# 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.

## Presentation on theme: "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."— Presentation transcript:

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? Photo credit (left and right): © 2009 Jupiterimages Corporation All true waves move or propagate through space, therefore the ripples on a sand dune are not waves.

Representing waves There are two main ways of representing a wave on a graph. graphing an oscillation in time: amplitude y t period 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.

Representing waves There are two main ways of representing a wave on a graph. graphing an oscillation in space: amplitude y x wavelength 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.

y t y x Waves in time and space
These two waveforms look the same, but they each give different information about the wave they represent. amplitude y t period amplitude y x wavelength Always label your axes!

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. Photo credit (top right): © Shutterstock 2009, Nola Rin Photo credit (bottom left): ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY

Using an oscilloscope Teacher notes
This activity could be used as a whole-class demonstration of how an oscilloscope and a signal generator are operated. The controls on both have been simplified to include only the most important features. Students could be asked to suggest suitable settings on the oscilloscope to display a signal of a chosen frequency and amplitude from the signal generator. It should be made clear to students that an oscilloscope displays a waveform based on a varying voltage signal; the image on the screen is not a true wave.

Wave speed A series of surface waves is moving across a pond. The peaks of the waves are 20 cm apart. 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? wave speed = number of waves passing per second × wavelength Photo credit: © Shutterstock 2009, Luca Bertolli 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.

Understanding waves and waveforms

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? 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. During an earthquake, transverse and longitudinal waves travel through solid rock away from the epicentre. Teacher notes This should encourage discussion. Sound and pressure waves are equivalent, but we usually call something a ‘sound wave’ when it is audible to the human ear, so it usually applies to waves in air, or occasionally waves in water. The frequency of the wave is another factor. Many of the pressure waves during an earthquake are of such a low frequency that they are undetected by the human ear. See the Boardworks GCSE Science (Physics) ‘Wave Properties’ presentation for more information on transverse and longitudinal waves. Photo credit: © Shutterstock 2009, Vladimir Zivkovic During an earthquake, a seismometer records the movements of the ground by tracing its oscillations against time. What is the difference between a sound wave and a pressure wave?

Pitch and loudness 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. Which of these traces shows the louder sound and which shows the sound with the higher pitch? higher pitch louder Not every note of the same pitch sounds the same. The waveform of a wave determines the quality of the sound.

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. 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. Photo credit: © Shutterstock 2009, Ritu Manoj Jethani Animals are thought to do this naturally, but it can also be done by a computer, such as in prenatal scanning.

Sonar distance = speed × time
One practical application of ultrasound is sonar. 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. Knowing the speed of sound in water, it is then possible to calculate the distance the sound has travelled using this equation: distance = speed × time

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

Sound imaging calculations

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? 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. The reflected waves are detected by a receiver. A computer turns the distance and intensities of these echoes into a 2-dimensional image. Photo credit: © Shutterstock 2009, Ngo Thye Aun

Further uses for ultrasound
Ultrasound has many uses in industry as well as medicine. Jewellers and watch repairers use ultrasound to clean delicate items. The dirt is shaken off by the air vibrations, leaving the mechanism unharmed. 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. Photo credit (top): © 2009 Jupiterimages Corporation Photo credit (bottom): © Shutterstock 2009, Carlos Caetano

Wave media

Sound waves

What is diffraction?

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 smaller than the wavelength, the waves cannot pass through the gap at all. 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.

Using diffraction Diffraction is useful for long distance communications. 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. Photo credit: © Shutterstock 2009, Hugo de Wolf Higher frequency waves are diffracted much less. Television signals, for example, have a much shorter range.

Understanding diffraction

What is interference?

Interference of sound What caused the pattern of loud and quiet spots?
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 constructive interference. This is destructive interference.

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. Photo credit: © 2009 Jupiterimages Corporation

Interference patterns in light
Teacher notes Monochromatic light is light of a single wavelength. The interference pattern that results from shining monochromatic light through a double slit depends on the wavelength of the light. Students could be asked what they would see if white light were used instead of red light. Photo credit (Sun): © Shutterstock 2009, lucwa Photo credit (Young’s fringes): GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY

Understanding interference

Polarization Electromagnetic waves (such as light) ‘oscillate’ in three dimensions, shown by the green and the blue waves below: unpolarized wave polarized wave polarizing filter 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.

Polarization of light 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! 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. Photo credit: © Shutterstock 2009, Antonov Roman Polaroid sunglasses are designed to block out this light, making it much easier to see the fish clearly.

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. Teacher notes See the ‘Reflection and Refraction’ presentation for more information on refraction. At frequencies over 30 MHz, radio waves can pass completely through the ionosphere and into space. These are called space waves. 36

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. 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. Teacher notes See the ‘Orbits’ presentation for more information on the different types of satellite orbit. 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.

Identifying wave behaviour
What is happening to the radio signals in this picture? Teacher notes This illustration contains several examples of wave phenomena, for example: Long-, medium- and short wave radio signals are transmitted from a central mast. The radio waves propagate through space. The long wave signal propagates as a ground wave and is received at a television aerial. The medium wave signal propagates through the atmosphere until it reaches the ionosphere. Here it refracts, changing direction and propagating back down through the atmosphere to be received as a radio station. The refraction through the ionosphere gives the appearance of reflection. This is a sky wave. The short wave signal propagates through the ionosphere to an orbiting satellite. It is a space wave. Another medium wave signal is being transmitted by the pirate radio station. This is on the same frequency as the radio signal from the central mast, so the two signals interfere, and the van driver cannot tune his radio properly.

39

Glossary Glossary amplitude – The maximum distance any point in a wave moves from its mean position. amplitude modulation (AM) – A form of radio transmission in which the information signal is encoded by creating variations in the amplitude of a carrier wave. carrier wave – A wave that can be modified by an information signal to encode the information for radio transmission. constructive interference – Two (or more) waves combine with their peaks and troughs in phase with each other, to produce a resultant wave with greater amplitude than the original waves. destructive interference – Two (or more) waves combine with their peaks and troughs out of phase with each other, to produce a resultant wave with a smaller amplitude than the original waves. diffraction – The spreading of a wave after it moves through a gap or past an obstacle. frequency – The number of waves passing a point each second. It is measured in hertz (Hz). ground wave – A component of a radio transmission that travels around the contours of the Earth, close to the ground. interference – The interaction of two or more waves of the same type when superimposed on each other. medium – The material through which a wave propagates. microphone – A device that converts sound waves into an electrical signal. monochromatic light – Light consisting of waves of one specific wavelength. oscilloscope – An instrument that displays an electrical signal on a screen in real time. pitch – The audible characteristic of a sound that is determined by its frequency. A higher frequency sound wave has a higher pitch. polarization – A property of transverse waves, which describes the plane in which the wave oscillates. Polarization can also refer to the process of restricting a wave to one plane. polaroid lens – A lens that polarizes light, such as that used in sunglasses to reduce glare from horizontal surfaces. propagation – A term for the movement of waves through a medium, which expresses the idea that the wave and its associated energy travel forward while there is no bulk movement of the medium. reflection – When a wave changes direction by ‘bouncing’ off a surface or a boundary between two media of different densities. refraction – When a wave changes direction when it passes through a boundary into a new medium (or zone) where its speed of propagation is different from that in the old. scattering – When a travelling wave is deflected from its course by one or more obstacles, such as dust particles in the atmosphere. signal generator – An instrument designed to produce a varying AS output with adjustable amplitude, frequency and waveform. sky wave – A radio wave with a frequency of between 3 and 30 MHz, which can be transmitted a greater distance than ground waves via refraction through the ionosphere. Sonar – The use of sound waves underwater for imaging and echolocation (originally ‘Sound Navigation And Ranging’) space wave – A radio wave with a frequency of over 30 MHz, which can be transmitted through the ionosphere into space. ultrasound – Sound above the frequency range of human hearing (>20 kHz). wavelength – The distance between two matching points on neighbouring waves. wave speed – The speed at which a wave propagates. Young’s fringes – The interference pattern observed when monochromatic light is passed through a pair of slits of similar width to the wavelength of the light.

Anagrams

Multiple-choice quiz Teacher notes
This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of waves. The questions can be skipped through without answering by clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by completion on the IWB.

Download ppt "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."

Similar presentations