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. The visible light part of the spectrum ranges in wavelengths from about 380 or 400 nm
to 700 or 780 nm depending on which reference you may use. In fact it also depends on the sensitivity of
a specific persons eyes. When all the waves are seen together, they make white light.
When white light shines through a prism, the white light is broken
apart into the colors of the visible light spectrum. Water vapor in the
atmosphere can also break apart wavelengths creating a rainbow. This is due to the property called
Dispersion.
Each color in a rainbow corresponds to a different
wavelength of electromagnetic spectrum.
How do we "see" using Visible Light?
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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.
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| This is a photograph taken from the space shuttle of
Phoenix, Arizona. |
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This is a true-color satellite image
of Phoenix, Arizona. Can you see a difference between this image and
the photo above it?
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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.
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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.
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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 show us?
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!
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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.
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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.
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RETURN TO THE ELECTROMAGNETIC SPECTRUM
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