Our eyes are sensitive to light which lies in a very small region of the
electromagnetic spectrum labeled "visible light". This "visible
light" corresponds to a wavelength range of 400 - 700 nanometers (nm) and
a color range of violet through red. The human eye is not capable of
"seeing" radiation with wavelengths outside the visible spectrum. The
visible colors from shortest to longest wavelength are: violet, blue, green,
yellow, orange, and red. Ultraviolet radiation has a shorter wavelength than
the visible violet light. Infrared radiation has a longer wavelength than
visible red light. The white light is a mixture of the colors of the visible
spectrum. Black is a total absence of light.
Earth's most important energy source is the Sun. Sunlight consists of the
entire electromagnetic spectrum.
(Wavelength image from Universe by Freedman and Kaufmann.)
The visible violet light has a wavelength of about 400 nm. Within the
visible wavelength spectrum, violet and blue wavelengths are scattered more
efficiently than other wavelengths. The sky looks blue, not violet, because our
eyes are more sensitive to blue light (the sun also emits more energy as blue
light than as violet).
The visible indigo light has a wavelength of about 445 nm.
The visible blue light has a wavelength of about 475 nm. Because the blue
wavelengths are shorter in the visible spectrum, they are scattered more
efficiently by the molecules in the atmosphere. This causes the sky to appear blue during the main part of the day,
when blue light is scattered into your eye no matter which direction you look.
The visible green light has a wavelength of about 510 nm. Grass, for example,
appears green because all of the colors in the visible part of the spectrum
are absorbed into the leaves of the grass except green. Green is reflected,
therefore grass appears green.
The visible yellow light has a wavelength of about 570 nm.
The visible orange light has a wavelength of about 590 nm. Low-pressure
sodium lamps, like those used in some parking lots, emit a orange-ish (wavelength
589 nm) light.
The visible red light has a wavelength of about 650 nm. At sunrise and sunset,
the light you see has traveled a longer distance through the atmosphere.
A large amount of blue and violet light has been removed as a result of scattering and the longwave colors,
such as red and orange, are more readily seen.
There are many wavelengths in the electromagnetic spectrum the human eye
Energy with wavelengths too short for humans to see
Energy with wavelengths too short to see is "more violet than violet".
Light with such short wavelengths is called "Ultraviolet" light.
The term "ultra-" means higher than.
How do we know this light exists? One way is that this kind of light causes
sunburns. Our skin is sensitive to this kind of light. If we stay out in this
light without sunblock protection, our skin absorbs this energy. After the
energy is absorbed, it can make our skin change color ("tan") or it
can break down the cells and cause other damage.
Energy with wavelengths too long for humans to see
Energy whose wavelength is too long to see is "redder than red".
Light with such long wavelengths is called "Infrared" light. The
term "Infra-" means "lower than".
How do we know this kind of light exists? One way is that we can feel energy
with these wavelengths such as when we sit in front of a campfire or when we
get close to a stove burner. Scientists like
Samuel Pierpont Langley passed light through
a prism and discovered that the infrared light the scientists could not see
beyond red could make other things hot.
Very long wavelengths of infrared light radiate heat to outer space. This
radiation is important to the Earth's energy budget. If this energy did not
escape to space, the solar energy that the Earth absorbs would continue to
heat the Earth.