"What is
Physics Good For?"
Color and
Temperature
Another thing physics is good for is making
connections between apparently unrelated subjects.
Physics is supposed to explain everything right? So
everyhing is related to everything else through
physics (sounds a bit religious doesn't it?).
Here goes.
Color is related to the frequency (or
wavelength) of light. The visible spectrum runs
from red, wavelength = 700 nm and f = 4.3 x
1014 to blue, wavelength = 400 nm and f
= 7.5 x 1014.
But temperature is also related to color. This is
because hot things radiate light (for instance, the
filament in an incandescent bulb). The temperature
of the object affects the color of the light that
is radiated. "Red hot" things glow red, "white hot"
things glow white. How does this work? There
are some complicated details, but here is the
basis: as we discussed in class, temperature is a
measure of the internal energy of a material. The
hotter a material is, the faster its atoms are
moving. But light is emitted when charged particles
vibrate. If an electron or an ion (the core of an
atom, including its inner electrons but not those
involved with bonding) is vibrating fast enough, it
may emit light. Clearly, the hotter an object is,
the more often this will happen, and the higher the
frequencies involved will be. Thus, a hot object
that appears a dim red is cooler than one which is
bright orange. An even hotter object will be
emitting radiation across the whole visible
spectrum resulting in a white appearance(for
instance, the filament in an incandescent bulb).
The exact distribution of colors emitted by a
material at a given temperature depends a bit on
the composition of the material. However, to a
great extent it does not. The "ideal" spectrum is
known as "Blackbody radiation" or the "blackbody
spectrum". For a given temperature blackbody, the
spectrum has a peak at some particular wavelength.
The hotter the body, the shorter ("bluer") the
wavelength at the peak. An example is shown in the
graph
The color/temperature relationship is extremely
useful, and many different processes and devices
make use of the relationship between temperature
and color. One example is photographic film and
lighting equipment. Color film contains
photosensitive chemicals that record the light
which strikes them. Different chemicals are
sensitive to different colors of light. The mixture
of these chemicals on the film affects how
sensitive the film is across the visible spectrum.
You want film that is "balanced", that is, it is
sensitive to different colors in the same
proportions that are present. But these proportions
depend on the light source! Film balanced for
sunlight is different than film balanced for flash.
Incandescent light is different still. The
manufacturers of film (Kodak, Fuji,
etc.) specify the balance of film by specifying the
temperature of the light. For instance, sunlight
has a temperature of 5500 K.
Scientists also use color as a visual
indicator of temperature, radiation intensity, or
other properties. Images that are acquired with
radio telescopes, infrared detectors or other
instruments may be "color coded" so that the data
may be more easily interpreted. The image to the
left shows data on the "El Nino" of 1997-98. This
phenomenon, a increase in the temperature of the
surface waters in the eastern Pacific ocean can
cause severe economic damage in the US and
elsewhere due to flooding and other climatic
changes.
Steelmakers and others who must heat materials
to specific temperature ranges use the
temperature/color relationship to visually
determine when a material is "hot enough." There is
a fairly standard nomenclature:
Temperature (°C) |
Color |
480 |
Barely red in the dark |
600 |
Dark red |
800 |
Cherry red |
950 |
Orange, barely visible in sunlight |
1100 |
Orange-yellow, visible in bright
sunlight |
1300 |
Light yellow, nearly blinding, welding
goggles required. |
1500 |
Nearly white, blinding |
A last example is a category of devices known as
"optical pyrometers." These are a type of
thermometer that measures the temperature of an
object by measuring the color of the light coming
from it. That is, it is a device which does
quantitatively the same chore that your eye does
qualitatively in using the scheme above.
You can get a lot more information about this
subject on the internet. Here are a few search
engines
1. Alta Vista
2.
Google
3. Yahoo
4.
AlltheWeb
5.
Dogpile
6. Ask Jeeves
7. Infoseek
And here are a few good links to get you
started.
1. 2. 3. 4.
This site is made
possible by funding from the National Science
Foundation (DUE-9981111).
©2007
IUPUI, all rights
reserved.
|