define what is meant by a black - body. sketch black-body spectra for two object?

Question

one at temperature T1 and the other T2 where T1 is bigger than T2.

Answer 1

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A black body is a perfect emitter, it emits eveything it absorbs. That's why black car seats get so hot in the Sun. Its spectrum looks a bit like the Maxwell-Boltzmann distribution, like a normal curve but skewed to the left if you are plotting intensity (y axis) against wavelength (x axis). For T1>T2, the peak on the graph for T1 will be further to the left than for T2 using Wien's law: [wavelength of maximum emission] x temperature = constant)

intensity

|

| /~\

| / -- \

| / --- -\

| - ------- -----------------

-----------------------------wavelength

HTH

Answer 2

according to wien's displacement law the wavelenght of a photon is inversely proportional to the temperature of the blackbody from which the photon is originating. ie L = W/T ---------------1 ( L is the wavelength in meters, W is wien's constant, T is temperature in Kelvins) now photon energy = 1.55eV = 1.55 * 1.602 * 10^-19 joules photon energy = h c/L ------- 2 (h is plank's constant, c is the speed of light, L is the wavelength) from equations 1 & 2 we get T = WE/hc -------3 ( E is photon energy in joules) h = 6.626 * 10^-34 Js, W = 2.93 * 10^-3 mK substitute values in equation 3 and you will get temperature as 3660 K or 3387*C remember this law holds only for perfect blackbodies. also since we are using the mean wavelength it is an approximation.