Waves & Radiation: Transverse, Longitudinal Waves & Electromagnetic Spectrum, Summaries of Physics

Download Waves & Radiation: Transverse, Longitudinal Waves & Electromagnetic Spectrum and more Summaries Physics in PDF only on Docsity! NGN5 Unit 3 - Waves and Radiation [ LDUNCANRIG SECONDARY SCHOOL |] Physics Departmen! Summary Notes Crest Amplitude A 2 3.1 Wave Characteristics A wave transfers energy from one place to another. Waves are made from particles vibrating. In this unit we will consider two types of waves, transverse waves and longitudinal waves. A transverse wave is one in which the vibrations making up the waves are at right angles to the direction of the wave. Direction of vibration of particles. Direction of wave Waves on a rope, water waves, light waves and all members of the electromagnetic spectrum are transverse waves. A longitudinal wave is one in which the vibrations are in the same direction as the wave travels. Direction of vibration of particles Direction of wave Sound is a longitudinal wave. N4 5 A wave analogy Suppose a goods train is coming out of a tunnel at a speed of 10 metres per second and the trucks are 5 metres long. 5 metres 10 m/s Every second, 10 metres of train exits the tunnel. Every second 2carriages exit the tunnel. The frequency of the carriages is 2 per second. Each “carriage length” is 5 metres. The speed of the train is 10 metres per second. The link between speed, frequency of the carriages and carriage length is: Speed = frequency x carriage length If the carriage length were replaced with wavelengths the equation becomes: Speed = frequency x wavelength v = f x λ A mathematical derivation If you are watching waves in a pond, you can time how long it takes a whole wavelength to pass a point by timing from one crest to the next. You can also measure the distance from one crest to the next to find the wavelength. Using these measurements the speed of the wave can be calculated from: v Since you measured one wave, d = λ (the wavelength) and t = T (the period). Therefore, But we know that Therefore, So N5 6 3.2 Sound Diffraction Waves are able to bend around obstacles. This bending of waves around corners is called diffraction. Long wavelength waves diffract more than short wavelength waves. Long wavelength Short wavelength As longer wavelengths diffract more than short wavelengths, radio transmissions from ground stations are more likely to be received than shorter wavelength TV waves. Sound waves can only be transmitted through solids, liquids and gases. Sound cannot travel through a vacuum as a vacuum does not contain particles. The speed of sound in air varies but is approximately 340 metres per second which is much slower than the speed of light in air at 300 000 000 metres per second.. Speed of sound in air We will consider two methods to measure the speed of sound in air. Method 1 An observer with a stopwatch stands a long distance away from a starter at an athletics meeting. When the starter fires their starting pistol, the observer sees the flash from the gun instantly and hears the sound after a short delay. The observer starts their stopwatch as soon as they see the flash and stop it when they hear the sound. Using the distance, d, travelled by the sound (which must be measured) and the time, t, for the sound to travel to them (from the stopwatch) the speed of sound can be calculated from: d = vt. d This is not a particularly accurate method as it relies on human reaction time. N5 N4 7 Speed of sound in air Method 2 d Hammer and microphones metal block The distance, d, is measured with a metre stick. The hammer is struck against the block. As the sound reaches the first microphone the timer is started, when it reaches the second microphone the timer is stopped. The equation is used to calculate the speed again. This is a much more accurate method. The exact value for the speed of sound in air can vary, however it is around 340 m/s. Electronic timer Amplitude and frequency An oscilloscope can be used to analyse wave patterns and what effect changing certain properties has on the shape of a wave. Waves with a low frequency would be low pitched and waves with a high frequency would be high pitched. Waves with a small amplitude would be quiet and those with a large amplitude would be loud. original sound frequency lower frequency higher no sound LOUD SOUND quiet sound N4 N4 10 Sound reproduction and noise cancellation Sound is an analogue signal. This means that it varies continuously over a range of values. Most recording technology nowadays uses digital technology. Digital signals can be one of two values with nothing in between. Analogue to digital converters are used to process the sound signal so that it can be transmitted easier, then a digital to analogue device allows the sound to be reproduced faithfully at the other end. If two waves travelling in opposite directions were to meet, the result would be that they cancel each other out. The same would happen any time a crest of one wave meets a trough of another. This effect is called interference of waves. We can make use of this effect in noise cancelling technology. Noise cancelling headphones use a technique called “active noise control”, to block out background sound to allow you to hear the sound fed through the headphones more clearly. For example:  blocking out aircraft engines when you are trying to listen to music on your MP3 player.  Blocking out rotor noise on a helicopter to allow you to speak to someone else inside the helicopter. This cancellation is done in an electronic circuit. The active noise cancellation works in the circuit by detecting the unwanted outside noise signal and generating the exact same noise signal but the inverse of it. Since the two signals are equal but opposite they cancel each other out. Since the circuit requires energy to work, the noise cancelling headsets must have their own power source such as a battery to work. Do some research of your own to find out about noise cancellation in Humvees. + = N4 11 3.3 Electromagnetic spectrum Electromagnetic spectrum There are a number of waves which travel at the speed of light. They are all part of the electromagnetic spectrum. These waves are all transverse waves and travel at 300 000 000 m/s (3 x 108m/s) in a vacuum. The different parts of the electromagnetic spectrum differ in wavelength and frequency Gamma rays X-rays Ultraviolet Visible light Increasing wavelength Infrared increasing frequency Microwaves TV and Radio The different parts of the electromagnetic spectrum can also be distinguished by their energy. Higher frequency electromagnetic radiation has a greater amount of energy than lower frequency electromagnetic radiation. Some information on each part of the spectrum is given below Type of e-m radiation Typical source Application Detector Possible hazard Radio & TV Electrical antennae Telecommunications Aerial Potential increased cancer risk Microwaves Cosmic sources, magnetron Cooking, Telecommunications Diode probe Heating of body tissues Infra-red Heat-emitting objects Thermograms Phototransistor, blackened thermometer Over heating of body tissues causing dehydration Visible light Stars Vision Eye, photographic film Intense light can damage the retina Ultraviolet Sunlight Treating skin conditions Fluorescent paint Skin cancer X-rays X-ray tube, cosmic sources Medical imaging Photographic plates Destroys cells which can lead to cancer Gamma rays Nuclear decay Treating tumours Geiger–Müller tube and counter Destroys cells which can lead to cancer N4 N5 12 Light Reflection The law of reflection states that the angle of incidence is equal to the angle of reflection. Remember that all angles in a ray diagram are measured from the normal. Refraction At the boundary between different types of materials, the speed of the light wave changes. This results in a change in wavelength, and can often cause the direction of a wave to change. The change in light speed when going from one medium into another is known as refraction. This effect is used in lenses. Above a certain angle of incidence, refraction no longer occurs, and instead the light wave is reflected back into the medium where it came from. This is known as total internal reflection. The minimium angle of incidence that causes the wave to undergo total internal reflection is called the critical angle. Total internal reflection is used in optical fibres. Optical fibres can be used for communication or in medical applications to allow doctors to see into the body. One bundle of fibres carries light into the body whilst another carries the light back out of the body. This instrument is known as an endoscope. N5 15 Long-sighted Short-sighted People who are long-sighted can focus on objects far away People who are short-sighted can focus on objects close up When people who are long-sighted try to look at an object close up the lens of their eye cannot adjust enough to focus the rays onto the retina of the eye. The rays focus behind the retina, like this: When people who are short-sighted try to look at an object far away the lens of their eye cannot adjust enough to focus the rays onto the retina of the eye. The rays focus in front of the retina, like this: This means that they see an unfocused (blurry) image. This means that they see an unfocused (blurry) image. To correct this problem we need a lens, which will converge the rays before they enter the eye – a convex lens will do this, like this: To correct this problem we need a lens, which will diverge the rays before they enter the eye – a concave lens will do this, like this: A convex lens can be used to correct long- sightedness. A concave lens can be used to correct short-sightedness. 16 3.4 Nuclear radiation The atom The above diagram shows a simple model of the atom (it is not to scale). Nuclear radiation Nuclear radiation is so called because it originates in the nucleus of an atom. Nuclear radiation can come from natural sources such as cosmic rays and naturally occurring radioactive materials such as uranium. It can also come from artificial sources such as man-made radioisotopes such as plutonium. Nuclear radiation can be used in medicine to sterilise instruments by killing germs and bacteria. It can also be used to kill the cells which make up a cancerous tumour, however care must be taken in this procedure as nuclear radiation can also kill or damage healthy cells . Nuclear radiation can also be used to examine the body through using radioactive materials in something called a tracer. This is a substance that is injected into the body and detected to analyse its progress through the body. We will look at three different types of nuclear radiation: alpha α beta β gamma γ type of radiation nature Minimum absorber alpha two protons and two neutrons (helium nucleus) sheet of paper, few centimetres of air beta fast-moving electron few cm of aluminium gamma electromagnetic wave Several cm of lead N4 N4 N5 proton neutron electron nucleus 17 Ionisation The process by which nuclear radiation damages cells is known as ionisation. This is where electrons are removed from or added to an atom to leave a charged particle called an ion. If the atom gains an electron it has an overall negative charge and if it loses an electron it has an overall positive charge. Alpha radiation causes more ionisation than beta or gamma radiation. Background radiation Nuclear radiation is always present in our environment. This is known as background radiation. This can come from natural sources e.g. radon gas, cosmic rays or from man-made sources e.g. nuclear fallout from weapons testing and accidents at nuclear power stations. Absorbed dose and equivalent dose The amount of energy received by a substance per unit mass is known as the absorbed dose. This can be calculated by using the equation where D is the absorbed dose in grays (Gy) E is the energy in joules (J) and m is the mass in kilograms (kg) This does not tell the whole story of how a person would be affected by nuclear radiation. It does not take into account the type of radiation encountered. The equivalent dose allows us to take the type of radiation into account. It is calculated by using the equation where H is the equivalent dose in sieverts (Sv) D is the absorbed dose in grays (Gy) and wR is the radiation weighting factor. Alpha radiation has a radiation weighting factor of 20, whereas beta and gamma radiation both have a radiation weighting factor of 1. N5 20 Energy and nuclear radiation There are two ways in which nuclear radiation can be used to generate energy. 1. Fission If a neutron is fired at a uranium 235 nucleus, it becomes unstable and separates into two smaller nuclei and releases some more neutrons. The mass of these nuclei and neutrons is slightly less than the mass of the original nucleus and neutron. Using the equation E= mc2 , where m is the mass lost and c is the speed of light, we can calculate the energy released in each fission reaction. If the neutrons that are released are captured by other uranium 235 nuclei, the process can be repeated. This is known as a chain reaction. In nuclear power stations, the energy released is used to heat water to produce steam to turn a turbine. This drives a generator which produces electricity. N5 21 Energy and nuclear radiation (continued) 2. Fusion Fusion is a process where two smaller nuclei are combined to create a larger nucleus. Again, the total mass of the products of this reaction is less than the total mass before the reaction, allowing us to calculate the energy released by using the equation E = mc2. It is thought that fusion would allow us to generate far more energy than fission at much lower risk, however we are currently unable to do this economically. Fusion is the process in which stars convert fuel to light and heat. Note: It is important that you do not misspell fusion or fission! Using nuclear radiation to produce electricity reduces the amount of carbon dioxide released into the atmosphere. Carbon dioxide is a greenhouse gas which helps contribute to global warming. However, nuclear reactors produce radioactive waste which needs to be stored for thousands of years before it is safe. N5