Heat And Thermodynamics
Kirchhoff's Law
(i) At a given temperature and for a given wavelength 
, for all bodies,
= constant = 
Where 
= spectral emissive power
= monochromatic absorptive power
= emissive power of perfectly black body
(ii) 
.
Hence good emitters are good absorbers also.
(iii) The various colours of bodies are explained on the basis of this law.
(iv) Fraunhofer lines are explained on the basis of this law.
(v) The law implies that, at a particular temperature, a body can absorb only those wavelength which it is capable of emitting.
Colours - primary and complementary:
Primary colours are those which do not get dispersed when passed through a prism.
The primary colours are red, blue and green.
Complementary colours are those two colours which when produce white light.
The colour triangle indicate these colours as below.
Red + green = yellow
Green + blue = cyan (turquoise)
Blue + red = magenta (purplish red)
Complementary colours:
yellow + blue = white
magenta + green = white
cyan + red = white
Red + yellow + blue = black
When a green body is heated in a dark room then it appears red because it emits all colours except green and the emitted colours are domineated by red colour. It is also true vice-versa.
Fraunhofer lines:
These are the dark lines present in the continuous spectrum of Sum.
These are due to absorption of radiations.
Photosphere, the central part of Sun, is at a temperature of the order of 
K. It emits continous light of all wavelengths. These radiations pass through chromosphere which is at 6000 K. It contains certain elements in vapour form. These elements selectively absorb certain wavelengths. These missing wavelengths are dark Fraunhofer lines.
About 20000 such dark lines have been detected so far.
These dark lines belong to hydrogen, helium, sodium, iron, calcium etc.
At the time of total solar eclipse, photosphere is covered by moon. The elements present in the chromosphere emit the characterstic wavelength they had absorbed. Therefore, Fraunhofer lines appear as bright lines at the time of total solar eclipse.
Kirchhoff's law explains this phenomenon.
These lines are not perfectly black but they appear dark, because of their lower intensity in comparison to remaining part of spectrum.
ThesThese lines were named as A, B, C, D .... etc.
Variation of colour with temperature :
Light red colour at temperature 525°C
Cherry red colour at temperature 900 °C
Orange red colour at temperature 1100°C
Yellow colour at temperature 1200° C
White colour at temperature 1600°C
Higher the temperature of the body, higher the frequency of colour or lower the wavelength of colour.
The white light intensity is concentrated in yellow - green region.