Answer
see explanation
Work Step by Step
When a piece of iron is heated up it emits radiation called thermal radiation. At normal temperatures this radiation falls in the infrared region, therefore is not visible to our eyes. As the temperature increases the radiation emitted will have shorter wavelength, therefore it will be in the visible spectrum. As the temperature increases the wavelength becomes shorter. Even though the energy is emitted in all the wavelengths, our brain will see a predominant colour: we will start to see a red glow that will shift into orange and yellow as the wavelength gets smalls.
At a certain point we will see all the wavelengths emitted, which our brain will interpret as white color.
