A piece of iron is heated in a flame. It first becomes dull red then becomes reddish yellow and finally turns to white hot. The correct explanation for the above observation is possible by using
AIPMT 2013, Easy
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According to $Wien's$ displacement law
${\lambda _m}T = constant$
${\lambda _m} = \frac{{constant}}{T}$
So when a piece of iron is heated, ${\lambda _m}$ decreases $i.e.,$ with rise in temperature the maximum intensity of radiation emitted gets shifted towards the shorter wavelengths. So the colour of the heated object will change that of longer wavelength $(red)$ to that of shorter $(reddish\,yellow)$ and when the temperature is sufficiently high and all wavelengths are emitted, the colour will become white.
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