The Electromagnetic Spectrum
- The electromagnetic spectrum is a family of seven radiations - radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, gamma rays.
- A beam of electromagnetic radiation contains 'packets' of energy called photons. Different radiations contain photons that carry different amounts of energy.
- The intensity of a beam of radiation depends on the number of these photons it delivers every second. The intensity of the beam also depends upon the amount of energy carried by each photon.
- The higher the frequency of the electromagnetic radiation, the more energy is transferred by each photon. e.g. emitter- the Sun, travels by visible light, detector-the eye.
- All electromagnetic waves travel through space (a vacuum) at the same very high speed (300000km/s)
- A general model of radiation describes how energy travels from a source that emits radiation to a detector that absorbs radiation.
- On the journey from emitter to detector the radiation can be transmitted, reflected or absorbed by materials. For example, on a cloudy day energy from the Sun is absorbed and reflected by the clouds and the amount of light received at the ground is less than it would be on a sunny day.
Intensity and Heat
- The intensity (or measure of strength) of electromagnetic radiation is the energy arriving at a square metre of surface per second.
- The intensity depends on the number of photons delivered per second and the amount of energy each individual photon contains.
- The intensity of a beam of radiation decreases with distance, so the further away from a source you are, the lower the intensity.
- The decrease in intensity is due to the photons reflection by other particles, photons absorbed by particles in the atmosphere and the photons spread out which means the number of photons arriving per second at a detector is reduced, resulting in a lower measured intensity.
- When a material absorbs radiation, it will heat up; the temperature increase depends on the intensity of the radiation.
- The amount of heating also depends on the duration of exposure.
- Some electromagnetic radiations (ultraviolet, x-rays, gamma rays) have enough energy to change atoms or molecules which can initiate a chemical reaction.
- Some materials (radioactive materials) emit ionising gamma radiation all the time. Ionising radiation has photons with enough energy to remove an electron from an atom or molecule to form ions.
- Ionising radiations are those with high enough photon energy to remove an electron from an atom or molecule. Ultraviolet radiation, x-rays and gamma rays are all examples of this.
- Ions are very reactive and can easily take part in other chemical reactions.
When living cells absorb radiation, damage can occur in different ways;
- The heating effect can cause damage.
- Ionising radiaton, such as ultraviolet radiation, can damage cells, causing ageing of the skin.
- Ionising radiation can cause mutations in the nucleus of a cell, which can lead to cancer.
- Different amounts of exposure can cause different effects, e.g. high intensity ionising radiation can kill cells, leading to radiation poisoning.
- Microwaves are strongly absorbed by water molecules, which means microwaves can be used to heat objects containing water.
- Microwave ovens have a metal case and a wire screen in the door - this reflects the microwaves and protects users by preventing too much radiation from escaping. The door screen also absorbs microwaves, protecting users from the radiation.
- There may be a health risk from the low-intensity microwaves of mobile phone handsets and masts, but this is disputed. A study in 2005 found no link from short-term use, but other studies have found some correlation between mobile phone masts and health problems. Further studies are underway to look in more detail at mobile phone masts and the long-term effects of mobile phone use.
- Other physical barriers are used to protect people from ionising radiation, e.g. sun-screens and clothing can absorb most of the ultraviolet radiation from the Sun and this helps to prevent skin cancer.
- X-rays are absorbed by dense materials, so they can be used to produce shadow pictures of bones in our bodies or of objects in aircraft passengers' luggage. Radiographers are protected from radiation by dense materials such as lead and concrete.
The Sun's Energy
- The Sun (and all other objects) emit electromagnetic radiation with a principal frequency that increases with temperature. The Earth is surrounded by an atmosphere that allows some of the electromagnetic radiation emitted by the Sun to pass through.
- This radiation warms the surface of the Earth when it is absorbed.
- The ozone layer is a thin layer of gas in the Earth's upper atmosphere. This layer of gas absorbs some of the ultraviolet radiation from the Sun before it can reach Earth.
- Without the ozone layer, the amount of ultraviolet radiation reaching Earth would be very harmful to living organisms, especially animals, due to cell damage.
- The energy from the ultraviolet radiation causes chemical changes in the upper atmosphere when it is absorbed by the ozone layer, but these changes are reversible.
- The Earth emits electromagnetic radiation into space which has a lower principal frequency than that emitted by the Sun.
- There are gases in the atmosphere that absorb or reflect some of this radiation. This keeps the Earth warmer than it would otherwise be and is known as the greenhouse effect.
- Carbon dioxide is a greenhouse gas and it makes up a small amount of the Earth's atmosphere - about 0.035%. Other greenhouse gases include water vapour and methane.
The Carbon Cycle
- Carbon dioxide is removed from the atmosphere by green plants to produce glucose by photosynthesis. Some is returned to the atmosphere by the plants during respiration.
- The carbon obtained by photosynthesis is used to make carbohydrates, fats and proteins in plants. When the plants are eaten by animals, the carbon becomes carbohydrates, fats and proteins in animals.
- Animals respire, releasing carbon dioxide into the atmosphere.
- When plants and animals die, other animals and microorganisms feed on their bodies, causing them to break down. Excretion also releases carbon.
- As the detritus feeders and microorganisms eat the dead plants and animals, they respire, releasing carbon dioxide into the atmosphere.
The Carbon Cycle explains....
- The amount of carbon dioxide in the atmosphere had remained roughly constant for thousands of years because it was constantly being recycled by plants and animals.
- The importance of decomposers, which are microorganisms that break down dead material and release carbon dioxide back into the atmosphere.
- The amount of carbon dioxide in the atmosohere has been steadily increasing over the last 200 years, largely due to human activity such as burning fossil fuels and deforestation.
- Fossil fuels contain carbon that was removed from the atmosphere millions of years ago and has been 'locked up' ever since. Burning fossil fuels for energy releases this carbon into the atmosphere.
- Burning forests (deforestation) to clear land not only releases the carbon they contain but also reduces the number of plants removing carbon dioxide from the atmosphere.
The increase of greenhouse gases in the Earth's atmosphere, especially carbon dioxide, means that the amount of absorbed radiation from the Sun also increases. This increases the temperature on Earth an effect known as global warming. As the Earth becomes hotter, there are some potential results;
- Climate change - it may become impossible to grow some crops in certain areas.
- Extreme weather conditions caused by increased convections and larger amounts of water vapour in the hotter atmosphere.
- Rising sea levels - the melting ice caps and higher ocean temperatures may cause sea levels to rise, which could cause flooding of low-lying land. Some Pacific islands have already been abandoned.
Climatologists collect data about how the Earth's temperature has changed over the years. The data collected is used with climate models to look for patterns. These computer models show that one of the main factors causing global warming is the rise in atmospheric carbon dioxide and other greenhouse gases, providing evidence that human activities are causing global warming.
Electromagnetic Waves Uses
Different electromagnetic waves have different frequencies. This affects their properties and the effect that other materials have on them. They can be used for different purposes, depending on how much they are reflected, absorbed or transmitted by different materials;
- Radio waves are used for transmitting radio and television programmes because they are not strongly absorbed by the Earth's atmosphere. They can travel long distances through the atmosphere and through space. Radio telescopes are used in astronomy to pick up radio waves from stars.
- Microwaves are used to transmit mobile phone signals because they are not strongly absorbed by the atmosphere. They are reflected well by metals so satellite dishes are made of metal and shaped to reflect the signal onto the receiver.
- Light and infrared radiation will travel huge distances down optical fibres without the signal becoming significantly weaker. This makes them very useful for carrying information, e.g. in computer networks and telephone conversations.