Perfectly black body - class-XI

Ideal black body absorbs all the radiation, no transmission, no reflection.

wein's and ferry's black bodies are well defined in there papers and sun can also be considered as black body because it is very good emitter. but coal is just a normal carbon compound and neither is it a good absorber or emmiter

• (A) The combination of radiation of all the visible wavelengths makes white light. Hence black  body radiation is white

Black body is a body which absorbs and emit all types of radiations completely. Since we know that lamp black is perfect black body so its absorptivity is $1.00$

Black Body radiation involves emission of a number of wavelengths, which by definition is continuous emission.

Ferry's black body is accurately represented by a fine hole in a double walled spherical cavity, evacuated and painted black.

a black body looks black because it absorbs light of all colors. so when it radiates energy as light, it will release all the colors which it had absorbed resulting in the emission of white light, because white light is the resultant of many component colors.

$Rediant Energy = \sigma T^2$

According to Wien's displacement law, lambda_max * T = constant. Initially, T1 = 3000K. If lambda_max changes by 9 microns, we need the initial lambda_max. Assuming lambda_max1 = 1 micron (typical for 3000K), then lambda_max2 = 10 microns. T2 = (1/10) * 3000 = 300K.

Stefan-Boltzmann law states E is proportional to T^4. T1 = 273 + 273 = 546K. T2 = 0 + 273 = 273K. Ratio E2/E1 = (273/546)^4 = (1/2)^4 = 1/16. Thus E2 = E/16.

Emissivity (e) is defined as the ratio of energy radiated by a body to that of a black body at the same temperature. For a black body, e = 1. For non-black bodies, 0 < e < 1.

An ideal blackbody is defined as an object that absorbs all electromagnetic radiation that falls on it, regardless of frequency or angle of incidence. Because it absorbs everything, it does not reflect any radiation.

Hollow enclosure blackened inside and having a small hole is a very good example of a black body.
Suppose once light enters inside it.
It may be absorbed or it may be reflected.
Since it is blackened from inside, there is a high probability that it will be absorbed.
Now if it is reflected, it will suffer multiple reflections and it is very unlikely that it will come out of the hole because the aperture of hole is too small. Moreover, with each reflection, more and more fraction of it will be absorbed. So, it will serve as a good black body.

we know that $\alpha +\rho +\tau =1$
${\alpha}= absorptivity$
${\rho}=reflectivity$
${\tau}=transmitivity$
so for black body ${\rho}\  and \ {\tau} \ will\  be \ zero$
so ${\alpha}=1$ so black hole has also ${\alpha}=1$ which is equivalent to black body.

It is given that the absolute temperatures of both the black body and the surroundings are constant with time, even after sunlight(radiation) is incident on it.

• When a body absorbs radiation, its temperature increases
• When a body emits radiation, its temperature decreases

Initially the black body is in bright environment. So, it appears dark and finally when its temperature becomes equal to temperature of furnace, it becomes invisible