Question
'If $\mathrm{r}=1$, then it is a white body'. Is this true ? Explain.
✓
Answer
No.
Since $a+r+t=1, a=0$ and $t=0$ for $r=1$. Therefore, by Kirchhoff's law, $e=a=0$ which is impossible as every body at temperature above $0 \mathrm{~K}$ does emit radiant energy, and $\mathrm{T}=0 \mathrm{~K}$ is impossible.A blackbody being a full radiator, when heated to high enough temperature it would emit thermal radiation at all the wavelengths and thus appear white. A perfect reflector $(r=1)$, on the other hand, is a poor emitter and thus would not necessarily appear white when heated.
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