Black body

Temperature and black body:

Physicists often separate a spectre into two components, one of which depends on the temperature and the other on the chemical composition. The black body spectrum is the first component. The second component contains emission or absorption lines which depend on the matter of which the object is made.

A black body spectrum is a theoretical model: it is the electromagnetic radiation from an object which emits no spectral lines ^?  but only emission due to its temperature.

A black body emits in all wavelengths, but the maximum emission wavelength depends on its temperature. The hotter it is, the more the maximum emission shifts towards the short wavelengths. The wavelength of red light is greater than that of blue light, a blue star is hotter than a red one.

Wien’s displacement law gives the maximum emission wavelength: $\lambda \text{(in m)} = \frac{0.003}{T}\text{K}$

By measuring the position of the maximum emission on its spectrum, the temperature of a body can be identified.

For example, the human body is at 37° Celsius, or $37 + 273 = 310^{\circ} \space \text{Kelvin}$. This temperature corresponds to a wavelength of $9.7 \times 10^{-6}$ meters. The human body emits at about 10 microns ^? , that is infrared radiation.

With a temperature of 5,780K, the Sun has a maximum emission in the “visible” part of the electromagnetic spectrum. But it would be more appropriate to say the opposite: the “visible”, in other words the sensitivity of the human eye, corresponds to the Sun’s maximum emission.