Interaction of electromagnetic radiation with 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, 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 primarily move through free
space, but may have a chance to encounter with the nucleus or an electron of an atom.
Because the encounters of photons with sub atomic 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 composition and
thickness of the medium. 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.
The interactions between electromagnetic radiation and matter cause changes in the
energy states of the electrons in matter.
Electrons can be transferred from one energy level to another, while absorbing or
emitting a certain amount of energy. This amount of energy is equal to the energy
difference between these two energy levels (E2
-E1
).
When this energy is absorbed or emitted in a form of electromagnetic radiation, the
energy difference between these two energy levels (E2
-E1
) determines uniquely the
frequency (λ) of the electromagnetic radiation:
( E) = E2
-E1 = h
1.4.1 Absorption
Absorption of electromagnetic radiation is the way in which the energy of a photon is
taken up by matter, typically the electrons of an atom. Atoms or molecules absorb
light, the incoming energy excites a quantized structure to a higher energy level. The
type of excitation depends on the wavelength of the light. Electrons are promoted to
higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light,
and rotations are excited by microwaves.
An absorption spectrum is the absorption of light as a function of wavelength. The
spectrum of an atom or molecule depends on its energy level structure, and absorption
spectra are useful for identifying of compounds.
Emission
The emission spectrum of a chemical element or chemical compound is the spectrum
of frequencies of electromagnetic radiation emitted due to an atom's electrons making
a transition from a high energy state to a lower energy state. Atoms or molecules that
are excited to high energy levels can decay to lower levels by emitting radiation
(emission or luminescence). For atoms excited by a high-temperature energy source
this light emission is commonly called atomic or optical emission, and for atoms
excited with light it is called atomic fluorescence. For molecules it is called
fluorescence if the transition is between states of the same spin (singlet to singlet
transition) and phosphorescence if the transition occurs between states of different spin
(triplet to singlet transition).
The emission intensity of an emitting substance is linearly proportional to analyte
concentration at low concentrations, and is useful for quantitating emitting species.
1.4.3 Transmission
Transmission happens when light goes through a surface or object. There are 3 types
of transmission: direct, diffuse or selective.
1. Direct transmission: it is when light goes through an object and no change in
direction or quality takes place. For example, through glass or air.
2. Diffuse transmission: it is produced when light goes through a transparent or semi-
transparent object with texture. For example, frosted glass or drafting paper. Light,
instead of going in one direction, is redirected to other directions. Light which is
transmitted in a diffused manner tends to be softer; it will have less contrast and
less intensity; it will generate clearer shades; and it will have a smoother transition
between highlights and shadows than direct light.
Selective transmission: it is produced when light goes through a coloured object. A
portion of light will be absorbed and another portion will be transmitted through this
object. In the example below, white light (red, green and blue) goes through a red
surface. The green and blue are absorbed and only red is transmitted. As a result, we
will only see red light on the other side of this surface.
Filters or gels, which we mentioned in the lesson about colour temperature, work
through selective transmission. Colour filters will only allow one colour to go through
(a blue filter allows only blue light go through) and it will absorb the rest of the
colours. A blue filter lets blue wave lengths through and absorbs red and green wave
lengths.
1.4.4. Reflection
Reflection happens when light reaches an object and it bounces or is reflected,
partially or totally, from this object. Light can be reflected directly or in a diffused
manner.
1. Direct reflection: it is produced when light is reflected from a flat or smooth
surface such as, for example, a mirror. Light will be reflected in the same angle as it
reached this surface (law of reflection).
