Good day, student of introductory astronomy!
Since you were interested enough to inquire, I have
decided
to pass along the following to you. This is in regards to our
discussions
of the concepts of light, color, and thermal (also known as blackbody)
radiation.
The first concept to keep straight in your mind is
the
difference between reflected light and emitted light.
We observe
our surroundings through the reflection of visible light,
emitted
by our Sun or man-made sources. Emitted light is composed of photons
generated
by the matter emitting the light - it's an intrinsic source of light.
Reflected
light consists of photons whose origin is elsewhere but reflected
(scattered) from
some
object. Light falling upon matter can be absorbed, reflected, or
transmitted
(meaning that the light and matter interact very little - the matter is
transparent),
or often some combination.
Absorption, emission, reflection, transmission
A photon of light that is absorbed by an atom or
molecule
must have had a particular energy that was capable of
internally ``exciting''
that atom or molecule. Once absorbed, the photon has given up ALL of
its energy
to the atom or molecule, and so ceases to exist. For example, the
electrons
bound to the nuclei of atoms can be excited to higher energy orbitals
through
the absorption of photons. The electrons of molecules can also
be
excited to higher energy orbitals (molecules can also be excited into
modes
of vibration and rotation). If that excited atom or molecule can
(spontaneously)
de-excite itself before colliding with another particle of matter, a
photon
will be emitted. The emitted photon will have an energy
corresponding
to the how much energy the atom or molecule lost in the process of
de-excitation.
The corresponding wavelength of light emitted is inversely proportional
to
the energy of that photon.
Each atom (e.g., hydrogen, carbon, oxygen) and
molecule
(e.g., H2O, CO2) of a given elemental composition has a unique set of these
internal
energy levels (or modes of excitation). Thus we can identify the
elements
and molecules in some material simply by studying the wavelengths of
light
that are absorbed or emitted.
If the light passing near the atom or molecule
hasn't
the right energy to be absorbed, there is a chance that it might just scatter
(or bounce) off one of the bound electrons, sending the photon off in
some
semi-random direction. If the photon doesn't interact much with the
matter,
then it will just pass on through - and we say that the light is transmitted
through the matter*.
Color
Color is our brain's interpretation of our eyes'
detection
of the different wavelengths of visible light. This is the regime of
light,
with wavelengths between 400 nanometers and 700 nanometers (1 nanometer
=
1 billionth of a meter = 10-9 meter),
that
our eyes are sensitive to. Our eyes have 3 ``color'' sensitive
detectors
called cones: red, green, and blue.
Each type of cone is sensitive to a broad range in wavelength (called a
response
curve) of visible light, but this sensitivity is roughly bell-shaped,
peaking
at a particular wavelength, with lower sensitivity for wavelengths to
either
side of the peak. These response curves may be seen here.
Our perception of color comes from the brain's interpretation of how
many
photons of particular wavelengths fall within the response curves of
each
of the 3 types of cones. For example, an equal mixture1
of red and green light is interpreted as yellow, equal mixture of red and blue light is
interpreted
as magenta, and equal mixture of blue and
green
light is interpreted as cyan. Red, green,
and
blue are called primary colors; magenta, yellow, and cyan are called
secondary
colors. Our brain interprets a roughly equal mixture of red, green, and
blue
light (or a roughly equal mixture of secondary color light) as white
light.
Other ``colors'' are possible with the mixture of primary and/or
secondary
color light with white light. For example, pink is the combination of
yellow
and magenta light, or equivalently due to the combination of red and
white
light. It is probably more accurate to speak of degrees of response
between
the red, green & blue cones, rather than mixtures of red, green
&
blue light as I described above. Go here
and
here
for
more information.
Because the cones' sensitivities overlap in
wavelength,
any colored source of light will appear white if it is intense
enough
(all three cone types will be fully activated). It is also true that
the cones
are insensitive to low light intensities; the least sensitive blue
cones
stop reacting first, followed by the red ones, then the green ones, as
the
intensity of light diminishes. At low light levels, another detector in
our
retina takes over - the rods - and these are ``color blind,''
and
their response curve is also shown here. So very dim sources of broad spectrum light will appear
dull
white or grey.
The color of reflected light
Note that so far we are discussing the mixing of
different
color light, which is an additive process, not the mixing of
dyes,
pigments, or filters which is a subtractive process. The
molecules
in dyes and pigments absorb light strongly at some wavelengths and
reflect
others; colored filters absorb strongly at some wavelengths and
transmit
others. When white light falls upon an object that absorbs blue (i.e.,
shorter
wavelength visible) light strongly, the color yellow results. An object
that
absorbs strongly red (i.e., longer wavelength visible) light, reflects
a
cyan colored spectrum. This is the reason for the colors of Uranus
and Neptune,
which have methane in their atmospheres that strongly absorbs longer
wavelength
visible (and infrared) light. Chlorophyll is a molecule that absorbs
both short and long wavelength visible light - a green dominated
spectrum
is what is reflected, and accounts for the color of leaves. Something
that
appears ``black'' absorbs visible light of all forms very effectively.
There is nothing special or fundamental about color.
It
is merely our body's physiological (eye-brain) reaction to the
detection
of light of various wavelengths between 400 nm and 700 nm.
Color and thermal radiation, stars
As the temperature of a hunk of metal in a blacksmith's
fire
is increased, the heated metal follows Wien's and Stefan's laws of
thermal
radiators. Wien's law says that the wavelength of the peak intensity in
the
broad spectrum of light emitted will shift to shorter wavelengths;
Stefan's
law says that the total amount of light emitted per square meter of its
surface
per second increases. Initially, the metal is at room temperature and
radiates
with a thermal radiation spectrum that peaks at about 10 microns (1
micron is 10-6m = 1000 nm). It is
invisible to
the human eye in a dark room. As the temperature
increases, the thermal radiation spectrum's maximum (or peak) intensity
shifts to
shorter wavelength infrared light, and our skin and nervous system
first
detect the radiation emitted from the warming metal when we place our
hand
in its vicinity. When the metal's temperature approaches 700 Kelvin or
so,
our eyes begin seeing a dim, red glow (in an otherwise dark room). As
the
temperature further increases, the metal glows ever more brightly and
its
``color''2 changes from red, to
orange,
to yellow, to yellow-white, to nearly white. The change in color is due
to
the thermal radiation spectrum shifting to shorter wavelengths with the
increase
in temperature. Ultimately, the object becomes ``white hot'' because
light
from the broad, thermal radiation spectrum is emitted roughly uniformly
across the
visible spectrum and has equally activated the red, green, and blue
cones
- sometimes to the extent of saturating them (which the brain also
interprets
as white).
When you look at the brighter stars at
night, look
closely and you will discern that stars appear to have slightly
different,
albeit subtle, colors! To my eye, the brighter stars can appear pale
blue-white,
white, pale yellow-white, pale yellow-orange to pale orange.3 This is a real effect! Your eye is crudely measuring the
temperature
of the star - the above color sequence is one of decreasing
temperature. Stars
are, to a good approximation, thermal (blackbody) radiators. However,
keep in mind
that the apparent subtle colors of the planets is not a
temperature
effect. The visible light from planets is due to sunlight reflected
from
their atmospheres or surfaces, and any coloration you perceive is due
to
the absorption of certain wavelengths of light, as discussed above.
Planets
do emit thermal radiation-like spectra due to their finite temperatures
(due to the
absorption of sunlight, as well as internal energy sources), but these
appear
in the infrared portion of the electromagnetic spectrum of light.
Color perception is actually a rather complex
phenomenon,
and there is much more to it than meets the eye (or than we have
discussed
here). However, I hope to have provided you with a flavor of what goes
on.
thanks for your interest,
Professor Korista
*A technical detail (and so for
only for the really curious). A low density gas' interaction with light
differs in detail from that of bulk matter's interaction with light of
a given wavelength. Reflection and refraction could (or should)
actually be thought of as phenomena of coherent scattering. Thus,
for example, light traversing all the way through transparent glass has
still interacted with the electrons in the glass. But this light didn't
have the correct energy to cause a change in the quantum state of the
electrons, and so the interaction time is insignificant.
1More acurately: an equal response from the red and
green cones
due to the absorption rate of photons of the appropriate energies (or
wavelengths).
2The colors of "thermal radiators" are never stark
or "saturated"
(such as a red
traffic
light), because visible blackbody radiation spans the full
visible
spectrum, even if it favors long over short
wavelength,
for example.
3Even the brighter stars in the night sky are dim enough
that
the color-blind rods are contributing significantly to your perception
of
them.
Kirk T. Korista
Associate Professor of Astronomy
Department of Physics
Western Michigan University
email: kirk.korista@wmich.edu
last edited: 9 October 2007
