The Electromagnetic Spectrum

In 1799, William Hershel set up dozens of different experiments to test the link between light and heat. In one, he formed a spectrum using a prism and screened out all but one of the colours. He let this light fall upon a thermometer and recorded the temperature that it showed. Hershel found that violet light gave the lowest reading on the thermometer whereas the highest reading was produced beyond the red end of the spectrum. He had discovered infra-red radiation. Hershel thought that infra red and light were very different forms of energy whereas Thomas Young thought they were similar and today we know that they are both part of the electromagnetic spectrum in which visible light is sandwiched between infra red and ultra violet. Electromagnetic radiation with a wavelength between approximately 400 nm and 700 nm is detected by the human eye and perceived as visible light. The light that the human eye is sensitive to is only a small portion of the electromagnetic spectrum. In 1801 Wilhelm Ritter investigated the light energy of different parts of the spectrum. He used strips of paper soaked in silver nitrate solution. When light falls on silver nitrate there is a chemical reaction that produces tiny grains of silver and the grains appear dark. Carl Wilhelm Scheele had previously noted that light of different colours darkened the paper at dfferent rates so blue light had more effect than red. Ritter decided to test beyond the visible end of the spectrum where no light could be seen. Here he found the reaction was even stronger and the invisible light he discovered became known as ultraviolet. Sunlight consists of an almost continous spectrum of waves but the majority of infra red and ultraviolet rays are filtered out by the atmosphere before reaching earth.