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8 Ionizing Radiation and Life
8.4.1 Production of Ultraviolet Light
Ultraviolet light (UV) is typically made by atomic transitions in atoms such as
mercury in fluorescent tubes or by heating gases to temperatures of above 4000
◦ C, such as in carbon arcs. If the inner surface of a fluorescent bulb is coated with a
phosphor, the UV can be converted to visible light. Alternatively, if the tube is made
of quartz glass, the UV radiation will pass out of the tube.
Very hot bodies, such as our white-hot Sun, produce ultraviolet light, and blue
stars make proportionately even more. Carbon arcs are sometimes used by welders
to melt metals. They also were once used in movie projectors to make intense white
light. They consist of two metal-coated carbon rods which are touched together,
letting a large current pass through. The current vaporizes the carbon which becomes
hot enough to emit a significant fraction of its thermal radiation in the visible part of
the electromagnetic spectrum, but also UV, causing nearby tissue to be sunburned.
A ‘black light’ is a light source which makes ultraviolet light in the range of UVA. This so-called ‘black light’ is produced by passing current through mercury vapor
in an inert gas, with a surrounding tube made of quartz. The quartz tube allows UV-A
to pass through, unlike ordinary glass. The electrons in the current created between
an anode and cathode collide and excite the gas atoms. Excited mercury atoms will
emit light at wavelengths of 365.4 nm in the UV-A spectrum, and of 184.45 nm and
253.7 nm in the UV-C spectrum,
UV-C is sometimes called ‘germicidal UV’, since bacteria are killed by exposure
to UV-C. Air exposed to UV-C will generate ozone. This will be evident by the
distinct smell accompanying ozone near an air purifier equipped with a Ultraviolet
Germicidal Irradiation (UVGI) device. UVGI devices are also used to help purify
water, although bacteria can survive if embedded in small particles in the water.
Light-emitting diodes are available which generate UV light at a variety of frequencies in the UV-A to UV-C ranges. They have application in finding fluorescent
materials, such as tagged biomolecules, and in the quick curing (polymerizing) of
dental resin-based composites.
8.4.2 Ultraviolet Light in Biology
Although humans cannot normally see UV, a wide variety of animals, including
many birds, reptiles, and insects, can see and use ultraviolet light. Flowers have
developed UV-reflecting pigmentations to attract insects.
Biologists can use phosphate and urine fluorescence (rat urine fluoresces yellow,
human glows blue-green) to find animal trails. Fluorescent materials can be used as
tracers and labels in studies of organic processes, even down to the molecular level.
8 Ionizing Radiation and Life
8.4.1 Production of Ultraviolet Light
Ultraviolet light (UV) is typically made by atomic transitions in atoms such as
mercury in fluorescent tubes or by heating gases to temperatures of above 4000
◦ C, such as in carbon arcs. If the inner surface of a fluorescent bulb is coated with a
phosphor, the UV can be converted to visible light. Alternatively, if the tube is made
of quartz glass, the UV radiation will pass out of the tube.
Very hot bodies, such as our white-hot Sun, produce ultraviolet light, and blue
stars make proportionately even more. Carbon arcs are sometimes used by welders
to melt metals. They also were once used in movie projectors to make intense white
light. They consist of two metal-coated carbon rods which are touched together,
letting a large current pass through. The current vaporizes the carbon which becomes
hot enough to emit a significant fraction of its thermal radiation in the visible part of
the electromagnetic spectrum, but also UV, causing nearby tissue to be sunburned.
A ‘black light’ is a light source which makes ultraviolet light in the range of UVA. This so-called ‘black light’ is produced by passing current through mercury vapor
in an inert gas, with a surrounding tube made of quartz. The quartz tube allows UV-A
to pass through, unlike ordinary glass. The electrons in the current created between
an anode and cathode collide and excite the gas atoms. Excited mercury atoms will
emit light at wavelengths of 365.4 nm in the UV-A spectrum, and of 184.45 nm and
253.7 nm in the UV-C spectrum,
UV-C is sometimes called ‘germicidal UV’, since bacteria are killed by exposure
to UV-C. Air exposed to UV-C will generate ozone. This will be evident by the
distinct smell accompanying ozone near an air purifier equipped with a Ultraviolet
Germicidal Irradiation (UVGI) device. UVGI devices are also used to help purify
water, although bacteria can survive if embedded in small particles in the water.
Light-emitting diodes are available which generate UV light at a variety of frequencies in the UV-A to UV-C ranges. They have application in finding fluorescent
materials, such as tagged biomolecules, and in the quick curing (polymerizing) of
dental resin-based composites.
8.4.2 Ultraviolet Light in Biology
Although humans cannot normally see UV, a wide variety of animals, including
many birds, reptiles, and insects, can see and use ultraviolet light. Flowers have
developed UV-reflecting pigmentations to attract insects.
Biologists can use phosphate and urine fluorescence (rat urine fluoresces yellow,
human glows blue-green) to find animal trails. Fluorescent materials can be used as
tracers and labels in studies of organic processes, even down to the molecular level.