The visible light spectrum
The pattern produced when white light shines through a prism is the visible spectrum. The prism seperates the mixtures of colours in white light into the different colours. The light waves are refracted as they enter and leave the prism.
The shorter the wavelength of the light the more it is refracted. Red light is refracted the least because it has the biggest wavelength, violet light is refrcted the most because it has the shortest wavelength.
Visible light is only part of the electromagnetic spectrum.
Photons and Ionisation
Electromagnetic radiation comes in tiny 'packets' called photons.
The photons deliver different quantities of energy, radio photons delivering the least energy and gamma photons delivering the most energy.
If the photons have enough energy they can break molecules into ions, this is ionisation. Ionising radiations remove electrons from atoms in their path.
In the electromagnetic spectrum there are 3 types of ionising radiation, ultra violet, x-rays and gamma rays. These have the photons with the most energy.
Ionising radiation and health
Ions produced when molecules in the body break up can take part in other chemical reactions. If these chemical reactions take part in body cells the cells can die or become cancerous.
So ioning radiation is damaging to health.
Energy and Intensity
The intensity of electromagnetic radiation is the energy arriving at a square metre of surface each second. Intensity depends on two things:
- The energy in each photon
- Number of photons arriving each second
All types of electromagnetic radiation deliver energy, this will heat the material that absorbs the radiation. The amount of heating depends on the intensity of the radiation, and the length of time it is absorbed for.
Radiation source- an object which gives out electromagnetic radiation.
Detector- something affected by the radiation.
The further from the source, the lower intensity of radiation the detector recieves. As photons spread out from the source, they are more thinly spread out when they reach the detector. Intensity may also decrease with distance due to partial absorbtion by the medium it travels through.
Ioning radiation includes:
- Ultraviolet radiation- which is found in sunlight
- X-rays- which are used in medical imaging machines
- Gamma rays- which are produced by some radioactive materials
Radiowaves, light and microwaves are not ionising.
Microwaves are used to heat materials such as food. The molecules in the material absorb the energy delivered by the microwaves. This makes them vibrate faster, so the material heats up. The heating effect increases if:
- the intensity of the microwave beam is increased
- the microwave beam is directed onto the material for longer
Food needs to be cooked for longer in a less powerful microwave oven. This is why they have different power ratings and food labels take this into account.
Some radiation of the electromagnetic sprectrum is absorbed by the atmosphere, but some is transmitted. Light, some infared, some ultraviolet and microwaves pass through the atmosphere and reach the earths surface. Gamma rays, x-rays, most ultraviolet and some infared light are absorbed by the atmosphere.
Infared reaches the earths surface from the sun and warms it. The earth emits some infared radiation and some is absorbed by gases in the atmosphere. This is the greenhouse effect. Without the greenhouse effect, the earth would be too cold for life.
Photosynthesis- light from the sun provides the energy for plants to produce food via photosynthesis.
Microwaves- the atmosphere transmits microwaves, these can be used to communicate with satellites.
Radiation and cell damage- microwaves
Microwaves in the environment may be harmful, but there is no agreement on this. They are not ionising so cannot cause cancers in the way that ultraviolet, x-rays and gamma rays do.
Microwave ovens work because the food contains water molecules which are made to vibrate by the water molecules. So food absorbs microwaves and gets hot. The rays cannot escape from the microwave because the metal case reflect microwaves back into the oven.
Some think that mobile phones, which recieve and transmit microwaves may be a health risk. Others think the intensity of the microwaves is too low to damage tissues by heating and microwaves are not ionising.
Radiation and cell damage- Ultraviolet and the Ozo
Ultraviolet light- Ultraviolet light from sunlight is damaging to health. Not much ultraviolet light reaches us because the ozone layer absorbs it. In the summer it is wise to use suncream and clothing to absorb ultraviolet and prevent it reaching the sensitive cells of the skin.
The ozone layer- the ozone layer absorbs ultraviolet because ultraviolet ionises the ozone, which then changes to oxygen. This chemical change is reversible, and the oxygen changes back to ozone.
Aerosols and fridges
Chemicals used in aerosol spray cans and fridges gradually made their way up to the ozone layer when released into the atmosphere, and removed some of it. This has increased the intensity of the ultraviolet radiation reaching the Earth. These chemicals are not used any more, and the ozone layer is gradually returning to normal. However, this will take a number of years more.
Benefits vs risks of activities e.g. sunbathing
Ulatraviolet radiation can cause skin cancer and is difficultto tell how much you are recieving.
Sunbathing produces a tan and some exposure to ultraviolet produces vitamin D in the skin.
Making a judgement
What is the chance of the outcome happening?
What is the consequence of that outcome?
'The precautionary principle'- avoid activity if serious harm could arise
The real risk may be different from the percieved risk.
Some gases in the atmosphere absorb infared radiation, including carbon dioxide. Although carbon dioxide only makes up 0.4% of the atmosphere, it is very important because it absorbs infared well.
1. The suns rays enter the earths atmosphere.
2. Heat is emitted back from the Earth’s surface at a lower principal frequency than that emitted by the Sun
3. Some heat passes back into space.
4. But some heat is absorbed by carbon dioxide, a greenhouse gas, and becomes trapped within the Earth’s atmosphere. The Earth becomes hotter as a result.
Even though only a trace of methane is present in our atmosphere, it is a very effiecient absorber of infared.
The carbon cycle
The amount of carbon dioxide in the atmosphere is controlled by the carbon cycle.
Processes that remove carbon dioxide from the air: photosynthesis, dissolving in the sea.
Processes that remove carbon dioxide from the atmosphere: respiration by plants, combustion, thermal decomposition of limestone.
Cellulose- all cells contain carbon, because they all contain proteins, fats and carbohydrates. For example plant cell walls are made of cellulose a carbohydrates.
Decomposers- such as microbes and fungi break down remains of dead plants and animals and in doing so release carbon dioxide by respiration.
The percentage of carbon dioxide in the atmosphere has increased because humans are burning fuels as energy sources and burning large areas of forests to create land, this means there is less photosynthesis removing carbon dioxide from the air.
Global warming - computer climate models
One piece of evidence that supports the view of scientists who blame human activities for global warming has been provided by 'supercomputers'. Computer generated climate models, based on different amounts of carbon dioxide in the atmosphere, produce the same changes as have been observed in the real world.
Infrared light, microwaves and radio waves are all used to transmit information such as computer data, telephone calls and TV signals.
Infared light- Information such as computer data and telephone calls can be converted into infrared signals and transmitted by optical fibres. Optical fibres are able to carry more information than an ordinary cable of the same thickness. In addition the signals they carry do not weaken so much over long distances. Television remote controls use infrared light to transmit coded signals to the television set in order to, for example, change channels or adjust the volume.
Microwaves- Microwave radiation can be used to transmit signals such as mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones which are 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.
Radiowaves- Radio waves are used to transmit television and radio programmes. Longer wavelength radio waves are reflected from an electrically charged layer of the upper atmosphere. This means they can reach receivers that are not in the line of sight because of the curvature of the Earth’s surface.
Analogue and digital information
Before a sound or piece of information is transmitted, it is encoded in the transmitter in one of the ways described below - analogue or digital. The receiver must then decode the signal to produce a copy of the original information or sound.
- Analogue signals vary continuously in amplitude, frequency or both.
- Digital signals are a series of pulses with two states - on (shown by the symbol ‘1’) or off (shown by the symbol ‘0’). Digital signals carry more information per second than analogue signals and they maintain their quality better over long distances.
All signals become weaker as they travel long distances. 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.
An important advantage of digital signals over analogue signals is that if the original signal has been affected by noise it can be recovered more easily. In analogue signals, when the signal is amplified to return to its original height, noise gets amplified as well.
Analogue vs digital
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 from the background noise, for example.
Noise also adds extra random information to digital signals. However, this noise is usually lower in amplitude than the 'on' states of the digital signal. 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. This 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. Another advantage of digital signals is that information can be stored and processed by computers.
Coding and storing information
Coding involves converting information from one form to another. All types of information can be coded into a digital signal.
Digital signals are a series of pulses consisting of just two states, ON (1) or OFF (0). There are no values in between. The sound is converted into a digital code of 0s and 1s, and this coded information controls the short bursts of waves produced by a source.
When waves are received, the pulses are decoded to produce a copy of the original sound or image.
The amount of information needed to store an image or sound is measured in bytes (B).
A megabyte is larger than a byte, and a gigabyte is larger than a megabyte.
To store one minute’s worth of music it would take about 1 megabyte, to store an average two hour movie it would take 1.5 gigabytes.
In general, the more information that is stored about an image or sound, the higher the quality.