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Where does Ultraviolet Light Come From?
For humans, the common natural source of UV light is the sun.
The sun gives off UVA, UVB and UVC light. UVC doesn't reach us
on the earth's surface, as it is blocked by the earth's atmosphere.
This is good, as UVC exposure is dangerous to human and other
animals. UVA, the light we use in this book, is the safest. UVB
is what causes us to tan.
Distant suns and planets give off UV light, some which makes
it to earth's, and much which is blocked by our atmosphere.
Humans manufacture UVA, UVB and UVC light for industrial,
commercial and scientific purposes. The human made UV products
include the black lights used in this book, suntan lamps, germicidal
lamps used to sterilize and currency counterfeit detectors. Even
the normal office and home lights give off some UV light.
How Was UV light discovered?
Ultraviolet light has existed since long before humans were
around. It wasn't invented by a human. but discovered. Ultraviolet
light was discovered in 1801 by scientist Johan Wilhelm Ritter,
a year after William Hershel discovered infrared light. These
were the first times light invisible to human eyes were discovered.
After hearing that Hershel discovered a form of light beyond
the color red, Rittner experimented to see if invisible light
also existed beyond the color violet. He discovered that silver
chloride turned black under ultraviolet light. Silver chloride
is used in much photography, as it turns dark under sunlight.
Using a prism, he spread apart light into its color spectrum,
and saw that an invisible light beyond the visible violet turned
black the silver chloride. This showed that an invisible form
of light existed beyond the violet end of the spectrum.
Why can't we see ultraviolet?
Because we can't.
Whether eyes, ears or human-made machine, all measuring devises
have limits to what they can detect and measure. The rods and
cones of the human eyeballscan only detect certain wavelengths
of light-- the wavelengths that fall within the visible light
range. Human eyes phyisically can't detect ultraviolet, infrared,
gamma and X-rays.
Just as some animals can hear sounds humans can't hear, some
animals can see light we can't see. Snakes can detect infrared
light, while bees, geese and butterflies can see ultraviolet.
Owls can detect lower levels of light than humans, and are able
to see things in the darkness of light that are invisible to
us.
There are other physical limits to human sight. We can't make
out detail that is too small or far away. We use optical aids
like binoculars and microscopes to aid our sight. We can't see
or see clearly objects that move too fast. A hand waved by our
eyes looks blurred. A speeding bullet is invisible. Sight variations
exist amongst humans. Some humans can see more colors than others,
are able detect light at lower levels and make out details others
can't. As humans get older, their night vision usually gets worse.
Though you can't see it, you can physically detect ultraviolet
and infrared light. Infrared light can be hot and ultraviolet
light can give you a suntan.
How Does Black Light Make Things
Fluoresce?
The fluorescence, or visible light that is emitted from a
material when black light is shined on it, happens at the atomic
level. When you are shining a black light on an object, you are
actually testing its atomic makeup!
Just as with light, heat and x-rays, black light is a form
of energy. When black light is shined on a material, whether
the material is glass, plastic or paper, energy is being added
to the atoms of the material. The atoms can only hold this extra
energy for a short amount of time before having to give it off.
The atoms give off the energy in a different form than received.
The atoms receive the energy as black light, but may give the
energy off as heat, ultraviolet light, infrared light, visible
light or, often, a combination of these. What form(s) of energy
the atoms gives off is dependant on the makeup of the atoms.
If visible light is emitted by the atoms, that is the fluorescence
we see. The color of this visible light is also dependant on
the atomic make up. If the atom gives off just heat, ultraviolet
light or infrared light, there will be no fluorescence. In a
darkened room this material will remain dark.
Phosophorescence : After glow
Phospheresnce is closely related to flouroescence. Like fluorescent
materials, phosphorsecant materials give off visible light when
excited by energy like UV light. However, while flouroesce material
quits emitting light when UV light is turned off, phosophorescant
material continues to give off light. The extra duration varies
from phosphorescent material to phospheresant material. Sometimes
phosopherescant material gives off light for a fraction of a
second longer, other material for hours or even days. The phosophorescance
can be caused by UV light, but also visible light, X-rays, infrared
and other light. As with fluorescence, the color, brightness
and duration of the phosphorescence is caused by the atomic make
up of the material.
As with fluorescent material, the added energy of UV or other
light excites the atoms in phosphorescent material, raising the
electorons to a higher orbital. While the electons move back
to their normal orbital right away with flouroscent material,
it takes longer with phosphorescent material. Thus the phosphorescent
glow lasts longer.
Next: How are black
lights made?
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