Page 18 - Veterinary Laser Therapy in Small Animal Practice

4 Veterinary Laser Therapy in Small Animal Practice

Electric field
2-D
Magnetic field Visible
3-D
Propagation
1-D

10 3 10 2 10 1 1 10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 10 -11 10 -12
Wavelength
(meters) longer shorter
km m mm µm nm
10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 19 10 20
Frequency
(Hz)
MHz GHz
keV MeV
Photon energy lower higher
(electron Volts)
10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 10 1 10 2 10 3 10 4 10 5 10 6
Radiowaves Microwaves Infrared Ultraviolet X-Rays ɣ-Rays
Figure 1.1 Structure of light as an electromagnetic wave (top). You can also see the scale of photon energy across the full
electromagnetic spectrum.

cones that can detect wavelengths from about 390–700 them called tissues. Inside the atom, electrons are
nm, which is what we call the visible part of the spec- orbiting the nucleus in a cloud that has some struc-

trum. The near-infrared spans from about 700–1100 ture to it and has an equilibrium state. Molecules are
nm. I’ll bounce between these descriptions (wavelength atoms connected by bonds, which are made up of some
and frequency) many times, but either one of them tells sharing of these electron clouds and which also have an
the whole story. equilibrium state. And most of the equilibrium states of

This is the full, classical picture of light as it pertains these entities are electrically neutral. This doesn’t mean
to the kinds of therapy discussed in this book, i.e. non- that there are no charged particles, but rather that there
ionizing (and in particular, visible and infrared) light are as many positively charged things as there are nega-

incident on living biological tissue. There is so much tively charged things.

more interesting stuff to learn about light, but not But light is an oscillating electromagnetic field, and

much of it will help you in the clinic, so we will not charged particles in this kind of field experience a force.

cover it here. So shining light on matter exerts a force on these equi-
OK, one more tidbit because it’s cool and head- librium states, and as it passes by, the matter creates

spinning and it will affect how we talk about light a little some recoil force, much like a spring (warning: we
later on: it turns out that the energy of light is directly physicists use the concept of springs to explain every-

proportional to its frequency (and therefore inversely thing). The force experienced by the matter is pro-
proportional to its wavelength). To compound this, the portional to the energy of the field (frequency of the

amount of energy any “piece” of light carries is discrete or light) and the mass and charge of the particles (atomic/

“quantized.” These individual packets of light are called molecular structure) of the matter.

photons and make up the quantum picture of light. So matter, in effect, is just a bunch of charged atomic
The main question remains: why does frequency, this nuclei tied together with a bunch of springs. The types

one characteristic of this phenomenon we call light, of interactions you get, then, are simply regulated by
determine such a broad range of implications, i.e. from the frequency of oscillation of the light and how this
destruction to manipulation, from imaging to therapy? corresponds to the frequency of oscillation of the bonds
Oddly enough, that question boils down to a differ- of the things they bump into.

ent kind of question: what happens when something
that wiggles bumps into something else that wiggles? 1.2 Absorbing light
We’ve already established that light can be described as

an oscillating field. Well, matter is made up of atoms, Absorption happens when the frequency of light is
groups of them called molecules, and big groups of close to the natural frequency of the thing it interacts

REDONDO PRINT (4-COL BLEED).indd 4 08/08/2019 09:46