Question

While reading structure of atom chapter no. 2 from ncert class 11th chemistry part 1 book, I got a doubt. In the book it says on page 40 that:

Hot objects emit electromagnetic radiations over a wide range of wavelengths. At high temperatures, an appreciable proportion of radiation is in the visible region of the spectrum. As the temperature is raised, a higher proportion of short wavelength (blue light) is generated. For example, when an Iron rod is heated in a furnace, It first turns to dull red and then progressively becomes more and more red as the temperature increases. As this is heated further, the radiation emitted becomes white and then becomes blue as the temperature becomes very high. This means that red radiation is most intense at a particular temperature and the blue radiation is more intense at another temperature. This means intensities of radiations of different wavelengths emitted by hot body depend upon its temperature.
So if on heating there is higher proportion and intensity of shorter wavelengths, then why red hot iron turns to white before blue as white light is consisting all wavelengths of visible colours?

Answer 1

As the temperature of an object increases, it emits radiation across different wavelengths of light. It first glows red, then as the temperature rises further the color changes to white because it emits light across the entire visible spectrum evenly, and finally appears blue at the highest temperatures due to a higher proportion of energy emitted at shorter wavelengths.

Step-by-step explanation:

The phenomenon in question involves the emission of electromagnetic radiation from heated objects, and is described by what is known as blackbody radiation. As the temperature of an object increases, the peak wavelength of the emitted radiation shifts to shorter values (from infrared towards ultraviolet). This corresponds to an increase in frequency and energy. As an object like an iron rod is heated, it first glows red, which is the longest wavelength of visible light. With increasing temperature, it starts to emit shorter wavelengths (higher frequencies) and hence, the color shifts to orange, then yellow, and finally to blue as the temperature increases.

The reason an object appears white before it turns blue is because, at the white-hot stage, it is emitting a more or less uniform spectrum across all visible wavelengths, resulting in the combined perception of white light. As the temperature is increased even further, the object emits a higher proportion of its energy in the blue end of the spectrum, giving it a blue appearance. The intensity of radiation at different wavelengths is represented on their respective graphs, with the height of the distribution indicating the intensity. The shift in intensity towards shorter wavelengths with higher temperatures causes the visible appearance of the object to go from red to white to blue.