A piece of iron is heated in a flame_ It first becomes dull red then becomes reddish yelJow and finally turns to white hot. The correct explanation for the above observation is possible by using

According to Wien's displacement lawλmT=constant λm=constantTSo when a piece of iron is heated, λm 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.