Greenhouse effect
The radiation from very hot objects like the sun is mostly in the form of light and short-wavelength infrared. The radiation from less hot objects, example a fire, is largely long-wavelength infrared radiation which, unlike light and short-wavelength infrared cannot pass through glass.
Light and short-wavelength infrared rays from the sun penetrate a glass house and are absorbed by the soil and plants, raising their temperature. These in turn emit infrared radiation but, because of their relatively low temperature, this has a long wavelength and is not transmitted by the glass. The green house thus acts as ‘heat-trap’ and its temperature rises.
Ultra-Violet Radiation
Ultraviolet rays have shorter wavelengths than that of light and can be detected just below the violet end of the spectrum by fluorescent paper. They cause suntan and produce vitamins in the skin, but an overdose can be harmful especially to the eyes.
Ultraviolet radiation causes teeth, finger nails and florescent paints to fluoresce (they glow by reradiating as light the energy of the ultraviolet radiation they absorb).
An ultraviolet lamp used for scientific or medical purposes contains mercury vapor and this emits ultraviolet radiation when an electric current passes through it. Florescent tubes also contain mercury vapor and their inner surfaces are coated with powders, which radiate light when struck by ultraviolet rays.
Radioactivity
The discovery of radioactivity in 1896 by the French scientist Becquerel was accidental. He found that uranium compounds emitted radiation which (a) affected a photographic plate even when wrapped in black paper and (b) ionized a gas. Soon afterwards Marie Curie discovered radium.
Today radioactivity is used widely in industry, medicine and research. Obviously we are all exposed to background radiation caused partly by radioactive materials in rocks, the air and our bodies, and partly by cosmic rays from our space.
Half-Life
A radioactive atom breaks up into an atom of a different element when it emits and a alpha (α) or a beta (β). It is said to decay. The change happens of its own accord, is unaffected by temperature and occurs whether the material is pure or combined chemically with other elements.
Every radioactive element has its own definite decay rate, expressed by its half-life. This is the time for half the atoms in a given sample to decay. Half-lives vary from millionths of a second to millions of years; that of radium is 1600 years. In some cases it would be impossible to find the ‘full-life’ of a sample; the half-life is a much more practical quantity which is readily found by experiment.
Radio Waves
Long medium and short waves diffract round obstacles so they can be received when hills, for example, are in their way. This allows their use for local radio broadcasts. They are also reflected by layers of electrically charged particles in the upper atmosphere (ionosphere), thus making long-distance reception possible as well.
Very-high frequency and ultra-high frequency waves have shorter wavelengths and need a clear, straight-line path to the receiver. Local radio and television use them. They pass through the ionosphere.
Microwaves are used for radar and also for international telephone and television links with geostationary communication satellites. These go around the equator at the same rate as the earth spins and so appear to be at rest. Signals are beamed by large dish aerials to the satellite where they are amplified and sent back to another dish aerial in another part of the world. Microwaves are also used for cooking since like all electromagnetic waves they have a heating effect when absorbed.
X-rays
X-rays have smaller wavelengths than those of ultraviolet rays. They can penetrate solid objects and affect a film. Very penetrating X-rays are used in hospitals to kill cancer cells. They also damage healthy cells so careful shielding of the X-ray tube with lead is needed. Less penetrating x-rays have longer wavelengths and penetrate flesh but not bone: they are used in dental X-ray photography.
Experiments to study the penetrating power, ionizing ability and behavior in magnetic and electric fields, shows that a radioactive substance emits one or more of three types of radiation: alpha (α), beta (β) and gamma (γ).
Alpha rays
Alpha rays can be stopped by a thin sheet of paper. They have a range in air of a few cm, since they cause intense ionization in a gas due to frequent collision with gas molecules and thus lose energy. They are deflected by electric and strong magnetic fields. Americium (241 Am) may be used as a pure source of alpha (α)
Beta rays
Beta rays are stopped by a few mm of aluminum. Some have a range in air of several meters, because their ionizing power per cm is much less than that of α particles. This suggests that they are lighter and faster than alpha (α) particles and consequently have fewer collisions with the molecules in the air over a given distance. As well as being deflected by electric fields, they are more easily deflected by magnetic fields Strontium (90Sr) emits rays only.
Gamma rays
Gamma rays are the most penetrating and are stopped only by many centimeters of lead. They ionize a gas even less than beta (β) particles do, and are not deflected by electric and magnetic fields. They give interference and diffraction effects and are electromagnetic radiation traveling at the speed of light. Their wavelengths are those of very strong X-rays, from which they differ only because they arise in atomic nuclei whereas X-rays come from energy changes in the electrons outside the nucleus. Cobalt (60Co) is a pure γ source. Radium (226Ra) emits α, β and γ rays.