All electromagnetic radiation is light.
Visible light is the only part of the spectrum you can see.
For all your life, your eyes have relied on this one narrow band
of EM radiation to gather information about your world.
Though our Sun's visible light appears white, it is really the combined light
of the individual rainbow colors with wavelengths ranging from violet
at 380 nanometers to red at 700 nanometers.
Before Isaac Newton's famed experiment in 1665,
people thought that a prism somehow colored the Sun's
white light as it bent and spread a sunbeam. Newton disproved this
idea by using two prisms. To show that white light is made up of
the bands of colored light, Newton used a second prism to show that the bands
of colored light combine to make white light again. Visible light
contains important scientific clues that reveal hidden properties of objects
throughout the Universe. Minute gaps in energy at specific
visible wavelengths can identify the physical condition and composition
of stellar and interstellar matter. Human eyes aren't nearly
sensitive enough to detect these faint peaks, but
scientific instruments can. Scientists can learn the composition of an atmosphere
by considering how atmospheric particles scatter visible light.
Earth's atmosphere, for example, generally looks blue
because it contains particles of nitrogen and oxygen which are just
the right size to scatter energy with the wavelength of blue light.
When the Sun is low in the sky, however, light travels through more
of the atmosphere and more blue light is scattered out of the beam of sunlight before
it reaches your eyes. Only the longer red and yellow wavelengths
are able to pass through, often creating breathtaking sunsets.
When scientists look at the sky, they don't just see blue,
they see clues about the chemical composition of our atmosphere.
However, visible light reveals more than just composition.
As objects grow hotter, they radiate energy with a shorter
wavelength, changing color before our eyes. Watch a
flame shift from yellow to blue as it is adjusted to burn hotter.
In the same way, the color of stellar objects tell scientists
much about their temperature. Our Sun produces more yellow
light than any other color because of its surface temperature.
If the Sun's surface were cooler, say 3,000 degrees celcius,
it would look reddish, like the stars Antares and Betelgeuse.
If the Sun were hotter, say 12,000 degrees celcius,
it would look blue like the star Rigel.
Like all parts of the electromagnetic spectrum, visible light data can also
help scientists study changes on Earth such as assessing damage from
a volcanic eruption. This NASA EO-1 image
combines both visible and infrared data to distinguish between snow
and volcanic ash and to see vegetation more clearly.
Since 1972, images from NASA's Landsat satellite
have combined visible and infrared data to allow scientists
to study changes in cities, neighborhoods, forests, and farms over time.
Visible light images taken by NASA's
Mars landers have shown us what it would look like to stand on another planet.
They have expanded our minds, our imagination, and our understanding.
NASA instruments can do more than passively sense
radiation, they can also actively send out electromagnetic waves
to map topography. The Mars Orbiting Laser Altimeter
sends a laser pulse to the surface of the planet and sensors measure the amount of
time it takes for this laser signal to return. The elapsed time
allows the calculation of the distance from the satellite to the surface.
As the spacecraft flies above hills, valleys, craters, and other surface
features, the return time varies and provides a topographic
map of the planet's surface. Back in Earth orbit,
NASA's ICESat mission uses the same technique to collect data about
the elevation of the polar ice sheets to help monitor changes in the amount
of water stored as ice on our planet.
Laser altimeters can also make unique measurements of the heights of clouds,
the top of the vegetation canopy of forests, and can 'see' the distribution
of aerosols from sources such as dust storms and forest fires.
Finally, visible light helps us to explore the
far reaches of the universe that humans could not hope to reach physically.
Using visible light, the Hubble Space telescope has created
countless images that spark our imagination, inflame our curiosity,
and increase our understanding of the Universe.