Physics- Black Body And The Black Body Radiation

Black Body And The Black Body Radiation 

R.M. Parvathavarthini, XII – 2C  (2021-22)

            

      Let us understand about black bodies and the black body radiation from the view of the scientists who created this concept.

       In the mid 19^th  century, physicists Rayleigh and Jean gave an equation which described, how the intensity of electromagnetic waves vary with their wave lengths. Now, we must know what is intensity. Though there are many equations and technical definitions, let’s simply say that intensity means brightness! Hence, Raylight and Jean explained how bright they  are when their wavelengths change. They said as the “wave length increases”’ they become “less brighter”.

    Since any new equation developed in physics needs both theoretical and experimental proof, the scientists did few experimental proof,  but to their surprise, it gave a different result! It said that the wavelength increases and intensity decreases only in the region between visible light and IR.

          But, in the UV – visible light region the intensity increases as the wave length increases! Scientists couldn’t believe this strange behaviour. They were wondering if there was any “mystery” in the UV region. But a German scientist Max Planck thought different and he asked “ what if we understood the basic ideas wrong?” i.e., We had missed out something about radiation and atoms? And also he resolved the confusion by stating that, “ATOMS ALSO EMIT AND ABSORB RADIATION” and “ENERGY IS QUANTIZED”.  'Quantized' means that energy is contained in “wave packets” and energy can be 1E, 1000E, 5196E but never 7.5 E or 3/2 E! He altered the Rayleigh – Jean’s equation, and provided the correct graph.      

  

      

          This is called the black body radiation. Generally, we require special equipments to do specific  experiments, eg. a thermometer to study about heat, a spring balance to learn weight as a force, likewise to experimentally find the relation between the intensity and wavelength of electro magnetic wave, scientists used a “black body”. As the name says, the object is completely black.

          Emit or absorb all the colours (all the rays in light). But why?

We know that an object of a specific colour absorbs all the colours except its own. That is, a blue coloured paper absorbs all six colours except  blue. A white paper is the mixture of 7 colours, therefore can it absorb any colour? Definitely No! So, it reflects it or allows light pass through it.

        What about black? It is not a colour, but represents the “absence of colour”, i.e., it has no colour. Hence, it could absorb all the colours of VIBGYOR! [That is why we use black coloured umbrellas in summer to protect ourselves from sunlight!] Therefore, if we take a box and coat it completely black it could absorb 80 – 90% of the radiation or light that hits it.

Why 80 – 90%? Because, there are always a few exceptions and no matter how much blackish the body is, it partly reflects some amount of light. Now, if the black body absorbs huge amount of radiation, it gets more and more hotter or we could say it gets more and more thermal energy. Now, what does thermodynamics say?

         Whenever an object is hotter than its surrounding, heat flows from hotter body to the colder region, until they both get in equal temperature. For us, since our black body is hotter, heat flows from it to the surrounding in the form of radiation itself! Remember, that is why a black body  in any object that emits or absorbs all radiation in thermal equilibrium! If you’re wondering about why does Planck’s graph show that strange wave and what was the mistake in Rayleigh.

·      Think about, what if a body which is not black in colour, behaves like a black body. Is it possible?

·      Sun is a very hot object and it emits enormous heat in form of radiation. Can it be a black body?

If you’ve found the answer for the 1^st question, then this will be  very easy!