When you use a remote control to change channels on
your TV, your remote is using light waves. But this light is beyond
the visible spectrum of light you can see. Back in 1800,
William Herschel conducted an experiment measuring the temperature changes
between the colors of the spectrum, plus one measurement beyond visible red.
When that thermometer registered a temperature warmer than all the other colors,
Herschel had discovered another region of the electromagnetic spectrum, infrared light.
This region consists of
short wavelengths around 760 nanometers to longer wavelengths
about 1 million nanometers, or about a thousand micrometers, in length.
We can sense some of this infrared energy as heat.
Some objects are so hot they also emit visible light,
such as a fire. Other objects such as humans,
are not as hot and only emit infrared waves.
We cannot see these infrared waves with our eyes alone. However
instruments that can sense infrared energy, such as night vision goggles
or infrared cameras, allow us to 'see' these infrared waves from
warm objects like humans and animals. Infrared energy
can also reveal objects in the Universe that cannot be seen with optical
telescopes. Infrared waves have longer wavelengths than visible
light and can pass through dense regions of gas and dust with lower
scattering and absorption. When you look up at the constellation Orion,
you see only the visible light. But NASA's Spitzer
telescope was able to detect nearly 2,300 planet-forming discs
in the Orion nebula by sensing the infrared glow of their warm dust.
Each disc has the potential to form planets and its own solar system.
Incoming ultraviolet, visible,
and a limited portion of infrared energy, together sometimes called
"shortwave radiation", from the sun drives our Earth system.
Some of this radiation is reflected off of clouds and some is
absorbed in the atmosphere. Larger aerosol particles in the atmosphere
interact with and absorb some of the radiation causing the atmosphere
to warm. The heat generated by this absorption is emitted
as long-wave infrared radiation, some of which radiates out to space.
The solar radiation that does pass through Earth's atmosphere
is either reflected off snow, ice or other surfaces
or is absorbed by the Earth's surface. This absorption of radiation
warms the Earth's surface and this heat is emitted as long-wave
radiation into the atmosphere which allows only a small amount to radiate
out to space. Greenhouse gases in the atmosphere,
such as water vapor and carbon dioxide, absorb most of this
emitted long-wave infrared radiation, and this absorption heats
the lower atmosphere. In turn, the warmed atmosphere emits
long-wave radiation, some of which radiates towards the Earth's surface
keeping our planet warm and generally comfortable.
The energy entering, energy reflected, energy absorbed, and energy
emitted by the Earth system constitutes the components of the
Earth Radiation Budget. A budget that's out of balance can cause the temperature of the
atmosphere to increase and eventually affect our climate.
For scientists to understand climate, they must also determine what
drives the changes within the Earth's radiation budget.
The CERES instrument aboard NASA's Aqua and Terra satellites can
measure the reflected shortwave and emitted long-wave radiation into space
accurately enough for scientists to determine the Earth's total radiation budget.
Other NASA instruments monitor the changes in other aspects
of the Earth's climate system, such as clouds, aerosol particles,
or surface reflectivity, and scientists are examining their many
interactons with the energy budget. A portion of solar radiation
from the Sun that is just beyond the visible spectrum is referred to
as near-infrared. Scientists can study how this radiation
reflects off the Earth's surface to understand changes in land cover
such as growth of cities or changes in vegetation. Our eyes
perceive a leaf as green because wavelengths in the green region
of the visible light spectrum are reflected while other visible wavelengths
are absorbed. Yet, the chlorophyll and the cell structure of the
leaf are also reflecting near-infrared light, light we cannot see.
This reflected near-infrared radiation can be
sensed by satellites, allowing scientists to study vegetation from space.
Using these data, scientists can identify some
types of trees, can examine the health of forests, and can even
monitor the health of vegetation such as forests infested with pine beetles
or crops affected by drought. Studying the emission and
reflection of infrared waves helps us to understand the Earth's system
and its energy budget. Near-infrared data can also help scientists
study land cover such as changes in snow, ice,
forests, urbanization, and agriculture.
Scientists are beginning to unlock the mysteries of cooler objects across
the Universe such as planets, cool stars, nebulae,
and much more using infrared waves.