PHYSICS

there are 4 - 6 layers of iridophores that collectively comprise an
area of up to 60 urn in thickness. Their presence increases the reflectance
by about 65%. In insects, color change generally involves
phase changes of li pids or other materials within the cuticle.
Often these alterations produce a change from nearly black to a
shiny metallic blue or green, depending on the ambient temperature.
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UV
UV-CI UV-BIUV-A
100 280 315
400 nm
Chapter 23. PHOTOBIOLOGY
Photobiology is the branch of biophysics concerned with the
effect of light on living organisms.
23.1. ELECTROMAGNETIC SPECTRUM
The entire frequency range of electromagnetic waves is called
the electromagnetic spectrum. Part of the spectrum is shown in
fig. 23.1. The upper part of the top bar shows two of the most
important sources of radiation (solar and thermal) and the lower
part names three important bands - ultraviolet (290 - 400 nm),
visible (400 - 700 nm) and near-infrared (700 - 2500 nm). The
second bar shows three bands of the ultraviolet (UV-C, UV-B
and UV-A) and the wave bands of the visible colors.
400
Visible
IR
700 nm
3
6 10 um
Fig. 23.1. Scale of electromagnetic waves in ultraviolet,
visible and infrared parts of spectrum
r
23.2. EFFECT OF ULTRAVIOLET RADIATION
ON LIVING ORGANISMS
23.2.1. Human and Animal Health
Ultraviolet radiation (UV) is generally divided into three
groups, based upon the biological effectiveness of the photons in
each. The UV-C range contains the shortest wavelengths « 280
nm) and the highest energy photons, which are capable of causing
ionizing reactions in the upper atmosphere. Due to absorption
by ozone in the stratosphere, UV-C radiation does not reach
the Earth's surface. UV-B radiation (280 - 320 nm), however,
does reach the biosphere in small and variable quantities. The radiation
is of sufficient energy to cause direct damage to chromophoric
regions of absorbing macromolecules such as nucleic acids
and proteins. UV-B radiation is responsible for damage to crop
plants. At longer wavelengths, UV-A radiation (320 - 400 nm)
absorption is usually less strong and is dominated by molecules
with conjugated double bonds, and cyclic and polycyclic molecules
(e.g., isoprenoids, flavonoids, quinines, alkaloids), metal
enzymes, and others. The increase in UV-B radiation entering the
biosphere is associated with stratospheric ozone depletion and is
likely to have a significant detrimental impact on human health.
Eye diseases. UV radiation can damage the cornea and lens of
the eye. Chronic exposure to UV-B is one of several factors clearly
associated with the risk of cataracts. Artificial sources of UV radiation
also cause corneal damage. For example, the injury from a
welder's arc is known as flash burn, welder's flash or arc eye.
Other sources of UV radiation injury include sun tanning beds,
carbon arcs, photographic flood lamps, lightning, electric sparks,
and halogen desk lamps. Prolonged exposure to UV radiation can
lead to several ocular surface disorders such as pinguecula, pterygium,
climatic droplet keratopathy, and even squamous metaplasia
and carcinoma.
Immune suppression. Some components of the immune system
are present in the skin. UV exposure decreases the immune response
to skin cancer, infectious agents, and other antigens.
Erythema. The normal acute effects of visible and ultraviolet
radiation arise by photochemical change in the epidermis due to
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