What Is Infrared?

When we talk about light, we may first think of visible light, like the glare of the sun on a sunny day or the soft glow of a light bulb at night. But the only visible light we can see only makes up a small part around us in the world.
Infrared light was first discovered by Sir William Herschel in 1800. He split light into a rainbow (called a spectrum) by passing sunlight by a prism, and then placed a thermometer in different colors in that spectrum. Unexpectedly, he found the thermometer showed a rise in temperature, even when placed in the dark area beyond the edge of the red light. He hypothesized that there must be more light beyond the color red that we simply could not see with our own eyes.
Infrared (IR) light is the part of the EM spectrum that people encounter most in our daily life, although much of it goes unnoticed. Infrared radiation is a kind of electromagnetic radiation, as are radio waves, ultraviolet radiation, X-rays and microwaves. It is invisible to human eyes, but people can feel it as heat. Infrared light falls just outside the visible spectrum, beyond the edge of what we can see as red. Infrared waves are longer than those of visible light, just beyond the red end of the visible spectrum. Infrared (IR) falls in the range of the (EM) spectrum between microwaves and visible light. It has frequencies from about 3 GHz up to about 400 THz and wavelengths of about 30 centimeters (12 inches) to 740 nano meters  (0.00003 inches), although these values are not definitive.

According to the research, incandescent bulbs convert only about 10 percent of their electrical energy input into visible light energy; about 90 percent is converted to infrared radiation. Household appliances such heat lamps and toasters use IR radiation to transmit heat, as do industrial heaters such as those used for drying and curing materials. These appliances generally emit black body radiation with a peak energy output below the wavelength of visible, though some energy is emitted as visible red light.