Contains the full lesson along with a supporting toolkit, including teachers’ notes.
What is radiation?
A candle radiates light and heat in all directions and can illuminate a room. From its source the light appears to travel in straight lines, called rays, and if something blocks the path a shadow is formed. Light is a form of radiation.
When sunlight is dispersed by a prism a spectrum is formed as Newton found in 1665. In 1800 Thomas Young showed that light was some kind of wave and he measured the wavelength of light of different colours; they range from about 750 nm (red) to about 400 nm (violet).
Also in 1800 Herschel discovered, using a thermometer, that the spectrum contained some heat radiation in the dark area beyond the red; we now call this infra-red radiation (IR). The following year (1801) Ritter showed that there was some form of radiation beyond the violet end of the spectrum which could cause chemical change faster than any of the visible colours. It is called ultra-violet radiation (UV).
What is ionising radiation?
We now know that visible light, IR and UV form a small part of a much wider spectrum of electromagnetic waves. Those with shorter wavelengths transmit more energy and often interact with matter as particles called photons.
Electromagnetic radiation whose wavelength is less than about 300 nm is called ionising radiation because it can knock electrons off some atoms and molecules causing ions to form, or to be more precise, ion pairs. Ionising radiation can damage bio-molecules such as DNA, causing cells to die or malfunction.
In the 1890s certain materials were found to emit a form of highly ionising radiation which, unlike light, was strongly affected by a magnet and so was different from electromagnetic waves. Some materials emitted positively charged particles called alpha particles. Others emitted negatively charged particles, or beta particles. Some materials also emitted high energy electromagnetic waves called gamma rays. All these materials are said to be radioactive.
In 1912 Victor Hess found — ascending above 5000 metres by balloon — that extremely ionising radiation increased with altitude and that it did not come from the Sun. This cosmic radiation comes from outside our solar system and at least some of it is extra-galactic It is by far the most strongly ionising radiation known. About 90% of it consists of protons, often travelling at over 99% of the speed of light. It also contains alpha particles and lesser amounts of heavier nuclei, neutrons and positrons.