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3 of 43© Boardworks Ltd 2009 What is a wave? A wave is an oscillation that moves through space, transferring energy from one place to another. Which of these is not an example of a wave? All true waves move or propagate through space, therefore the ripples on a sand dune are not waves.
4 of 43© Boardworks Ltd 2009 Representing waves There are two main ways of representing a wave on a graph. graphing an oscillation in time: period amplitude t y This graph represents how y changes with time. It could be an oscillation of voltage, displacement, pressure, or any other suitable variable, depending on the context.
5 of 43© Boardworks Ltd 2009 Representing waves graphing an oscillation in space: There are two main ways of representing a wave on a graph. wavelength amplitude x y This graph represents how y changes along an axis x in space. It could be a wave of displacement or pressure, or any other suitable variable, depending on the context.
6 of 43© Boardworks Ltd 2009 Waves in time and space period amplitude t y wavelength amplitude x y Always label your axes! These two waveforms look the same, but they each give different information about the wave they represent.
7 of 43© Boardworks Ltd 2009 Studying waveforms in the classroom An oscilloscope is an instrument that detects a varying voltage from an input, such as a microphone, and plots its waveform against time. A signal generator produces an alternating voltage at a chosen frequency and amplitude. It can also produce a range of different waveforms.
8 of 43© Boardworks Ltd 2009 Using an oscilloscope
9 of 43© Boardworks Ltd 2009 Wave speed A series of surface waves is moving across a pond. The peaks of the waves are 20 cm apart. wave speed = × wavelength number of waves passing per second wave speed = frequency × wavelength The waves are passing the duck at a rate of 20 × 2 = 40 cm/s This formula can always be used to find wave speed. A duck is disturbed by the waves and bobs up and down twice a second as the waves move past it. At what speed are the waves travelling across the water surface?
10 of 43© Boardworks Ltd 2009 Understanding waves and waveforms
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12 of 43© Boardworks Ltd 2009 Waves in a medium Most types of waves are disturbances that propagate through a medium. Sound waves travel through the air as variations in pressure and density. Can sound travel through any other medium? What is the difference between a sound wave and a pressure wave? During an earthquake, transverse and longitudinal waves travel through solid rock away from the epicentre. Transverse waves can travel across a water surface. These are known as surface waves. Longitudinal pressure waves can also travel through a body of water.
13 of 43© Boardworks Ltd 2009 Pitch and loudness Which of these traces shows the louder sound and which shows the sound with the higher pitch? The shorter the wavelength of a sound, or the higher the frequency, the higher the pitch to the human ear. The loudness of a sound depends on the amplitude of the wave. Not every note of the same pitch sounds the same. The waveform of a wave determines the quality of the sound. higher pitch louder
14 of 43© Boardworks Ltd 2009 What is ultrasound? The range of human hearing is 20–20,000 Hz. Any sound above 20 kHz is called ultrasound. Whales and dolphins communicate using ultrasound. Many of them also use it for echolocation. Animals are thought to do this naturally, but it can also be done by a computer, such as in prenatal scanning. Echolocation works by timing how long a wave takes to reflect from a surface and return to its source. That information is then used to calculate the distance it has travelled.
15 of 43© Boardworks Ltd 2009 Originally an acronym for ‘SOund Navigation And Ranging’, sonar uses the same principle as echolocation in animals. A signal is sent out, and the time taken for it to return to its source after reflecting from a surface, such as a lake bed, is measured. Sonar One practical application of ultrasound is sonar. distance = speed × time Knowing the speed of sound in water, it is then possible to calculate the distance the sound has travelled using this equation:
16 of 43© Boardworks Ltd 2009 Depth evaluation using ultrasound A ship has recorded the following trace while using sonar to map the bottom of a lake. The traces are 0.01 s apart, and sound travels at 1500 m/s in water. How deep is the lake? transmitted signal reflected signal distance = speed × time = 1500 × 0.01 = 15 m However, this is not the depth of the lake! This is the distance the sound has travelled, down to the lakebed and back up again. depth of the lake = 15 2 = 7.5 m
17 of 43© Boardworks Ltd 2009 Sound imaging calculations
18 of 43© Boardworks Ltd 2009 How does ultrasound imaging work? We have seen how to calculate a simple distance to a boundary using sonar. How can ultrasound be used to create more complex images, such as of an unborn child? The reflected waves are detected by a receiver. A computer turns the distance and intensities of these echoes into a 2-dimensional image. Waves are not only reflected from solid surfaces: they are reflected from any boundary between different media. When transmitted into the body, ultrasound is reflected to varying degrees by all the different tissue boundaries present.
19 of 43© Boardworks Ltd 2009 Further uses for ultrasound Jewellers and watch repairers use ultrasound to clean delicate items. The dirt is shaken off by the air vibrations, leaving the mechanism unharmed. Ultrasound has many uses in industry as well as medicine. The reflection of sound waves from any boundary also makes ultrasound useful for finding flaws in mechanical structures or the raw materials used to build those structures.
20 of 43© Boardworks Ltd 2009 Wave media
21 of 43© Boardworks Ltd 2009 Sound waves
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23 of 43© Boardworks Ltd 2009 What is diffraction?
24 of 43© Boardworks Ltd 2009 Describing diffraction When waves pass through a gap they diffract. This means they spread out on the far side of the gap, changing shape as they pass through it. Maximum diffraction occurs when the gap size is equal to the wavelength of the waves. If the gap is much larger than the wavelength, only the edges of the waves diffract. When waves pass an obstacle on only one side, only those edges are diffracted. If the gap is much smaller than the wavelength, the waves cannot pass through the gap at all.
25 of 43© Boardworks Ltd 2009 Using diffraction Long wave radio waves are diffracted by hills and mountains because the wavelength is of a similar size to the obstacle. This allows them to travel around these obstacles, providing coverage over a large area. Higher frequency waves are diffracted much less. Television signals, for example, have a much shorter range. Diffraction is useful for long distance communications.
26 of 43© Boardworks Ltd 2009 Understanding diffraction
27 of 43© Boardworks Ltd 2009 What is interference?
28 of 43© Boardworks Ltd 2009 Interference of sound What caused the pattern of loud and quiet spots? This is constructive interference. Both speakers produce identical sounds. When the sound from one speaker meets the sound from the other, the two waves interact with each other. This is known as interference. If the waves are in phase, they reinforce each other. If the waves are out of phase, they cancel each other out. This is destructive interference. + = + =
29 of 43© Boardworks Ltd 2009 Using interference The phenomenon of interference has many uses. Some car manufacturers put microphones into the engine bay, delay the sound by half a wave, and play it back to the passengers. The effect of this is that the noise of the engine is cancelled out, and the journey is much quieter. The same idea is used on helicopters, to remove the incredibly loud rotor noise and allow the pilot to communicate more effectively.
30 of 43© Boardworks Ltd 2009 Interference patterns in light
31 of 43© Boardworks Ltd 2009 Understanding interference
32 of 43© Boardworks Ltd 2009 Polarization Electromagnetic waves (such as light) ‘oscillate’ in three dimensions, shown by the green and the blue waves below: When these waves pass through a polarizing filter, only one plane is able to get through (the blue one in this case). The other parts of the wave are blocked. This is polarization. If another slit at 90 degrees is placed in the waves path, then none of the wave can get through. polarized wavepolarizing filterunpolarized wave
33 of 43© Boardworks Ltd 2009 Polarization of light On a sunny day, light reflecting from a water surface can cause glare. However, only light in one plane is reflected from the flat surface of the water. In 1938, Edwin Land developed the polaroid lens. Today they are used in most sunglasses, as well as microscopes and LCD screens, but they were originally used to help with fishing! Polaroid sunglasses are designed to block out this light, making it much easier to see the fish clearly.
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35 of 43© Boardworks Ltd 2009 AM radio
36 of 43© Boardworks Ltd 2009 Sending signals into space Some radio waves follow the contours of the Earth. These are called ground waves. These are typically waves with a frequency of 3–3,000 kHz because their long wavelength means they diffract around the curves of the Earth’s surface. Radio waves that refract through the ionosphere and return to Earth, giving the impression of reflection, are called sky waves. Their frequencies are 3–30 MHz. At frequencies over 30 MHz, radio waves can pass completely through the ionosphere and into space. These are called space waves.
37 of 43© Boardworks Ltd 2009 Communicating with satellites Space waves are used to communicate with satellites. These waves are known as microwaves because of their short wavelength compared to other radio waves. Radio waves with a frequency of greater than 30 GHz are easily absorbed and scattered by dust and water in the atmosphere, so they have little practical use. Microwaves only diffract by a small amount due to this short wavelength, so they can be sent to a satellite in a thin beam to save energy. The satellite can then send a second beam back to earth in response.
38 of 43© Boardworks Ltd 2009 Identifying wave behaviour What is happening to the radio signals in this picture?
39 of 43© Boardworks Ltd 2009 Characteristics of radio waves
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41 of 43© Boardworks Ltd 2009 Glossary
42 of 43© Boardworks Ltd 2009 Anagrams
43 of 43© Boardworks Ltd 2009 Multiple-choice quiz
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