THE ELECTROMAGNETIC SPECTRUM

This table indicates the different parts of the electromagnetic spectrum.

The spectrum of electromagnetic waves ranges from very low frequency radio waves to extremely high frequency gamma rays.

Low frequency (long wavelength) radio waves have low energy and are not harmful to humans.

High frequency (short wavelength) gamma rays have very high energy and are extremely harmful to humans

The waves of all parts of the spectrum:

• are caused by the acceleration of charged particles.

• are transverse electromagnetic waves made up of magnetic and electric fields vibrating at right angles to each other.

•have particle characteristics, with the energy of each quantum being directly proportional to the frequency of the wave according to Planck’s equation: E = hf

• travel in a vacuum at 3 X 108 m.s-1

RADIO WAVES

• Radio waves are produced by oscillating (vibrating) electric currents in a transmitting aerial.

• Radio waves are easily diffracted because they have long wavelengths.

• Medium wave radio is easily transmitted. Radio 702 transmits on a frequency of 702kHz ( λ = 427m).

•Short wave radio waves are reflected by the ionosphere resulting in erratic reception of short wave radio.

•Very high frequency radio waves are used for FM (frequency modulated) radio transmission.

RADIO WAVES

RADIO WAVES

• VHF (very high frequency) and UHF (ultra high frequency) waves are used for television.

• VHF and UHF waves are not easily diffracted and so cast shadows behind buildings. This is why so many transmitting aerials are needed for TV transmission.

MICROWAVES

• The wavelength of these waves is a few centimeters.

• Microwaves are used for satellite communication, for telephone and television. The microwaves travel in straight lines over long distances from the satellite to the receiver. Receivers are in the form of a parabolic dish which focuses the radiation onto a detector.

•Microwaves are used to cook food in microwave ovens. Water molecules absorb the energy from these wavelengths. This causes the water molecules to vibrate, thereby heating the water and cooking the food.

Most food contains a great deal of water which causes the food to cook very quickly. In addition, the food cooks throughout its volume and not just from the outside, as in conventional cooking.

INFRARED RADIATION

• The particles of all objects vibrate and as a result emit radiation in the infrared region of the spectrum.

• Infrared radiation affects some photographic film. We can take infrared photographs of objects in the absence of light.

• Infrared binoculars allow us to view objects, like animals, at night.

VISIBLE LIGHT

• When objects are heated they first glow red (emit light of low frequency) but as they become hot they begin to emit higher frequencies, until they are “white hot” and emit all the frequencies that make up the visible spectrum.

•Objects emit white light when the temperature reaches 2000oC . This is also the temperature of the sun.

• Visible light is referred to as white light. White light is made up of the seven colours of the spectrum. The human eye is unable to distinguish between the colours under normal conditions.

ULTRAVIOLET RADIATION

• These rays are emitted by very hot objects, like the sun and electric arc welders.

• Ultraviolet light causes some chemicals to glow or fluoresce. These chemicals are added to washing powders where they absorb ultraviolet rays and release them as visible light. Washing then appears to be “whiter than white.”

•The ultraviolet rays of the sun can cause skin cancer. The ozone layer of our atmosphere protects us from these rays by absorbing them before they reach us.

•Ultraviolet rays can kill harmful bacteria and are used for this purpose in some air-conditioning units.

X RAYS

• These rays are emitted when high-speed electrons bombard a metal plate. The electrons slow down and their energy is transferred as high energy electromagnetic radiation.

• X rays were discovered by Wilhelm RÖntgen in 1895.

•These rays pass through soft tissue but not bones and so are used to photograph bones.

•These rays are dangerous and have

high penetrating ability because of their

high energy.

• X rays do not penetrate lead so radiographers wear thin lead aprons to protect themselves from the radiation.

GAMMA RAYS

• These rays are emitted by naturally occurring radioactive materials and are a by-product of nuclear reactions.

• Gamma rays have the greatest penetrating ability of all electromagnetic radiation, have the greatest energy and are the most dangerous to man.