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7 Light in Biology and Medicine
7.18 Infrared Biodetectors
People have infrared sensitivity on their skin. Next time you are on the beach at
night with some exposed skin, stand far enough away from a bonfire so that you
are not warmed by the temperature of the air. Close your eyes. As you rotate your
body, your skin will tell you the direction of the fire. Thermoreceptors (both cold
and hot) in your skin send information to the brain about the temperature of the
skin. Temperature sensitivity of nerves occurs when active channels for the inward
flow of ions such as Na + and/or the outward flow of K + ions are constructed to be
strongly temperature dependent.
Pit vipers have evolved organs to detect infrared radiation, useful for catching
mice in the dark. These organs can detect radiation in the wavelength range from 5
to 30 μm, with acuity sufficient to see various glowing body parts of the mouse.
Directional focusing is performed by head position relative to two pit organs,
one on each side of the head, and by the shape of pit as well as the interior
sensitive membrane and cavities. Pythons, boas, and vampire bats also have evolved
directional IR detectors.
7.19 Production of Visible Light
To make visible light, one need only to employ charges made to oscillate with
frequencies in the range 10 14 –10 15 cycles/s, or use Doppler shift of non-visible light
into the visible spectrum. There are several techniques for visible light generation
in non-biological systems.
7.19.1 Thermal and Atomic Generation of Light
Visible light is evidently made by sufficiently hot bodies (above about 1000 K)
including our Sun and tungsten filaments in old light bulbs. Some atomic and
molecular transitions of electrons from excited states to lower energy states will
make visible-light photons. If the excited states are above some intermediate states,
then de-excitation may occur step wise: From the excited state to an intermediate
state, and then from the intermediate state to the lowest state (‘ground state’). (See
Fig. 7.19.) The material is then call ‘fluorescent’. Fluorescent bulbs have mercury as
a gas inside a partially evacuated glass tube. The mercury is excited by an electric
current. But its radiation is largely in the UV part of the spectrum. On the inside
surface of the tube is a set of ‘phosphors’, chemicals such as zinc sulfide which
have intermediate states between those which are excited by UV. Transitions from
these intermediate states make visible colors.
7 Light in Biology and Medicine
7.18 Infrared Biodetectors
People have infrared sensitivity on their skin. Next time you are on the beach at
night with some exposed skin, stand far enough away from a bonfire so that you
are not warmed by the temperature of the air. Close your eyes. As you rotate your
body, your skin will tell you the direction of the fire. Thermoreceptors (both cold
and hot) in your skin send information to the brain about the temperature of the
skin. Temperature sensitivity of nerves occurs when active channels for the inward
flow of ions such as Na + and/or the outward flow of K + ions are constructed to be
strongly temperature dependent.
Pit vipers have evolved organs to detect infrared radiation, useful for catching
mice in the dark. These organs can detect radiation in the wavelength range from 5
to 30 μm, with acuity sufficient to see various glowing body parts of the mouse.
Directional focusing is performed by head position relative to two pit organs,
one on each side of the head, and by the shape of pit as well as the interior
sensitive membrane and cavities. Pythons, boas, and vampire bats also have evolved
directional IR detectors.
7.19 Production of Visible Light
To make visible light, one need only to employ charges made to oscillate with
frequencies in the range 10 14 –10 15 cycles/s, or use Doppler shift of non-visible light
into the visible spectrum. There are several techniques for visible light generation
in non-biological systems.
7.19.1 Thermal and Atomic Generation of Light
Visible light is evidently made by sufficiently hot bodies (above about 1000 K)
including our Sun and tungsten filaments in old light bulbs. Some atomic and
molecular transitions of electrons from excited states to lower energy states will
make visible-light photons. If the excited states are above some intermediate states,
then de-excitation may occur step wise: From the excited state to an intermediate
state, and then from the intermediate state to the lowest state (‘ground state’). (See
Fig. 7.19.) The material is then call ‘fluorescent’. Fluorescent bulbs have mercury as
a gas inside a partially evacuated glass tube. The mercury is excited by an electric
current. But its radiation is largely in the UV part of the spectrum. On the inside
surface of the tube is a set of ‘phosphors’, chemicals such as zinc sulfide which
have intermediate states between those which are excited by UV. Transitions from
these intermediate states make visible colors.