of Electromagnetic Radiation and Matter
It is well known that all matter is comprised of atoms. But subatomically,
matter is made up of mostly empty space. For example, consider the hydrogen
atom with its one proton, one neutron, and one electron. The diameter of a single proton has been measured to
be about 10-15 meters. The diameter of a single hydrogen
atom has been determined to be 10-10 meters, therefore the
ratio of the size of a hydrogen atom to the size of the proton
is 100,000:1. Consider this in terms of something more easily
pictured in your mind. If the nucleus of the atom could be enlarged
to the size of a softball (about 10 cm), its electron
would be approximately 10 kilometers away. Therefore, when electromagnetic
waves pass through a material, they are primarily moving through
free space, but may have a chance encounter with the nucleus
or an electron of an atom.
Because the encounters of photons with atom particles are by
chance, a given photon has a finite probability of passing completely
through the medium it is traversing. The probability that a photon
will pass completely through a medium depends on numerous factors
including the photon’s energy and the medium’s composition
and thickness. The more densely packed a medium’s atoms,
the more likely the photon will encounter an atomic particle.
In other words, the more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur Similarly, the more material a photon must cross through, the
more likely the chance of an encounter.
When a photon does encounter an atomic particle, it transfers
energy to the particle. The energy may be reemitted back the way
it came (reflected), scattered in a different direction or transmitted
forward into the material. Let us first consider the interaction
of visible light. Reflection and transmission of light waves occur
because the light waves transfer energy to the electrons of the
material and cause them to vibrate. If the material is transparent,
then the vibrations of the electrons are passed on to neighboring
atoms through the bulk of the material and reemitted on the opposite
side of the object. If the material is opaque, then the vibrations
of the electrons are not passed from atom to atom through the
bulk of the material, but rather the electrons vibrate for short
periods of time and then reemit the energy as a reflected light
wave. The light may be reemitted from the surface of the material
at a different wavelength, thus changing its color.
X-Rays and Gamma Rays
X-rays and gamma rays also transfer their energy to matter though
chance encounters with electrons and atomic nuclei. However, X-rays and gamma rays have enough energy to do more than just make the electrons
vibrate. When these high energy rays encounter an atom, the result is an ejection of energetic
electrons from the atom or the excitation of electrons. The term "excitation" is used to describe an interaction where
electrons acquire energy from a passing charged particle but are
not removed completely from their atom. Excited electrons may
subsequently emit energy in the form of x-rays during the process
of returning to a lower energy state.
Each of the excited or liberated electrons goes on to transfer
its energy to matter through thousands of events involving interactions
between charged particles. With each interaction, the energy may
be directed in a different direction. The higher the energy of
a photon, the more likely the energy will continue traveling in
the same direction. As the radiation moves from point to point
in matter, it loses its energy through various interactions with
the atoms it encounters. If the radiation has enough energy, it
may eventually make it through the material.
Photon Interaction with Matter is Key
From the previous paragraph, it can be deduced that the energy of X- and Gamma ray photons is
largely responsible for their penetrating power. Einstein linked
the energy of a photon to its frequency and wavelength when he
postulated that each photon carries an energy of the frequency
of the wave times Planck’s constant (E = hƒ).
The frequency of an EM wave equals the speed of light divided
by the wavelength (ƒ =c/λ ). However, it should be understood
that the wavelength or frequency of electromagnetic radiation
does not in itself makes the EM wave more or less penetrating.
The key is its interaction with matter, or more specifically,
whether the photon's energy is right to excite some transition
of a charged particle. For instance, microwaves penetrate glass
very easily, but they are strongly absorbed by water. Move up
to slightly higher frequency, and infrared is strongly absorbed
by both glass and water, but both substances transmit visible light.
Ultraviolet is stopped by glass, but not so readily by water.