How do we determine the color of heated glass? If a glass of certain colour is heated, then how can we determine the corresponding colour that it will glow with?
For eg: I saw a question that asked "A blue glass when heated will glow with which colour?" and the answer was stated as "yellow" because "Blue glass appears blue at ordinary temperature as it absorbs all other colours. When it is heated, it emits white radiation deficient of blue colour, i.e., yellow coloured radiation."
How was this obtained? How is white light deficient of blue light yellow? How can we predict the same for a different case?
 A: The spectral emissivity, $e_{\lambda}$, of a surface is the power it emits in a narrow band of wavelengths centred on $\lambda$, expressed as a fraction of the power emitted by a black body of the same area in the same band.
The spectral absorptivity, $a_{\lambda}$, of a surface is the fraction of the incident radiant power it absorbs in a narrow band of wavelengths centred on $\lambda$.
Kirchhoff's law of radiation states that for any surface at any one temperature, $e_{\lambda}=a_{\lambda}$.
For blue glass  $a_{ \approx 470\ \text {nm}}$ is low compared to $a_{\text{other}\lambda s}$ so $e_{\text{other}\lambda s}$ is high compared with $e_{ \approx 470\ \text {nm}}$. In other words the glass emits other wavelengths much better than blue light.
A: you consider it a black body radiator, and Know the max of the spectrum at different temperatures, for practical use people use a color scale with temperatures versus color and compare the color to what they see. so you can adjust for different material , but the difference is not large for example between glass and iron
