BLACK BODY RADIATION

•         As we already saw in our blog ‘How do we see colors’, “It’s the material, Not the color! – how do we see color?”, black body is an ideal object.
•         Black body is an object that can absorbs all frequencies of radiation.
•         Due to this the object get heated up and when its temperature equals to the surrounding temperature, the object starts to radiate energy in the form of electromagnetic radiation.
•        Depending upon the temperature of the body the intensity of the radiation of different frequency varies.
•         Almost all the object behave like an black body (but not like an ideal one), example: sun, human body, hot objects…
•         In 18^th century when scientist thought that they found almost everything in the field of physics, black body concept came as a challenge in front of them.
•         Radiation from sun is observed and a graph was plotted.

•        From the graph, it is found that most of the radiation emitted by sun fall on the wavelength range of visible light.
•         Also when the temperature of the object decreases the curve move towards right and peak height decreases.
•         According to Classical physics energy is continuous. By using this concept scientist were unable to predict the behavior of the radiation emitted by sun.
•         Wien’s Displacement law showed that the wavelength emitted is inversely proportional to the temperature of the body.
•         Stefan – Boltzmann law showed that the intensity of radiation emitted is proportional to the fourth power of temperature of the body.
•         Rayleigh Jean’s formula showed that the intensity of radiation emitted is inversely  proportional to the fourth power of wavelength for a given temperature of an object.

•         But Rayleigh Jean’s formula was unable to explain the curve properly.
•         When wavelength decreases and tends to zero, the intensity goes to infinity.
•         This is called ultraviolet catastrophe.
•         Max Planck also worked on the black body.
•         Planck found that Classical ideas are not enough to explain the black body radiation curve.
•         Planck assumed certain conditions:

1.      Source of radiation are atoms.

2.      Energy is quantized.  E=nhv

3.      The emission or absorption of energy occur only when oscillator jumps from one energy state to another.

•         Using this assumptions , the energy radiated per unit volume is found to be
  
•         Here h is Planck’s constant, k is Boltzmann constant.
•         Planck initially called his theory “An act of desperation”.
•         To find a solution, an equation to explain the graph he made some assumption and introduced a constant (h).
•         After his work, Einstein while working on photoelectric effect proved that the assumptions made by Planck were correct.
•         Then a new field so physics came.
•         Quantum physics was born.