A wave is characterized by a wavelength. The wavelength is the distance from crest to crest or from trough to trough. We classify electromagnetic waves according to their wavelength. A graphical representation of the electromagnetic spectrum is shown in the figure below. The visible part of the spectrum may be further subdivided according to color, with red at the long wavelength end and violet at the short wavelength end, as illustrated in the following figure.
Hot objects emit radiation with a distribution of wavelengths.
But the average wavelength of the radiation decreases as the temperature of the object increases. Most thermal radiation lies in the infrared region of the spectrum. We cannot see this radiation, but we can feel it warming our skin. Different objects emit and absorb infrared radiation at different rates. Black surfaces are generally good emitters and absorbers, while silvery metal surfaces are poor emitters and absorbers.