[Physics Class Notes] on Black Body Radiation Wien Displacement Law Pdf for Exam

We know that to survive or stay in thermal equilibrium there must be a black body which must emit radiation at the same rate as it absorbs on the same hand and so it must also be a good emitter as well of radiation. The waves which are emitting Electromagnetic waves can absorb all the frequencies.  The emitted radiation by the blackbody is called blackbody radiation.

Wien’s Displacement Law

In this topic we will discuss the Wien’s law that is the displacement Wien’s law which states that the black-body radiation curve for the temperature which are different and will peak at different wavelengths is also that is they are inversely proportional to the temperature. The shift of peak is said to be a consequence of direct law of Planck radiation. This tells us  the spectral brightness of black-body radiation as a function of wavelength at any given temperature. He has described the whole shift of the black-body radiation spectrum which is shorter toward  wavelengths as temperature increases.

Observing we see that the displacement law of Wien’s states that the radiance of spectral which is of a body which is black and which has radiation per unit wavelength peaks at the wavelength that is denoted by the symbol  λpeak  and the equation is given by the following :

 λpeak = b/T

Here we can see that the capital letter that is  T is denoted as the temperature which is absolute. The Small letter b is said to be a constant of proportionality which is called as the Wien’s displacement constant, which is again equal to the number = 2.897771955…×10−3 m⋅K, so here we can say  that b ≈ 2898 μm⋅K. If we are considering the peak of the body which is black which is emitting per unit frequency constant as well. The formed law here remains the same: that is we can say that the wavelength of peak is said to be inversely proportional to temperature which is T, and the peak frequency is said to be directly proportional to temperature that is T.

Wien’s displacement law can also be referred to as the “Wien’s law”, a term which is used for the approx.

Example

From the above information we can easily deduce that a fire in the wood which is approximately 1500K hot, gives out radiation of peak at 2000 nm. This means that the radiation which is in majority from the wood fire is beyond the human eye’s visibility. This is why we consider that the campfire is an excellent source of warmth but a very poor source of light.

The sun’s temperature on its surface is 5700 K. Using the displacement of Wien law; we   the radiation of peak radiation output at a wavelength of 500 nm. This basically lies in the portion which is the green portion of the visible light spectrum. This seems that our eyes are highly sensitive to this wavelength of particular visible light. We really should be appreciative of the fact that a rather unusually large portion of the radiation of sun’s radiation which falls in a fairly small visible spectrum.

When a metal piece is heated it becomes ‘red hot’. This is the visible longest wavelength. On heating further, it moves from the red colour to orange colour  and then yellow colour. At its hottest level the metal will be seen to be glowing white in colour. This is the wavelength which is shorter wavelengths dominating the radiation.

If we Stay tuned with to learn more about black body radiation, the source of light sources and much more.

Discovery of Wien’s Displacement 

The law is named after the great scientist Wilhelm Wien, who in 1893 derived it first  based on an argument of thermodynamics. Wien had considered the process of adiabatic expansion of a cavity which is containing waves of light in equilibrium which is thermal. A principle which is very general of thermodynamics is that a thermal equilibrium state, when it is expanded very slowly, stays in thermal equilibrium. 

The principle of the process of adiabatic substances which are allowed by Wien to conclude that for each mode, which is said to be of the adiabatic invariant frequency or energy is only a function of the other adiabatic invariant. That is we can consider that the  frequency or we can say temperature is equal. A variant which is modern of Wien’s derivation can be found in the textbook by the name Wannier.

When mr. Max Planck formulated the correct black-body radiation function later it did not explicitly include Wien’s constant that is b. Rather the constant of  Planck’s that is h was created and introduced into his new formula. From the constant of Planck’s h and the Boltzmann constant that is denoted as k, Wien’s constant that is b can be obtained.

Maxima Differ According to Parameterization

If we notice this for a given temperature, it can be seen that parameterization by frequency implies a very different maximal wavelength than parameterization by wavelength alone. If we take the example that too by using temperature which is denoted by capital letter T = 6000 K and parameterization by wavelength as well. The wavelength for maximal radiance of spectral is denoted as  λ = 482.962 nm with frequencies which are corresponding frequency that is ν = 620.737 THz. 

These functions of the radiance are density functions, which we can simplify and say that are probability density functions which are scaled to give units of radiance. The  density function has very different shapes for the different parameterizations. which is depending on relative stretching or we can say the compression of the abscissa, which is always said to measure the changes which are  in probability density relative to a linear change. Sincerely know that the frequency and wavelength have a reciprocal relation, they represent significantly that is non-linear which shifts in probability density relative to one another.