Question

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

(a)  Kirchhoff's Law

(b)  Newton's Law of cooling

(c) Stefan's Law

(d) Wien's displacement Law

Answer 1

Correct option (d) Wien's displacement Law

Explanation:

According to Wien's displacement law  λ_m T = constant

 λ_m = constant/T

So when a piece of iron is heated,  λ_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.