Our eyes are detectors, which detect visible light waves. Visible light is one of the few types of radiation that can go through Earth's atmosphere and be detected on Earth's surface. There are other forms of light, which we cannot see. Humans only see a very small part of the entire electromagnetic spectrum.
The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The difference between these types of radiation is their wavelength and frequency. Wavelength increases and frequency decreases from gamma rays to radio waves. All forms of radiation travel at the speed of light. .
Infrared radiation lies between the visible and microwave sections of the electromagnetic spectrum. Infrared waves have wavelengths longer than visible and shorter than microwaves, and have frequencies lower than visible and higher than microwaves. Infrared is broken into three categories. Near, mid and far infrared. Near infrared refers to the infrared spectrum part closest to visible light. Far infrared refers to the part closer to the microwave region. Mid-infrared is between these two.
The primary source of infrared radiation is heat or thermal radiation. This is due to the radiation produced by the motion of atoms and molecules in an object. The higher the temperature, the more the atoms and molecules move hence the more infrared radiation they produce. Any object with an temperature above absolute zero (-273.15oC) radiates infrared. At absolute zero all atomic and molecular motion ceases. .
E.g. of infrared: Humans feeling heat from sunlight is from infrared. Our eyes cannot see it, but the nerves in our skin feel it (as heat).
History of Infrared.
Sir Frederick William Hershel (1738-1822) was born in Hanover, Germany. He became well known as a musician and an astronomer. He emigrated to England in 1757. He then started constructing telescopes to survey the night sky.