Page 19 - Veterinary Laser Therapy in Small Animal Practice

Light is just the catalyst 5

with. Just like pushing a child on a swing, if you push in In general, then, targeting something in tissue with
sync with the natural rhythm, you can transfer the most your laser means finding something in the cells of that
energy of your push to the child. But as I mentioned tissue that does something productive when it absorbs
above, light is “quantized,” so this energy transfer is an light, determining the resonant frequency of that some-
all-or-nothing kind of thing: you don’t absorb a piece of thing, then using the color of light that coincides with
a photon. If a photon of light transfers all of its energy that frequency. Simple. So what in the body absorbs
to the incident matter, we call that an absorption event. light and what happens once it does?
On the atomic scale, the transitions between elec-
tron energy levels correspond to light’s frequency in 1.3 Light’s bio-targets
the X-ray region of the spectrum, which is why X-rays
interact strongly with atoms, so much so that individ- Here I need to remind you that this is not a textbook
ual electrons can be knocked completely out of orbit on light. Instead it is a useful guide to laser therapy. For
in what is called ionization. This kind of process leads that reason, the following discussion is limited to the
to all sorts of sporadic and dangerous effects, because interactions of light in the visible-to-near-infrared por-
when an atom is missing an electron, it does whatever tion of the spectrum, where therapy lasers (all of them)
it can to steal one from another atom, and the chain live. And the focus is on things that are present in suf-
continues. Molecular bonds can be broken and whole ficient quantity in the body to be worth mentioning.
molecules can be destroyed this way. In fact, more Luckily for us, there are only a few.
than two-thirds of all light-induced mammalian DNA The most prolific substance in the body happens to
damage happens when water becomes ionized into be one of the main absorbers of infrared light in the
what’s called a hydroxyl radical. body: water. Volumes of textbooks have been written
On the molecular level, though, the bonds are much on water and its role in the sustenance of life, but the
more flimsy, i.e. they have much slower natural frequen- fact is, the molecule itself is actually very simple: two
cies (which makes sense because bigger things move hydrogen molecules bond (i.e. share electrons) with
more slowly), and so light in the visible and infrared one oxygen molecule. If you can get this pair of bonds
range takes over the interactions. And because lower bouncing and wiggling around, water can do some
frequency means less energy, the interactions are often wonderful things. We’ll explore more of this soon, but
not as catastrophic to the matter. Instead, the molecu- for now, target number one is water.
lar bonds that absorb the incident light’s energy wiggle Absorber number two of light in the body is melanin.
and twist and stretch and contract. Though smaller in This is the pigment in the skin and hair/fur that gives it
magnitude, these types of molecular manipulations the appearance of color. In the visible region of light,
lead to all types of changes in chemistry (more on this the amount of dark or light you see tells you which kind
in section 1.3). and how much light that material absorbs or reflects.
Absorption, even of visible and infrared light, is not Let me explain. Black is not really a color. To the con-
always calm, though. If matter has a particular bond trary, black is the lack of color. So if you see something
with a natural frequency that coincides with the inci- that is black, that material is absorbing most of the
dent light, and a lot of light is used, the absorption events visible light shining on it, and therefore reflecting very
lead to re-enforced vibrations that create heat and can little of it back for your eye to see. White is the combi-
literally shake molecules apart. This is why putting nation of all light. So when you see something white,
things that contain water (e.g. most of your food) in much less of that light is being absorbed; rather most
the microwave, which uses light with a frequency (and of it is being reflected back to your eye. How about
therefore energy) that is 200,000 times less than visible something in the middle?
light, heats your food much better than holding that Grass is green. That is to say, it has lots of chloro-
day-old pizza next to your desk lamp, however bright phyll that needs to absorb light for photosynthesis.
it may be. The bonds in water have a huge absorption So would shining green or, say, red light be better for
peak (a resonant frequency of bending/twisting) in the plant growth? Well, if the grass is green, then it reflects
microwave region of the spectrum. This same idea is most of the green light back to your eye, which means
used in surgical lasers to ablate tissue in a very efficient, it doesn’t absorb green very well. If you used red light
localized way. instead, a greater proportion would be absorbed. Why

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