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?

Here is the link to download book: https://ncert.nic.in/textbook/pdf/kech1dd.zip


1 Answer 1


White is not a color. It is the signal sent by the eye to the brain when the light contains the same proportion of red, orange, yellow, green, blue and violet as in the sun light. If the light entering the eye contains more blue than other colors, the eye sends the "blue" information to the brain.

So consider what light is emitted when the temperature of any body is increasing. Before 500°C, hot bodies do not emit any visible light. Maybe they emit some infra-red, but no visible light.

When hot objects are at around 600°C, they begin emitting a little light, but with more red light than any other color. At around 1500°C, they are brighter. They still emit all colors, but the light contains more orange rays than any other color. They look orange, but overall they look very bright. It is necessary to wear spectacles to look at such a hot body.

At still hotter temperatures, near 6000°C, the hot bodies look extremely bright, but they look yellow. Well ! This yellow is nearly similar to the sun light. So the light looks white, like the Sun, which is a little yellow.

Blue light emitting bodies are not easy to discover on Earth, because we are unable to produce the high enough temperatures. At these high enough temperatures, all ovens will melt and be vaporized. But blue lights can be observed on some stars, which are much brighter and thousand times hotter than the sun. But they emit a huge proportion of ultra-violet light.

Iron can never be hot enough to produce light perceived as blue. Hot iron may produce red light. Orange is the maximum.

  • 3
    $\begingroup$ Rather, iron can produce blue light (430-470 nm), but cannot produce light perceived as blue (having majority of emission in short visible wavelengths) $\endgroup$
    – Poutnik
    Commented Jun 11, 2023 at 19:56
  • 3
    $\begingroup$ Note that the Sun, observed from orbit, is like white. It becomes yellow observed from surface, after Rayleigh dispersion of large portion of violet/blue light in atmosphere. $\endgroup$
    – Poutnik
    Commented Jun 11, 2023 at 20:16
  • $\begingroup$ OK Poutnik. All you say is correct. But I have simplified the description of the phenomena. I am not sure our correspondant would have been happy by reading an answer containing the change of the black body radiation spectrum when the temperature is increasing. $\endgroup$
    – Maurice
    Commented Jun 13, 2023 at 20:39

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