William Herschel (1800) and Johann W. Ritter (1801) discovered that there were regions of the light spectrum invisible to the naked human eye.
Herschel used a prism and thermometer to measure the temperature of each of the seven colors that emitted after the light passed through the prism and split into seven colors of the rainbow (violet, indigo, blue, green, yellow, orange and red). He also wanted to use a control for his experiment and hence measured the temperature beyond the visible spectrum. To his amazement he found that the region beyond the red portion of the spectrum had the highest temperature. This area of the spectrum could not be seen but felt and was called the infrared region.
Ritter a year later discovered electro- magnetic emissions in the region beyond the violet end of the visible light spectrum. In his experiment he found that the silver chloride normally darkened when exposed to visible light and darkened even more when exposed to the light beyond the 'violet'end of the spectrum; this was called the ultraviolet region.