Glow Color Change and Temperature
If an iron or metal rod is heated, why does it change
color at different temperatures?
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
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.
The reason it changes color has to do with how the radiation (energy spectrum) changes
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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Update: June 2012