why piece of iron heated in a hot flame first becomes dull red , then reddish yellow and finally white hot ?

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Solution

The colour of iron or any substance changes on heating to higher temperatures because on heating the energy radiated by the body depends on the temperature of the body ( E directly proportional to fourth power of temperature). As the temperature of body increases so the energy radiated. Now, radiated energy frequency depends on the wavelength also. So as energy changes correspondingly there occurs a change in frequency of wavelength of emitted light and thus the colour of body.

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