Two important concepts for quantum physics are thermal radiation and blackbody. From now on we will study each of them.

## Thermal radiation

Any surface of a body that is above absolute zero emits electromagnetic radiation. Since this energy is related to temperature, it is called **thermal radiation**.

The thermal radiation emitted by the surface of a body at room temperature is infrared, an invisible radiation. If we raise the temperature of a metal plate to 600 ° C, for example, the emitted radiation will still be infrared, but now we will be able to “perceive it” if we move our hands closer to the plate. Increasing the temperature further to around 700 ° C will not only bring out intense infrared radiation, but we will also be able to observe the emission of reddish light.

If the temperature of the metal plate continues to rise and assuming that the melting temperature is not met, we will observe increasingly intense infrared radiation, and the plate gradually changes from red to orange, then to yellow, and so on. onwards, tending to white.

As blue light comes out, its mixing with the other lights gives us the sensation of white, as with the lit filament of an incandescent light bulb. If a body that has already reached white coloration continues to have increased temperature, it will tend to acquire a bluish coloration. That's why blue stars are the hottest.

## Stefan-Boltzmann Law

In 1879 Stefan obtained, empirically, the equation that Boltzmann demonstrated mathematically in 1884. The expression is:

(Stefan-Boltzmann Law)

Where:

Pot = total power radiated from the outer surface of a body at an absolute temperature T;

e = emissivity or emission power of the body, depending on the nature of the emitting surface and may assume values between 0 and 1 (dimensionless quantity);

σ = Boltzmann constant, whose value is σ = 5.67x10^{-8} W / m^{2}K^{4};

A = emitting surface area.

It is important to point out that, according to Stefan-Boltzmann's Law, the radiated power depends on the absolute surface temperature of the body in the fourth power, thus being the determining factor in the equation.

Stefan-Boltzmann's law can also be expressed as follows:

Where** I** is the total intensity of thermal radiation emitted by the body, ie the total amount of energy emitted per unit of time and per unit area of the outer surface of the body.

According to classical electromagnetic theory, thermal radiation is emitted by electrical charges from the body, oscillating at various frequencies near the surface due to thermal agitation. Thus, radiation is emitted in a continuous range of frequencies, that is, in a continuous spectrum:

According to Classical Physics, as thermal radiation shines on a body, the electric charges near the body's surface are agitated, so that some of the incident energy in the body is absorbed by it. In 1859 Gustav Kirchhoff realized that the absorption power of a body is equal to its emission power. Mathematically:

Therefore, a good body absorbing thermal radiation (bad reflector) is also a good emitter. Similarly, a bad absorber (good reflector) is also a bad emitter.