diffraction by a slit using laser light. Study hearing superposition using a signal generator and two loudspeakers. Study superposition of microwaves. Determine the wavelength of laser light with a double-slit. Determine the wavelength of light from an LED using a diffraction grating. Demonstrate stationary waves using a slinky spring, tubes and microwaves. Determine the speed of sound in air by formation of stationary waves in a resonance tube. Mr Powell 2012 Index Connection • • • Connect your learning to the content of the lesson Share the process by which the learning will actually take place Explore the outcomes of the learning, emphasising why this will be beneficial for the learner Demonstration • Use formative feedback – Assessment for Learning • Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex • Offer different ways for the students to demonstrate their understanding • Allow the students to “show off” their learning Activation Consolidation • Construct problem-solving challenges for the students • Use a multi-sensory approach – VAK • Promote a language of learning to enable the students to talk about their progress or obstacles to it • Learning as an active process, so the students aren’t passive receptors • Structure active reflection on the lesson content and the process of learning • Seek transfer between “subjects” • Review the learning from this lesson and preview the learning for the next • Promote ways in which the students will remember • A “news broadcast” approach to learning Mr Powell 2012 Index Further Research.... Radio waves: Radio waves Microwaves: Microwaves Infrared Infrared Visible Light: Light Natural sources produce EM radiation across the spectrum. EM radiation with a wavelength between approximately 400 nm and 700 nm is directly detected by the human eye and perceived as visible light. Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light, especially when visibility to humans is not relevant. Ultraviolet: Ultraviolet X-rays: X-rays Gamma rays: Gamma rays Mr Powell 2012 Index