An introduction to waves
Light travels as waves. Waves can be described by their amplitude, wavelength and frequency. The speed of a wave can be calculated from its frequency and wavelength. Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd. The wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn.
Some waves must travel through a substance. The substance is known as the medium, and it can be solid, liquid or gas. Sound waves and seismic waves are like this. They must travel through a medium. It is the medium that vibrates as the waves travel through. Other waves do not need to travel through a substance. They may be able to travel through a medium, but they don't have to. Visible light, infrared rays, microwaves and other types of electromagnetic radiation are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.
An introduction to waves
As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top and bottom of a wave. The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.
The frequency of a wave is the number of waves produced by a source each second. It is also the number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz). It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies. For example, most people cannot hear a high-pitched sound above 20kHz, radio stations broadcast radio waves with frequencies of about 100MHz, while most wireless computer networks operate at 2.4GHz.
The speed of a wave - its wave speed - is related to its frequency and wavelength, according to this equation: wave speed (metre per second) = frequency (hertz) × wavelength (metre) For example, a wave with a frequency of 100Hz and a wavelength of 2m travels at 100 × 2 = 200m/s.
Amplitude and wavelength
The electromagnetic spectrum
Electromagnetic radiation travels as waves and transfers energy from one place to another. All electromagnetic waves can travel through a vacuum, and they all travel at the same speed in a vacuum.
The electromagnetic spectrum is a continuous range of wavelengths. The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their wavelength and frequency.
White light can be split up using a prism to form a spectrum. A prism is a block of glass with a triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.
Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic radiation, some with longer wavelengths than visible light and some with shorter wavelengths than visible light.
The main types of electromagnetic radiation
Highest frequency and shortest wavelength
- Gamma Radiation is typically used for killing cancer cells.
- X-Rays are typically used for medical images of bones.
- Ultraviolet is typically used for detecting forged bank notes by fluorescence.
- Visible light is typically used for seeing.
- Infared is typically used for optical fibre communication.
- Microwaves are typically used for cooking.
- Radio waves are typically used for television signals.
Lowest frequency and longest wavelength
Gamma radiation and X-rays
Gamma waves have a very high frequency. Gamma radiation cannot be seen or felt. It mostly passes through skin and soft tissue, but some of it is absorbed by cells. Gamma radiation is used, among other things, to sterilise surgical instruments, kill harmful bacteria in food and kill cancer cells (note that lower doses of gamma radiation could lead to cells becoming cancerous).
X-rays have a lower frequency than gamma radiation. Like gamma rays, they cannot be seen or felt. X-rays mostly pass through skin and soft tissue, but they do not easily pass through bone or metal.
X-rays are used to produce photographs of bones to check for damage such as fractures. They are also used in industry to check metal components and welds for cracks or other damage.
Lower doses of X-rays can cause cells to become cancerous, so precautions are taken in hospitals to limit the dose received by patients and staff when X-ray photographs are taken.
Ultraviolet radiation and infrared radiation
Ultraviolet radiation is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal cells to become cancerous.
Ultraviolet radiation is used in sun beds, security pens and fluorescent lights (coatings inside the tube or bulb absorb the ultraviolet light and re-emit it as visible light).
Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and grills. It is also used for television remote controls and in optical fibre communications.
Microwave radiation has lower frequencies and longer wavelengths than visible light. Microwaves with certain wavelengths are absorbed by water molecules and can be used for cooking. Water in the food absorbs the microwave radiation, which causes the water to heat up and cook the food. The water in living cells can also absorb microwave radiation. As a result, they can be killed or damaged by the heat released.
Microwave radiation can also be used to transmit signals such as mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones in their range. Certain microwave radiation wavelengths pass through the Earth's atmosphere and can be used to transmit information to and from satellites in orbit.
Television and radio
Radio waves have lower frequencies and longer wavelengths than microwaves. They are used to transmit television and radio programmes. Television uses higher frequencies than radio.
A radio programme receiver does not need to be directly in view of the transmitter to receive programme signals. For low frequency radio waves diffraction can allow them to be received behind hills, although repeater stations are often used to improve the quality of the signals.
The lowest frequency radio waves are also reflected from an electrically charged layer of the upper atmosphere, called the Ionosphere. This means that they can reach receivers that are not in the line of sight because of the curvature of the Earth's surface.
Microwaves and radio waves in the atmosphere
Origins of the Universe
The foremost theory of the origin of the universe is the Big Bang theory. It suggests that the universe began several billion years ago in an explosion that caused it to expand, and to continue expanding. Some of the evidence for the Big Bang comes from studying the red shift of light received from distant galaxies. Telescopes allow us to observe the universe.
Scientists have gathered a lot of evidence and information about the universe. They have used their observations to develop a theory called the Big Bang. The theory states that originally all the matter in the universe was concentrated into a single incredibly tiny point. This began to enlarge rapidly in a hot explosion, and it is still expanding today. This explosion is called the Big Bang, and happened about 13.6 billion years ago (that's 13,600,000,000 years using the scientific definition of 1 billion = 1,000 million).
Astronomers have even detected a cosmic background radiation that is thought to be the heat left over from the original explosion.
Evidence of the Big Bang
There are two key pieces of evidence for Big Bang theory. These are red shift and the Cosmic Microwave Background radiation.
You may have noticed that when an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away. This effect, where there is a change in frequency and wavelength, is called the Doppler effect. It happens with any wave source that moves relative to an observer. This happens with light too. Our sun contains helium. We know this because there are black lines in the spectrum of the light from the sun, where helium has absorbed light. These lines form the absorption spectrum for helium.
Spectrum of the sun
When we look at the spectrum of a distant star, the absorption spectrum is there, but the pattern of lines has moved towards the red end of the spectrum, as you can see below.
Spectrum of a distant star
This is called red shift. It is a change in frequency of the position of the lines. Astronomers have found that the further from us a star is the more its light is red shifted. This tells us that distant galaxies are moving away from us, and that the further a galaxy is the faster it is moving away. Since we cannot assume that we have a special place in the universe this is evidence for a generally expanding universe. It suggests that everything is moving away from everything else. The Big Bang theory says that this expansion started billions of years ago with an explosion.
Cosmic Microwave Background radiation
Scientists discovered that there are microwaves coming from every direction in space. Big Bang theory says this is energy created at the beginning of the universe, just after the Big Bang, and that has been travelling through space ever since. A satellite called COBE has mapped the background microwave radiation of the universe as we see it. Big Bang theorists are still working on the interpretation of this evidence.
The light from other galaxies is red-shifted meaning that the light from other galaxies is red-shifted.
The further away the galaxy, the more its light is red-shifted meaning that the most likely explanation is that the whole universe is expanding. This supports the theory that the start of the universe could have been from a single explosion.
Cosmic Microwave Background meaning that the relatively uniform background radiation is the remains of energy created just after the Big Bang.
Distant stars and galaxies are too far away for us to reach. We cannot go to them to study them. So everything we know about distant stars and galaxies comes from analysing the radiation they produce. Telescopes are devices used to observe the universe. There are many different types and some are even sited in space.
Optical telescopes observe visible light from space. Small ones allow amateurs to view the night sky relatively cheaply but there are very large optical telescopes sited around the world for professional astronomers to use. Optical telescopes on the ground have some disadvantages: they can only be used at night and they cannot be used if the weather is poor or cloudy.
Radio telescopes detect radio waves coming from space. Although they are usually very large and expensive, these telescopes have an advantage over optical telescopes. They can be used in bad weather because the radio waves are not blocked by clouds as they pass through the atmosphere. Radio telescopes can also be used in the daytime as well as at night.
X-rays are partly blocked by the Earth's atmosphere and so X-ray telescopes need to be at high altitude or flown in balloons. Objects in the universe emit other electromagnetic radiation such as infrared, X-rays and gamma rays. These are all blocked by the Earth's atmosphere, but can be detected by telescopes placed in orbit round the Earth.
Telescopes in space can observe the whole sky and they can operate both night and day. However, they are difficult and expensive to launch and maintain. If anything goes wrong, only astronauts can fix them.
An atom of any given element consists of a nucleus containing a number of protons and neutrons. The nucleus is surrounded by electrons.
The half-life of a radioactive isotope is the time taken for half its radioactive atoms to decay. There are three main types of radiation, called alpha, beta and gamma radiation, which all have different properties. Radiation can damage cells and make them cancerous. Very high doses of radiation can kill cells. It can be detected using photographic film or a Geiger-Muller tube. Radiation badges are used to monitor the level of radiation that people who work with radioactive sources are exposed to.
Radiation has many practical uses. It can be used in medicine to trace where certain chemicals collect in the body, indicating disease, and also in industry, where it can be used to control measuring equipment.
Atoms and isotopes
Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom, and the electrons are arranged in energy levels or shells around the nucleus. All the atoms of a given element have the same number of protons and electrons. However, the number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element. The diagram shows three hydrogen isotopes.
The different isotopes of an element have identical chemical properties. Some isotopes, however, are radioactive. This means that they give out radiation from their nuclei. This happens all the time, whatever is done to the substance. For example, the radiation is still given out if the substance is cooled down in a freezer, or takes part in a chemical reaction.
Structure of the atom
Types of radiation
There are three main types of radiation emitted from radioactive atoms. These are alpha, beta and gamma radiation.
Alpha radiation consists of alpha particles. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons.
Helium atom (2 protons, Alpha particle (2 protons,
2 neutrons,2 electrons) 2 neutrons, 0 electrons)
Beta and Gamma radiation
Beta radiation consists of high energy electrons emitted from the nucleus. These electrons have not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. The electron then shoots out of the nucleus at high speed.
Gamma radiation is very short wavelength - high frequency - electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and X-rays, which can travel long distances.
Penetrating properties of radiation
Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air and gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.
Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper. Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium. Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.
Penetrative properties of different types of radia
Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral. This means that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is not deflected.
Remember that opposite charges attract. Beta particles are negatively charged so they will be attracted towards a positively charged plate. And positive alpha particles will be attracted towards a negatively charged plate.
Because they consist of charged particles, alpha radiation and beta radiation can also be deflected by magnetic fields. Just as with electric fields, gamma radiation is not deflected by magnetic fields.
Human senses cannot detect radiation, so we need equipment to do this.
Photographic film goes darker when it absorbs radiation, just like it does when it absorbs visible light. The more radiation the film absorbs, the darker it is when it is developed. People who work with radiation wear film badges, which are checked regularly to monitor the levels of radiation absorbed. The diagram shows a typical radiation badge when it is closed and what the inside looks like when it is opened.
There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.
The Geiger-Muller tube detects radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate. The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Muller tube is absorbing.
Hazards of radiation
When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems. The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms. The hazard symbol is shown on containers of radioactive substances to warn of the danger. The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.
If the radioactive source is inside the body, perhaps after being swallowed or breathed in alpha radiation is the most dangerous because it is easily absorbed by cells, but beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it. If the radioactive source is outside the body: alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body, but beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside. These effects are opposites and make sure you get them the right way around.
The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.
It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope.
There are two definitions of half-life, but they mean essentially the same thing: the time it takes for the number of nuclei of the isotope in a sample to halve or the time it takes for the count rate from a sample containing the isotope to fall to half its starting level.
Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,715 years, but the half-life of francium-223 is just 20 minutes.
It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time. The graph shows the decay curve for a radioactive substance. The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days. In the next two days, it drops from 40 to 20 - it halves. In the two days after that, it drops from 20 to 10 - it halves again - and so on. The decay curve for a radioactive substance:
Radiation is used in smoke detectors,for sterilising medical instruments,for killing cancer cells, for dating rocks and materials such as archaeological finds, in chemical tracers to help with medical diagnosis and for measuring the thickness of materials in, for example, a paper factory.
Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body. When a radioactive chemical is used in this way it is not normally harmful, because: it has a short half-life and so decays before it can do much damage and it is not poisonous. Emitters of beta radiation or gamma radiation are used because these types of radiation readily pass out of the body, and they are less likely to be absorbed by cells than alpha radiation.
Radiation is used in industry in detectors that monitor and control the thickness of materials such as paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material.
Optical fibres can carry information coded in light or infrared signals. Optical fibres can carry more information than an ordinary cable of the same diameter. Analogue signals vary continuously in amplitude, frequency or both. Digital signals are a series of pulses with two states – on or off. Digital signals carry more information per second than analogue signals, and they maintain their quality better over long distances. An optical fibre is a thin rod of high-quality glass. Very little light is absorbed by the glass. Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end.
Information such as computer data and telephone calls can be converted into electrical signals. These can be carried through cables, or transmitted as microwaves or radio waves. However, the information can also be converted into either visible light signals or infrared signals, and transmitted by optical fibres. Optical fibres can carry more information than an ordinary cable of the same thickness. The signals in optical fibres do not weaken as much over long distances as the signals in ordinary cables.
Recognising analogue signals
Music and speech vary continuously in frequency and amplitude. In the same way, analogue signals can vary in frequency, amplitude or both. You may have heard of FM radio and AM radio – Frequency Modulated radio and Amplitude Modulated radio. The diagram shows a typical oscilloscope trace of an analogue signal.
Oscilloscope trace of an analogue signal.
Recognising digital signals
Digital signals are a series of pulses consisting of just two states, ON (1) or OFF (0). There are no values in between. DAB radio is Digital Audio Broadcast radio – it is transmitted as digital signals. The diagram shows a typical oscilloscope trace of a digital signal. Digital signals carry more information per second than analogue signals. This is the same whether optical fibres, cables or radio waves are used. Digital signals maintain their quality over long distances better than analogue signals. You will notice far less noise and crackle from a DAB radio programme than in an ordinary FM or AM radio programme.
Oscilloscope trace of a digital signal.
Analogue versus digital
All signals become weaker as they travel long distances, and they may also pick up random extra signals. This is called noise, and it is heard as crackles and hiss on radio programmes. Noise may also cause an internet connection to drop or slow down, as the modem tries to compensate.
Noise adds extra random information to analogue signals. Each time the signal is amplified, the noise is also amplified. Gradually, the signal becomes less and less like the original signal. Eventually, it may be impossible to make out the music in a radio broadcast against the background noise, for example.
Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the amplitude of the ON states. As a result, the electronics in the amplifiers can ignore the noise and it does not get passed along. This means that the quality of the signal is maintained, which is one reason why television and radio broadcasters are gradually changing from analogue to digital transmissions. They can also squeeze in more programmes, because digital signals can carry more information per second than analogue signals.