Electromagnetic radiation

ELECTROMAGNETIC RADIATION (EMR) 1

Electromagnetic radiation (EMR)
Electromagnetic radiation (EM radiation or EMR) is a form of energy emitted and absorbed by charged
particles, which exhibits wave-like behavior as it travels through space. EMR has
both electric and magnetic field components, EMR carries energy—sometimes called radiant energy—
through space continuously away from the source (this is not true of the near-field part of the EM
field). EMR also carries both momentum and angular momentum. These properties may all be
imparted to matter with which it interacts. EMR is produced from other types of energy when created,
and it is converted to other types of energy when it is destroyed. The photon is the quantum of the
electromagnetic interaction, and is the basic "unit" or constituent of all forms of EMR. The quantum
nature of light becomes more apparent at high frequencies (or high photon energy). Such photons
behave more like particles than lower-frequency photons do.
In simple words,Electromagnetic radiation refers to the transfer of energy through electromagnetic
waves. These waves can transfer energy by traveling in a vacuum, and (in a vacuum) they travel at
around,
v = 3.00*10^8m/s
You may have identified this speed as the speed of light. Indeed, light is electromagnetic radiation.
Visible light forms a small part of the electromagnetic spectrum, a graphical representation of the
relationship between frequency and wavelength in electromagnetic waves.
Electromagnetic waves include radio waves, infrared, visible light, and gamma rays (there are a few
more). They all differ in frequency and wavelength. The speed of a wave is given by the equation,

velocity = (wavelength) x (frequency)
As you can see, wavelength and frequency are inversely proportional. That means that as one
decreases, the other increases (given, of course, a constant velocity, which has the magnitude I
mentioned before). Radio waves have the longest wavelength (and thus the less frequency).

Characteristics of Electromagnetic Radiation.

Electromagnetic energy or radiation is the medium for transmitting information from an object to
sensor. The information is propagated by electromagnetic radiation at the velocity of light from the
source (earth surface) directly through free space or indirectly by reflection, scattering and radiation to
the sensor. The electromagnetic radiation is looked at as sinusoidal waves which are composed of a
combination of two fields. An electric field (which we will use, in this course, to explain absorption

MICHAEL HEMBROM


and emission of radiation by analyses) and a magnetic field at right angle to the electric field (which
will be used to explain phenomena like nuclear magnetic resonance in the course of special topics in
analytical chemistry offered to Chemistry students only). However, if we look at the models describing
electromagnetic radiation, we can present the classical wave model and the quantum mechanical
particle model.

The classical wave model describes electromagnetic radiation as waves that have a wavelength,
frequency, velocity, and amplitude. These properties of electromagnetic radiation can explain classical
characteristics of electromagnetic radiation like reflection, refraction, diffraction, interference, etc.
However, the wave model cannot explain the phenomena of absorption and emission of radiation,
which leads to the quantum mechanical model.

An electromagnetic beam can be described as sinusoidal electric fields at right angle to magnetic fields.
If we look at one wave only at a certain plain (monochromatic plane polarized wave) we can picture
the wave as follows:

We will only deal with the electric field of the electromagnetic radiation and will thus refer to an
electromagnetic wave as an electric field having the shape of a sinusoidal wave. The arrows in the
figure above represent few electric vectors while the yellow solid sinusoidal wave is the magnetic field
associated to the electric field of the wave.

MICHAEL HEMBROM


Wave Parameters

There are some parameters which can describe the wave. These include the followinf:

1. Wavelength (l)

The wavelength of a wave is the distance between two consecutive maxima or two consecutive minima
on the wave. It can also be defined as the distance between two equivalent points on two successive
maxima or minima. This can be seen on the figure below:

2. Ammplitude (A)

The amplitude of the wave is represented by the length of the electrical vector at a maximum or
minimum in the wave. In the figure above, the amplitude is the length of any of the vertical arrows
perpendicular to the direction of propagation of the wave.

MICHAEL HEMBROM


3. Frequency (n)

The frequency of the wave is directly proportional to the energy of the wave and is defined as the
number of wavelengths passing a fixed point in space in one second.

4. Period (p)

The period of the wave is the time in seconds required for one wavelength to pass a fixed point in
space.

5. Velocity (v)

The velocity of a wave is defined as the multiplication of the frequency times the wavelength. This
means:
V=nl

The velocity of light in vacuum is greater than its velocity in any other medium. Since the frequency of
the wave is a constant and is a property of the source, the decrease in velocity of electromagnetic
radiation in media other than vacuum should thus be attributed to a decrease in the wavelength of
radiation upon passage through that medium.

MICHAEL HEMBROM


6. Wavenumber (n)

The reciprocal of wavelength in centimeters is called the wave number. This is an important property
especially in the study of infrared spectroscopy.
n=kn

7. Radiation Power and Intensity

The power of the radiation is related to the square of the amplitude. The intensity is also related to the
square of the amplitude. However, the power and intensity have different concepts but will be used
synonymously in this course.

MICHAEL HEMBROM


ELECTROMAGNETIC SPECTRUM
The electromagnetic radiation covers a vast spectrum of frequencies and wavelengths. This includes
the very energetic gamma-rays radiation with a wavelength range from 0.005 – 1.4 Ao to radio waves
rays
in the wavelength range up to meters (exceedingly low energy). However, the region of interest to us in
this course is rather a very limited range from 180 780 nm. This limited range covers both ultraviolet
ry 180-780
and visible radiation. It is of interest to indicate at this point that each region of electromagnetic
spectrum requires a special set of instrumental components. This can therefore make it clear for us that
therefore
the type of instrumental components we will deal with will be extremely limited but, at the same time,
will be representative enough.
In general, EM radiation (the designation 'radiation' excludes static electric and magnetic and near
a
fields) is classified by wavelength into radio, microwave, infrared, the visible spectrum we perceive as
)
visible light, ultraviolet, X-rays and gamma rays. he behavior of EM radiation depends on its
rays, .
frequency. Lower frequencies have longer wavelengths, and higher frequencies have shorter
wavelengths, and are associated with photons of higher energy.

Wavelength region and their specification
The types of electromagnetic radiation are broadly classified into the following classes:

1. Gamma radiation
2. X-ray radiation
3. Ultraviolet radiation
4. Visible radiation
5. Infrared radiation
6. Microwave radiation
7. Radio waves

MICHAEL HEMBROM


Gamma rays
discovered by Paul Villard in 1900. Radioactive materials (some natural and others made by man in
things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to
help them understand what matter is made of can sometimes generate gamma-rays. But the biggest
gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.
Wavelength rangest between 10-11-10-8.
X-rays
After UV come X-rays, which, like the upper ranges of UV are also ionizing. However, due to their
higher energies, X-rays can also interact with matter by means of the Compton effect. Hard X-rays
have shorter wavelengths than soft X-rays. As they can pass through most substances, X-rays can be
used to 'see through' objects, wavelength ranges between 0.3x10-8cm – 3x10-6cm.
Ultraviolet light
The Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to
burn! Stars and other "hot" objects in space emit UV radiation.The wavelength of UV rays is shorter
than the violet end of the visible spectrum but longer than the X-ray.
UV in the very shortest range (next to X-rays) is capable even of ionizing atoms (see photoelectric
effect), greatly changing their physical behavior. Wavelength ranges between 0.3 µ-0.4 µ.

Visible radiation (light)
his is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to
stars ... also by fast-moving particles hitting other particles. This is the range in which the sun and
other stars emit most of their radiation and the spectrum that the human eye is the most sensitive to.
Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and
atoms that move from one energy level to another. The light we see with our eyes is really a very small
portion of the electromagnetic spectrum.

MICHAEL HEMBROM


Visible radiation has three types.
1. Blue-0.4 µ -0.5 µ.
2. Green- 0.5 µ-0.6 µ.
3. Red- 0.6 µ-0.7 µ
Infrared radiation
The infrared part of the electromagnetic spectrum covers the range from roughly 0.7 µ to 1 µ. It can be
divided into three parts:
1. near-infrared -0.7 µ-1.3 µ
2. Mid infrared -1.3 µ-3.0 µ
3. Far or thermal infrared- 3.0 µ-0.3mm
Microwave radiation
Microwaves are waves that are typically short enough to employ tubular metal waveguides of
reasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solid
state diodes such as Gunn and IMPATT devices.wavelength ranges between 1mm-1m.

Radiowave
Radio waves generally are utilized by antennas of appropriate size (according to the principle
of resonance), with wavelengths ranging from hundreds of meters to about one millimeter. They are
used for transmission of data, via modulation. Television, mobile phones, wireless networking,
and amateur radio all use radio waves. The use of the radio spectrum is regulated by many
governments through frequency allocation. Wavelength ranges between 30cm – 3km.

MICHAEL HEMBROM


MICHAEL HEMBROM

Electromagnetic radiation

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