Visible Light Waves
Visible light waves are
the only electromagnetic waves we can see. We see these waves as the
colors of the rainbow. Each color has a different wavelength. Red
has the longest wavelength and violet has the shortest wavelength.
When all the waves are seen together, they make white light.
When white light
shines through a prism or through water vapor like this
rainbow, the white light is broken apart into the colors of
the visible light spectrum.
How do we "see" using
Cones in our eyes are
receivers for these tiny visible light waves. The Sun is a
natural source for visible light waves and our eyes see the
reflection of this sunlight off the objects around us.
The color of an object
that we see is the color of light reflected. All other
colors are absorbed.
Light bulbs are another
source of visible light waves.
This is an
photograph taken from the space shuttle of Phoenix, Arizona.
This is a
true-color satellite image of Phoenix, Arizona. Can you see
a difference between this image and the photo above it?
There are two types of
color images that can be made from satellite data - true-color and
false-color. To take true-color images, like this one, the satellite
that took it used sensors to record data about the red, green, and
blue visible light waves that were reflecting off the earth's
surface. The data were combined later on a computer. The result is
similar to what our eyes see.
Here is a
false-color image of Phoenix. How does it compare to the
true-color and space shuttle images on this page?
A false-color image is
made when the satellite records data about brightness of the light
waves reflecting off the Earth's surface. These brightnesses are
represented by numerical values - and these values can then be
color-coded. It is just like painting by number! The colors chosen
to "paint" the image are arbitrary, but they can be chosen to either
make the object look realistic, or to help emphasize a particular
feature in the image. Astronomers can even view a region of interest
by using software to change the contrast and brightness on the
picture, just like the controls on a TV! Can you see a difference in
the color palettes selected for the two images below? Both images
are of the Crab Nebula, the remains of an exploded star!
Here's another example -
the below pictures show the planet Uranus in true-color (on the
left) and false-color (on the right).
The true-color has been processed
to show Uranus as human eyes would see it from the vantage point of
the Voyager 2 spacecraft, and is a composite of images taken through
blue, green and orange filters. The false color and extreme contrast
enhancement in the image on the right, brings out subtle details in
the polar region of Uranus. The very slight contrasts visible in
true color are greatly exaggerated here, making it easier to
studying Uranus' cloud structure. Here, Uranus reveals a dark polar
hood surrounded by a series of progressively lighter concentric
bands. One possible explanation is that a brownish haze or smog,
concentrated over the pole, is arranged into bands by zonal motions
of the upper atmosphere.
What does Visible Light
It is true that we are
blind to many wavelengths of light. This makes it important to use
instruments that can detect different wavelengths of light to help
us to study the Earth and the Universe. However, since visible light
is the part of the electromagnetic spectrum that our eyes can see,
our whole world is oriented around it. And many instruments that
detect visible light can see father and more clearly than our eyes
could alone. That is why we use satellites to look at the Earth, and
telescopes to look at the Sky!
This is a
visible light image of Phoenix, Arizona, taken from the GOES
satellite. We often use visible light images to see clouds
and to help predict the weather.
We not only look
at the Earth from space but we can also look at other
planets from space. This is a visible light image of the
planet Jupiter. It is in false color - the colors were
chosen to emphasize the cloud structure on this banded
planet - Jupiter would not look like this to your eyes.