The Elecromagnetic Spectrum
The electromagnetic spectrum is a range of energies travelling as two transverse waves of pure energy. An electrical energy wave and a magnetic energy wave moving together but at right angles to each other.
The electromagnetic spectrum covers a range of frequencies and wavelengths from the very short wave, high energy gamma rays to long wave, low energy radio waves. All travel at the speed of light, (3 x 108m/s) so as the wavelength changes so does the frequency in order to maintain a constant speed.
The colour band in the middle is the visible light part of this spectrum. The values for frequency are in Hertz (Hz) and the wavelengths are given in meters.
Just like visible light, these are all transverse waves and can be reflected, refracted and focuses by a lens. They all carry energy, the longer the wave the lower the energy. Radio Waves pass through us all the time carrying everything from wifi to TV signals, they do us no harm. Gamma rays would be very harmful to be exposed to, they carry enough energy to burn the skin and kill.
Types of EMR
Types of Electromagnetic Radiation
Although the spectrum is divided into sections to identify different types of EM radiation, it is important to recall that this is a continuous spectrum. As such, a radio wave with a short wavelength has almost as much energy as a microwave with a long wavelength.
|Electromagnetic Radiation||Wavelength range|
|Gamma||Less than 1nm|
|X-rays||1 - 10 nm|
|Ultraviolet||10 - 400 nm|
|Visible light||400 - 700 nm|
|Infrared||700 - 5x105 nm|
|Microwaves||1x105 to 1x108 nm|
|Radio waves||Greater than 1x108 nm|
As the wavelength increases the frequency will decrease, as will the energy levels. Notice too that the visible part of the spectrum is only a small fraction of total range.
It is important to recall that all are transverse waves and just like the visible light they can be reflected and refracted by materials, as well as absorbed or transmitted. Radio waves can be reflected off the atmosphere to help send signals across the world by radio-waves.
Bricks and glass are transparent to microwaves, if this was not true then our mobile phones would not work indoor. They are also transparent to radio-waves, otherwise your wi-fi router would only work in one room. Glass will however absorb Ultraviolet radiation.
Applications of EMR
__Radio-Waves: __Telecommunications is the main application for radio waves. They are used to broadcast TV and radio signals, to carry information between airplane and the ground, ship to shore communications. They are also used by routers to broadcast wifi and Bluetooth™ devices also use radio waves.
Microwaves: These can be used for cooking food in a microwaves oven. In this application the microwaves are absorbed by the water molecules in the food. This added kinetic energy to the molecules and so heats up the food.
Microwaves are also used for radar, originally radio waves were used, but microwaves are more powerful, can travel further without loosing power and can detect smaller objects.
Mobile phone communications for voice and mobile data usage, rely on microwaves to carry the information from the phone to the nearest mast.
__Infrared: __These are used in night vision cameras by the military and rescue teams looking for survivors in collapsed buildings. The TV remote uses an infrared signal to communicate with the TV. Some lasers use infrared for accurate cutting of metals.
__Visible Light: __As well as for seeing, we use visible light in many lasers, the scanner at the supermarket and the lasers in your DVD and Blu Ray players for example. A DVD uses a red laser and a Blu Ray a blue laser.
__Ultraviolet: __Certain security features on banknotes are only visible under UV lights. UV is also used to kill bacteria in water. In some parts of the world were access to clean drinking water is difficult, this cheap method can help improve water quality without the need for expensive chemicals.
X-Rays: The main application is for medical examination. This works because X-rays are transmitted by the soft tissue of the body, but absorbed by the hard bones.
__Gamma: __Medically they are used to treat cancers. A targeted beam of gamma rays are fired at the tumour to kill the cancerous cells. The high energy of this part of the spectrum makes them very dangerous to life. They are used to sterilise surgical equipment and even to kill bacteria on food before transport to the shops.
Dangers of EMR
Although there are many applications for electromagnetic radiation, iis is still a form of energy and in the case of the shorter wavelengths, the energy levels can be very high. So much so that they can cause serious harm and even death.
Microwaves are used for cooking food and this means they can do the same thing to the internal body cells. The metal casing and mess grill in the glass of the door on a microwave cooker will absorb any microwaves, however if this is damaged then the microwaves can escape and cause harm.
Infrared is a form of heat radiation and can cause skin burns.
Ultraviolet has enough energy to damage surface skin cells and the eyes, leading to skin cancer and eye conditions, such as cataract. The very high UV from the sun is absorbed by the atmosphere but the UV a to c that reaches the surface can be very harmful and accounts for the rise in skin cancers over the last 40 years as more people have gone on holiday abroad.
X-rays and gamma rays have the potential to cause cancers and in high doses they can kill instantly. The damage to the cells, notably the DNA, is the main reason that there is a limit on the number of X-ray you are allowed each year.
When electromagnetic waves strike a surface and are absorbed the energy of the wave is transferred to the object. What effect this has will depend upon the material and the wavelength of the wave.
__Ionisation: __High energy waves (short wavelength, UV, X-rays and Gamma), can have enough energy to ionise atoms. This means that the absorbed energy pushes electrons out of their atomic orbits. This can lead to changes in the structure of proteins and DNA that lead to cancers.
__Heating effect: __All of the radiations in this spectrum can be absorbed by a material and the energy transferred to the vibration energy of the atoms and molecules. As kinetic energy of a material is linked to the materials temperature, the absorbed wave energy results in an increase in the temperature of the material.
__Re-radiation: __In some situations the wave energy is absorbed by the electrons in the outer orbits of an atom. This gives the electron an energy boost and it jumps up an orbit. However, this doesn’t last and the electron quickly returns to its original lower energy orbit. As it does so it releases the energy as a wave of electromagnetic radiation of a longer wavelength (less energy) than it originally absorbed. UV pigments work this way, they take in UV light, which we can not see, but re-radiate visible light in the blue/violet end of the visible spectrum which we then can see.
All objects emit radiation. The type, or more commonly, the combination, of radiation emitted is linked to the temperature of the object. The higher the temperature the more energetic the radiation emitted.
There is a balance between the energy an object absorbs and the energy it radiates. To remain at a constant temperature the absorption energy must equal the radiated energy. If the radiation is greater than the absorption then the object will cool down.
This simple balancing act applies to all object, no matter how large or small, including the earth.
The incoming infrared for example is shortwave radiation and can pass through our atmosphere to be absorbed by the earth. This heats the surface up and it then starts to radiate infrared too, but at a much lower wavelength. This longer wavelength infrared is reflected by the upper atmosphere and can not escape. This is due to the CO2in the atmosphere. The carbon dioxide there is in the atmosphere the less IR escapes into space and so gradually the earth heats up, as it is absorbing more than it radiates back into space.
Radio Waves can be produced by an electrical circuit, and be absorbed by them too. They result in oscillations in the electrical current, this is detected sometimes as interference. Good quality, well maintained electrical devices have shielding to prevent accidental escape of radio waves that might affect other local electrical circuits. However, sometime if the electrical system is old or damaged it can produce local interference. This can cause a break up in a TV signal, or affect the wifi in a house.
Living near power lines and thunderstorms can also have a similar effect as they emit radio waves too and these can be absorbed by your electrical devices.
- Give an application for a hazard of Ultraviolet radiation.
- Your answer should include: kill / bacteria / skin cancer
Explanation: UV can be used to kill bacteria in water making it safer to drink. It can be absorbed by the skin and increase the risk of skin cancers.
- Explain the relationship between the wavelength of electromagnetic radiation and its degree of hazard to human health?
- Your answer should include: short / shorter / more / energy / change / DNA
Explanation: The shorter the wavelength the more energy it carries, above that of radio waves, this can be harmful to human health if the radiation is absorbed and changes to molecules in the cells in such as way as to effect their function. Notably the change in DNA to cause cancers.
- Explain the relationship between the temperature of a radiating body and the wavelengths of radiation emitted.
- Your answer should include: energy / temperature / radiation
Explanation: The higher the temperature of the radiating body the higher the energy of the emitted waves. Higher temperature bodies can therefore emit radiation of shorter wavelengths.