Infrared radiation was discovered in 1800 by astronomer Sir William Herschel. When visible light passes through a prism, a rainbow of colours is seen. This is called a spectrum.
Herschel discovered an invisible radiation in the spectrum that was lower in energy than the red light, through its effect on the thermometer. This was the first time that a form of light beyond visible light had been detected.
Infrared radiation has a longer wavelength than visible light. It has applications in industrial, scientific, military, law enforcement, and medical fields. For instance, night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected.
Ultraviolet (UV) radiation was discovered in the year 1801 by the German physicist Johann Ritter. He observed invisible rays just beyond the violet end of the visible spectrum which darkened silver chloride-soaked paper more quickly than violet light itself. Initially named oxidizing rays by Ritter, these later came to be known as ultraviolet light. The word ‘ultra’ means ‘beyond.’
Through their discoveries Ritter and Herschel proved that there are invisible forms of light beyond both ends of the visible spectrum.
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