The Electromagnetic Spectrum

The Electromagnetic Spectrum:

Electromagnetic waves are electric and magnetic disturbances that transfer energy from one place to another.

Electromagnetic Scale (low to high frecquency):

• Radio
• Microwave
• Infared
• Visable
• Ultra Violet
• X-ray
• Gamma Ray

Wavelength:

• The wavelength is the distance form one wave peak to the next wave peak along the waves
• Radio waves have the longest wavelengths of more than 1km.
• X-rays and gamma rays have the shortest wavelengths of less than a millionth of a millionth of a metre (= 0.000,000,000,001m)

 Frequency:

• Frequency of an electromagnetic wave is the number of complete waves passing a point each second. 
• This is measured in Hertz (Hz), Kilohertz (kHz) and Megahertz (MHz) 
• I Hertz = 1 complete wave per second 
• Gamma rays have the highest frequency  
• Radio waves have the lowest frequency 

The speed of electromagnetic wave:

• All electromagnetic wave travel at the speed of light (300million m/s) through space or a vacuum.
• We can work out the wave speed using frequency and wavelength:

               Wave speed    =    Frequency    X   wavelength

                   (m/s)                     (Hz)                     (m)

Other equations:

Frequency   =                 1

                  Time period of 1 wave

Time of 1 wave   =          1

                                 Frequency

Radio waves:

Radio wave frequencies range from 300,000 to 300 million Hz. They are used to carry radio, TV and mobile phone signals. This works by:

• Radio waves are emitted from an aerial when we apply an alternating voltage to it. The frequency of the radio waves produced is the same as the frequency of the alternating voltage.
• When the radio waves pass across the receiver aerial, they cause a tiny alternating voltage in the aerial.

Microwaves:

They are called microwaves because they are shorter in wavelengths than radio waves. They are used for:

• Heating food in a microwave oven
• Communications – they can pass through the earths atmosphere and reach satellites. We use them to beam signals from one place to another because they don’t spread out as much as radio waves.

Infra-red:

All objects emit infra-red radiation. It can be absorbed by the skin. This damages the cells because it heats them up. It can be used for:

• Heaters
• Infra-red scanners
• Optical fibres
• Remote control handsets

Ultraviolet:

We cannot see ultraviolet radiation. But we can tell if it is there because it makes certain chemicals emitting light. The chemicals absorb the UV rays and emit them as light It is harmful to human eyes and can cause blindness. This is because UV wavelengths are smaller than visible light so carry more energy. To much exposure to UV rays can cause skin cancer. UV radiation is used for:

• Sun beds
• Security marking

X-rays:

• X-rays pass through soft tissue but are absorbed by bones and thick metal plates.
• X-rays are used to take pictures of bones.
• X-rays from x-ray tubes are directed at the patient and a light proof cassette containing photographic film is placed the other side.
• When the x-ray tube is switched on, x-rays pass through the patient’s body. They leave a shadow image on the film showing the bones.
• When the film is developed, the parts exposed to x-rays are lighter than the surrounding tissue so the bones appear lighter.
• Too much exposure x-rays can cause the same damage as gamma rays. People who work with x-rays must wear a film badge. If the badge is over-exposed, its wearer cannot continue working with the equipment.

Gamma Rays:

It is electromagnetic radiation from radioactive substances. Gamma rays are used for:

• Killing harmful bacteria in food
• Sterilising surgical instruments
• Killing cancer cell – they direct a narrow beam of gamma radiation at the cancerous cells. The beam is aimed at it from different directions on order to kill the tumour but not the surrounding tissue

Gamma rays can also be dangerous because they can cause cancer as well as killing it. Small doses of gamma radiation can cause cell mutation by damaging the DNA. High doses can kill living cells.

Communications:

The radio and microwave spectrum is split into frequency bands. The different uses of each band depend on its frequency range. This is because the higher the frequency of waves the more information they carry. The shorter their range (due to absorption by the atmosphere) the less they spread out.

The ionosphere is a layer of gas in the upper atmosphere, which reflects radio waves with frequencies less than 30Hz.

The ionosphere is stronger in the summer. This is why you can listen to distant radio stations because the radio wave bounce back and forth between the ionosphere and the ground

Optical fibres – they are very thin glass fibres and they are used to transmit signals by light of infra-red.