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7 Light in Biology and Medicine
Above the visible light frequencies, there are ‘near ultraviolet’ (400–300 nm),
‘mid ultraviolet’ (300–200 nm), ‘far ultraviolet’ (200–121 nm) ‘vacuum UV’ (200–
10 nm), ‘extreme UV’ (121–10 nm), and soft X-rays (10–0.01 nm), hard X-rays
(also called low-energy gamma rays) (0.01–0.0001 nm), and high-energy gamma
rays (0.0001 nm and smaller). The term ‘vacuum UV’ comes from the fact that solar
UV-rays with wavelengths smaller than about 200 nm are strongly absorbed by the
ozone in our upper atmosphere, and so are important in solar exposure only in the
vacuum of outer space or if the ozone layer is missing or depleted. For photobiological applications, the UV spectrum is alternatively divided into the ranges: soft
ultraviolet (UV-A) (400–315 nm), hard ultraviolet (UV-B) (315–280 nm) and severe
ultraviolet (UV-C) (280–100 nm). UVC is almost totally absorbed by atmospheric
ozone; over 90% of UVB is absorbed by atmospheric ozone, while most of the UVA
passes through the atmosphere and reaches the Earth’s surface. Near ultraviolet is
responsible for the production of vitamin D in our skin. 6 Far ultraviolet and extreme
ultraviolet electromagnetic radiation will ionize biological molecules, and therefore
exposure to these will have a much higher chance of producing cancer of the skin
than near UV. Never-the-less, UVA erythemal exposure can lead to melanomas and
skin aging. If not treated, some melanomas can be invasive into the body.
Sunscreen creams are often rated by an SPF, Sunscreen Protective Factor. The
number represents the inverse of the fraction of UVB reaching the skin through an
expected layer of lotion. So, an SPF of 50 means 1/50th of incoming UVB would
penetrate the expected layer of lotion on the skin. This is 2/100, i.e. only 2% of
UVB reaches the skin. The SPF does not apply to UVA protection. Rather, a broadspectrum sun blocker (UVA and UVB), such as ones with zinc oxide, does stop
some UVA, with ratings designated by UVA-PF (UVA protection factor) or UV-PA
showing‘ protecting grade’ or by an indication of at least 1/3 for the blockage of
UVA in ratio to UVB.
Since the beginnings of quantum theory at the turn of the twentieth century, we
have recognized that light can be seen to behave both as a wave and as a particle,
depending on the method of detection. Max Planck and Albert Einstein found that
light is emitted and absorbed only by a ‘quantum’ of energy proportional to the
frequency of the light, according to
E = h f ,
(7.10)
where h is called Planck’s constant. The quantum of light is now called the ‘photon’.
The interaction of matter with light may always be described by the absorption and
emission of photons. A large number of photons at nearly one energy and with
6 People who came from Africa to inhabit the northern latitudes of the Earth survived better if they
had lighter skin (less melanin), to produce needed vitamin D.
7 Light in Biology and Medicine
Above the visible light frequencies, there are ‘near ultraviolet’ (400–300 nm),
‘mid ultraviolet’ (300–200 nm), ‘far ultraviolet’ (200–121 nm) ‘vacuum UV’ (200–
10 nm), ‘extreme UV’ (121–10 nm), and soft X-rays (10–0.01 nm), hard X-rays
(also called low-energy gamma rays) (0.01–0.0001 nm), and high-energy gamma
rays (0.0001 nm and smaller). The term ‘vacuum UV’ comes from the fact that solar
UV-rays with wavelengths smaller than about 200 nm are strongly absorbed by the
ozone in our upper atmosphere, and so are important in solar exposure only in the
vacuum of outer space or if the ozone layer is missing or depleted. For photobiological applications, the UV spectrum is alternatively divided into the ranges: soft
ultraviolet (UV-A) (400–315 nm), hard ultraviolet (UV-B) (315–280 nm) and severe
ultraviolet (UV-C) (280–100 nm). UVC is almost totally absorbed by atmospheric
ozone; over 90% of UVB is absorbed by atmospheric ozone, while most of the UVA
passes through the atmosphere and reaches the Earth’s surface. Near ultraviolet is
responsible for the production of vitamin D in our skin. 6 Far ultraviolet and extreme
ultraviolet electromagnetic radiation will ionize biological molecules, and therefore
exposure to these will have a much higher chance of producing cancer of the skin
than near UV. Never-the-less, UVA erythemal exposure can lead to melanomas and
skin aging. If not treated, some melanomas can be invasive into the body.
Sunscreen creams are often rated by an SPF, Sunscreen Protective Factor. The
number represents the inverse of the fraction of UVB reaching the skin through an
expected layer of lotion. So, an SPF of 50 means 1/50th of incoming UVB would
penetrate the expected layer of lotion on the skin. This is 2/100, i.e. only 2% of
UVB reaches the skin. The SPF does not apply to UVA protection. Rather, a broadspectrum sun blocker (UVA and UVB), such as ones with zinc oxide, does stop
some UVA, with ratings designated by UVA-PF (UVA protection factor) or UV-PA
showing‘ protecting grade’ or by an indication of at least 1/3 for the blockage of
UVA in ratio to UVB.
Since the beginnings of quantum theory at the turn of the twentieth century, we
have recognized that light can be seen to behave both as a wave and as a particle,
depending on the method of detection. Max Planck and Albert Einstein found that
light is emitted and absorbed only by a ‘quantum’ of energy proportional to the
frequency of the light, according to
E = h f ,
(7.10)
where h is called Planck’s constant. The quantum of light is now called the ‘photon’.
The interaction of matter with light may always be described by the absorption and
emission of photons. A large number of photons at nearly one energy and with
6 People who came from Africa to inhabit the northern latitudes of the Earth survived better if they
had lighter skin (less melanin), to produce needed vitamin D.