Blackbody Radiation

The concept of Blackbody Radiation comes under the topic Radiation in Heat and Mass transfer. In this topic, we will be covering a general idea of what is black body radiation; Laws associated with black body radiation such a Plancks law, Wiens Law and Stefan-Boltzman Law.

By defn.Black body radiation is a type of electromagnetic radiation emitted by a black body at constant and uniform temperature.

Any body above absolute zero temperature or with a finite temperature emits energy in form of electromagnetic wave, which we call as Radiation.Every object in this room emit it own radiation, even we humans do emit radiations.

We see plants that are green in color. because in the presence of light, it absorbs almost all the colors from light source and emits only green color. This how we can see the color of any objects. So what about the objects that appear black. Black objects absorb all the radiation falling on it but it re-radiates energy. One such object is called a Blackbody. Then there is another question. What happens to radiation of objects when there is no light.

That is when objects are placed inside a black box with a small hole and observed. Any objects placed inside the black box always black. In most cases, it actually emits infrared light which is not visible to human eyes.

Then the objects are heated according to classical physics and observed. With increase in temperature, every object began to radiate colors but in the same order. First red, then yellow, white, blue and violet. All the colors that constitutes to visible spectrum in the order of low frequency to high frequency.

According to classical theory equations of a black body, the reading were supposed to be like this. But the actual readings from experiment appeared this.

This is nothing but the graph between Emissive power of light radiated to the wavelength of light at respective temperatures.

To find a solution for this issue, various scientists conducted so many experiments. And finally Max Plank came up with a brilliant solution. He said energy emitted is not emitted in form of waves but in form of energy packets called quanta and supported his theory with very famous equation E=hv. That is one of the greatest breakthroughs in science and happens to be the origin for Quantum mechanics.

Using Plancks equation the intensity of black body radiation is calculated and it is found that the intensity is

And its emissive power is calculated as

For a non black surface, when undergone same experiments, the emissive power is calculated as

Observing the graph we can tell that the maximum value of emissive power occurs at smaller wavelength as T increases.Simple example is a laser pointer.It is explained by Wiens disp law. Differentiate planck

Stefan-Boltzman law Total emissive power of a black body. Integrate Planck