The electromagnetic spectrum is a family of seven radiations including visible light. A beam of electromagnetic radiation contains 'packets' of energy called photons.
Different radiations contain photons that carry different amounts of energy.
The electromagnetic Spectrum includes:
- High energy photons
- Gamma Rays
- Ultraviolet rays
- VISIBLE SPECTRUM
- Infra-red rays
- Radio waves
- Low energy photons
The general model of radiation shows how much energy travels from a source which emits radiation, to a detector which absorbs radiation.
On the journey from emitter to detector, materials can transmit, reflect or absorb radiation.
For example, clouds absorb and reflect the Sun's energy, so on a cloudy day we receive less light than on a clearer day.
Intensity and Heat
The intensity of electromagnetic radiation is the energy arriving at the surface per second.
Intensity depends on the number of photons delivered per second and the amount of energy each packet contains i.e. the photon energy.
The intensity of a beam decreases with distance, so the further from a source you are the lower the intensity.
When a material absorbs radiation, heat is created. The amount of heat depends on it's intensity.
Intensity and Heat
The decrease in intensity is due to three factors
- Photons spread out as they travel.
- Some photons are absorbed by particles in the substances they pass through
- Some photons are reflected and scattered by other particles.
These factors combine to reduce the number of photons arriving per second at a detector. This results in a lower measured intensity.
When a material absorbs radiation, heat is created; the amount of heat depends on it's intensity.
The amount of heat created depends on the intensity of the radiation beam and the duration of the exposure.
Ionising radiation (electromagnetic radiation with a high photon energy) can break molecules into bits called IONS. Ultraviolet radiation, X-Rays and gamma rays are examples of ionising radiation.
Ions are very reactive and can easily take part in other chemical reactions.
Radiation damages living cells in different ways:
- the heating effect can damage the skin e.g. sunburn
- Ionising radiation can age the skin, it can also mutate DNA, which can lead to cancer
- different amounts of exposure can cause different effects e.g. high intensity ionising radiation can destroy cells, leading to radiation poisoning.
Microwaves can heat materials by causing the water particles to vibrate. There may be a health risk from the low intensity microwaves of mobile phones and masts, but this is disputed. One study found no link from short-term use but other studies have found some correlation.
Microwave ovens have a metal case and a wire screen in the door to absorb microwaves and stop too much radiation escaping.
Other physical barriers are used to protect people:
- X-Ray technicians use lead screens
- Sunscreens and clothing can absorb ultra violet radiation to help prevent skin cancer.
- Nuclear reactors are encased in thick lead and concrete.
People going into areas of high radiation must wear a radiation suit as a shield.
The sun and the ozone layer
Light radiation from the sun..
- warms the Earth's surface
- is used for plants for photosynthesis.
Photosynthesis counteracts respiration - it removes carbon dioxide and adds oxygen.
The ozone layer is a thin layer of gas in the Earth's upper atmosphere. It absorbs some of the Sun's ultra-violet radiation before it reaches Earth.
Without the ozone layer, the amount of radiation reaching Earth would be very harmful. Living organisms, especially animals, would suffer cell damage.
The energy from ultraviolet radiation causes chemical changes in the upper when it's absorbed by the ozone layer. These changes are reversible.
The greenhouse effect
The Earth emits electromagnetic radiation into space. Gases in the atmosphere absorb some of the radiation and this keeps Earth warmer than it would be. This is known as the greenhouse effect.
Carbon dioxide (a green house gas) makes up a small amount of Earth's atmosphere.
Other greenhouse gases include water vapour and trace amounts of methane.
The carbon cycle
The carbon cycle is an example of a balanced system.
- Plants remove carbon dioxide from the atmosphere.
- Carbon from photosynthesis makes carbohydrates, fats and proteins.
- Animals and micro-organisms feed on dead animals to break them down.
- Micro-organisms and animals respire, releasing carbon dioxide.
Carbon dioxide levels once remained constant - they were recycled by plants and animals. Levels have risen because of human activity e.g. deforestation.
Using the carbon cycle
The carbon cycle can be used to explain several points:
- Carbon dioxide (C O 2 ) levels in the Earth's atmosphere remained roughly constant for thousands of years because it was being recycled by plants and animals.
- Decomposers are important micro-organisms that break down dead material and release Carbon Dioxide.
- C O 2 levels in the atmosphere have been steadily increasing, largely due to human activity e.g. burning fossil fuels and deforestation.
- Burning fossil fuels releases carbon that was removed from the atmosphere millions of years ago and had been 'locked up' ever since.
- Burning forests not only release carbon, but also reduces the number of plants removing C O 2 from the atmosphere.
The increase in greenhouse gases in the Earth's atmosphere means the amount of absorbed radiation from the Sun increases. This causes the Earth's temperature to increase, an effect known as global warming, which may lead to
- climate change - crops may not be able to grow in some areas.
- extreme weather - e.g. hurricanes
- rising sea levels - melting ice caps and higher sea temperatures may cause sea levels to rise, flooding low lying land.
Data about the Earth's changing temperature is collected and used with climate models to look for patterns in the possible causes of global warming.
These computer models show that one of the main global warming factors is the rise in the carbon dioxide levels in the atmosphere, providing evidence that human activity is to blame.
Risk and benefit
All new advances have the potential for risk. Radiation advances are unlikely to be risk free.
For example, until a correlation between mobile phones and cancer be proved, people need to make their own decisions and evaluate the risks against the benefits.
Another example is X-Rays. Although X-Rays allow doctors to make a much more accurate diagnosis, their exposure times have to be controlled.
X-Ray radiation can destroy cancerous cells, but can harm healthy cells too.
A study may show a correlation between a factor and an outcome but this doesn't mean that the factor will cause the outcome.
For example, there may be a link between mobile phones (factor) and cancer (outcome) but using a mobile phone will not always lead to cancer.
Some people say its better to take a precautionary measure e.g. limit usage, especially for young people.
Weighing the risk.
In weighing up the risk it's important to consider the chance of the outcome and any consequences. Although a risk may seem low, the out come could be very serious.
For example, although there's evidence that prolonged exposure to ultraviolet light increases the risk of skin cancer, many people still sunbathe. Some reasons for that may be..
- sunlight is needed for good health and is a source of vitamin D
- sunlight can help prevent SAD (seasonal affective disorder) and skin conditions such as eczema
- people think a tan looks healthy or attractive
- people think it won't happen to them
A L A R A
Actual risk is a scientific measure of the dangers of something. Perceived risk is how dangerous people think it is.
These values can be very different. Factors that affect perceived risk include..
- media coverage and personal bias
- social influence, e.g. opinions of family.
The A L A R A (As low as reasonably achievable) principle is used as a guideline for risk management. It states that measures should be taken to make the risks and small as possible, whilst still providing the benefits and taking into account the social, economic and practical implications. For example, this is used in radiology units to protect staff, and to control the dose of radiation given in each treatment.