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Question:
If an iron or metal rod is heated, why does it change color at different temperatures?



Replies:
This is a deceptively simple question with a deceptively complicated "answer". The "answer" lies at the heart of the properties of a "black body", which is not necessarily "black" in the usual sense of the term "black". This in turn lies at the fundamentals of the quantum theory of radiation, which is certainly not simple. I will try to take a middle ground, which risks being unsatisfactory from either point of view. First: Stefan-Boltzmann's Law says that the TOTAL radiative energy of a black body is: Q = s x T^4 The constant "s" = 5.67x10^-8 (watts x m^2 x K^-4) Second: Wien's Law says that the maximum in this distribution of radiation Lmax = c' / T where Lmax is the maximum wavelength, T is the absolute temperature in kelvins and c' = 2898 micrometers x kelvins. Third: The eye (visible light) is only a "slice" of the TOTAL radiative energy but at a temperature of T = 1000 kelvins, Lmax ~ 3 micrometers, which puts it in the in the "hot" part of the visible spectrum (which is roughly 4 to 7 micrometers) -- not at the maximum but certainly "hot" enough so that the eye interprets the light to be "white" hot with a blue cast. This is because the details of the emitted radiation obeys "Planck's Law" which is a rather "fat" distribution. Planck's Law is a fairly messy function of the wavelength and the temperature -- too messy to attempt to write out here in all its glorious detail.

The "bottom line" is that as the temperature increases the eye sees no response, although the nerves can sense the "heat" of the infrared. As the temperature increases the source (its distribution is independent of the material unless it ignites) becomes more "red", then more "yellow", then "blue", then "white"hot.

The understanding of these laws of radiation lie at the roots of quantum mechanics. It would be unfair to give a glossed over, over simplified "explanation" of such a fundamental phenomenon.

Vince Calder


The reason it changes color has to do with how the radiation (energy spectrum) changes with temperature.

Not only does an object emit more light as the temperature is increased, but the most probable/likely photons to be emitted also changes. When the iron is not hot (say at room temperature), it is still emitting some light(radiation). However, it is not emitting much and the light it does emit is typically to low in energy to be seen by our eyes (think infra-red). As the iron is heated not only does it begin emitting more light, but the energy of the photons also tends to increase. Once there are enough photons of high enough energy we begin to perceive the glow. It starts as a red, but shifts towards orange and yellow (which are higher in energy than red) as the temperature increases.

Michael S. Pierce



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