Calorific rays were discovered in 1800 by William Herschel when he used a prism to refract sunlight. Using a thermometer, he noticed an increase in temperature beyond the red segment of the visible spectrum. The wavelength of visible light lies in the range between 0.38 (violet) and 0.78 (red) microns, with infrared being ‘just beyond red’ and found between 0.78 – 1000 microns (μm). Over time, these calorific rays became known as ‘infrared’.