- Created by: Abi Hiks
- Created on: 20-02-11 19:33
Principles of Waves
A wave is a way of transferring energy from one place to another
Amplitude - Height of a wave
Wavelength (metres) -
Frequency (Hertz) - Number of complete waves passing any point in one second
WAVE EQUATION: WAVESPEED = FREQUENCY X WAVELENGTH
WAVES CAN BE ABSORBED, REFLECTED, REFRACTED AND DEFRACTED
The Electromagnetic Spectrum
It is a range of radiation. In order, from the top; lowest frequency, lowest danger but biggest wavelength, to the bottom; biggest frequency, biggest danger but smallest wavelength. They all travel at the SAME speed in a vaccum (or space)
Radio Waves (TV and FM signals)
Micro Waves (Communication- eg. Mobiles. Heating up food)
Infa-red (Remote Controls)
Ultra Violet (For security- checking for invisible lighting, tanning)
X-ray (To take x-rays)
Gamma Ray (used to kill bacteria in food, radio therapy for cancer patients)
Fibre Optics - Using glass fibres rather than copper wires to improve communication.
Pulses of red light travel down the fibres due to a process called 'Total internal reflection', losing VERY LITTLE energy on the way.
ADVANTAGES- less booster stations required along its journey, more secure and less prone to interference.
Analogue and Digital Signals
Analogue signals - Music and speech vary continuously in frequency and amplitude. In the same way, analogue signals can vary in frequency, amplitude or both. There are two types of radio; FM radio and AM radio – Frequency Modulated radio and Amplitude Modulated radio.
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.
ADVANTAGES OF DIGITAL SIGNALS- carry more information per second than analogue signals (this is the same whether optical fibres, cables or radio waves are used). 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)
Why do Digital Signals maintain their quality bett
Noise- All signals become weaker as they travel long distances, and they may also pick up random extra signals (CALLED NOISE). 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.
Analogue Signals- 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. Eg. Eventually, it may be impossible to make out the music in a radio broadcast against the background noise.
Digital Signals- 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 - MEANING the quality of the signal is maintained, hence television and radio broadcasters are gradually changing from analogue to digital transmissions. Also, they can squeeze in more programmes, because digital signals can carry more information per second than analogue signals.
Isotopes- All atoms of a certain element have the same number of protons and electrons, HOWEVER, the number of neutrons they have may vary. Atoms of the same element that have different numbers of neutrons are called ISOTOPES of that element. Some isotopes are very RADIOACTIVE, which means they give out radiation from their nuclei. This is done constantly, no matter what happens to the substance (freezing, ect.)
MAIN TYPES OF RADIATION EMITTED FROM RADIOACTIVE ATOMS:
Alpha- consists of alpha particles, 2 protons, 2 neutrons, 0 electrons
Beta- consists of HIGH ENERGY electrons emitted from the nucleus (formed when a neutron splits into a proton and an electron) - Electron shoots out of the nucleus at high speed.
Gamma- SHORT WAVELENGTH, HIGH FREQUENCY = VERY DANGEROUS
Penetrating Properties of Radiation
Radiation can be absorbed by substances in its path. Eg. Alpha radiation travels 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- LEAST PENETRATING; stopped/absorbed by a sheet of paper
Beta- Can penetrate air and paper; stopped by a thin sheet of aluminium
Gamma- MOST PENETRATING; Even small levels can penetrate air, paper and thin metal. (Higher levels can only be stopped by many centimetres of lead or metres of concrete
Electric Fields: Alpha particles (positively charged), Beta particles (negatively charged) and Gamma radiation (electrically neutral) mean that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is NOT deflected. 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).
Magnetic Fields: 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.
Hazards of Radiation
In living cells: 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 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: Inside the body (eg. from being swallowed or breathing in) : Alpha (MOST DANGEROUS as it's easily absorbed by cells). Beta and Gamma (NOT AS DANGEROUS as they are less likely to be absorbed by a cell and can usually just pass right through it). Outside the body: Alpha (NOT AS DANGEROUS as its unlikely to reach living cells inside the body). Beta and gamma (MOST DANGEROUS SOURCES as they can penetrate the skin and then damage the cells inside)
Radioactive decay is RANDOM - we cannot predict which atoms will decay.
Half life is the TIME TAKEN FOR THE ACTIVITY TO DECREASE BY HALF
THE TIME TAKEN FOR THE NUMBER OF ACTIVE NUCLEI TO HALF
eg. 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.
Uses of Radiation
- 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
- for measuring the thickness of materials in, for example, a paper factory
The Big Bang theory
The foremost theory of the origions of the universe
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 - Red Shift
Two key pieces of evidence for Big Bang theory are red shift and the Cosmic Microwave Background radiation.
Red Shift: 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. 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. 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.
Evidence of the Big Bang - Cosmic Microwave Backgr
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 all of this evidence:
The light from other galaxies is red-shifted (The other galaxies are moving away from us)
The further away the galaxy, the more its light is red-shifted (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 (The relatively uniform background radiation is the remains of energy created just after the Big Bang)
Devices used to observe the universe; there are many different types, 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. DISADVANTAGES: they can only be used at night and they cannot be used if the weather is poor or cloudy.
Other telescopes: Radio telescopes detect radio waves coming from space. 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. HOWEVER, they are very large and expensive.
Space telescopes: Objects in the universe emit other electromagnetic radiation. These are all blocked by the Earth's atmosphere, but can be detected by telescopes placed in orbit round the Earth. Can observe the whole sky and operate both day and night. HOWEVER, expensive to launch and maintain and only astronaughts can fix them.