Electromagnetic
UNIT 1 ELECTROMAGNETIC RADIATION Radiation
Structure
1.1 Introduction
Objectives
1.2 What is Electromagnetic Radiation?
Wave Mechanical Model of Electromagnetic Radiation
Quantum Model of Electromagnetic Radiation
1.3 Consequences of Wave Nature of Electromagnetic Radiation
Interference
Diffraction
Transmission
Refraction
Reflection
Scattering
Polarisation
1.4 Interaction of EM Radiation with Matter
Absorption
Emission
Raman Scattering
1.5 Summary
1.6 Terminal Questions
1.7 Answers
1.1 INTRODUCTION
You would surely have seen a beautiful rainbow showing seven different colours
during the rainy season. You know that this colourful spectrum is due to the separation
or dispersion of the white light into its constituent parts by the rain drops. The rainbow
spectrum is just a minute part of a much larger continuum of the radiations that come
from the sun. These are called electromagnetic radiations and the continuum of the
electromagnetic radiations is called the electromagnetic spectrum. In the first unit of
this course you would learn about the electromagnetic radiation in terms of its nature,
characteristics and properties. Spectroscopy is the study of interaction of
electromagnetic radiation with matter. We would discuss the ways in which different
types of electromagnetic radiation interact with matter and also the types of spectra
that result as a consequence of the interaction.
In the next unit you would learn about ultraviolet-visible spectroscopy ‒ a
consequence of interaction of electromagnetic radiation in the ultraviolet-visible range
with the molecules constituting the matter.
Objectives
After studying this unit, you should be able to:
• describe the wave nature of electromagnetic radiation,
• define the parameters that characterise wave form of electromagnetic radiation,
• outline and explain the properties of the electromagnetic radiation that arise due
to its wave nature,
• describe the quantised or particle nature of electromagnetic radiation,
• explain the phenomena of absorption, emission and scattering of
electromagnetic radiation, and
• explain the origin of line spectrum and band spectrum.
7
,Molecular Spectroscopic
Methods-I 1.2 WHAT IS ELECTROMAGNETIC RADIATION?
You feel hot while sitting close to a fire. You would say it is obvious, what is so
special about it? Fine! But have you ever wondered what makes you feel hot or in
other words how does the energy as heat reaches you from the source? This is in the
form of electromagnetic (EM) radiation. Heat energy reaches us in the form of infra-
red radiations which is a type of electromagnetic radiation. Let us learn about EM
radiation in general.
An electromagnetic radiation may be defined as the radiant energy which is
transmitted through space at enormous velocities. It exists in various forms; the visible
light and radiant heat being the easily recognised forms which we experience in
daytoday life too. The EM radiation, unlike sound, does not require a medium for
transmission and pass readily through vacuum and in vacuum, the velocity of radiation
is 3 × 108 m s . The study of different ways in which EM radiation can interact with
‒1
matter is of great importance in Chemistry. In order to understand these interactions it
is therefore necessary to have knowledge of the properties of EM radiation. The
properties of an electromagnetic radiation could be explained in terms of both classical
wave model (wave mechanics) and particle model (quantum mechanics). Let us learn
about these models of EM radiation and the properties associated with them.
1.2.1 Wave Mechanical Model of Electromagnetic Radiation
In the wave mechanical model, the electromagnetic radiation is considered to be
sinusoidal in nature, i.e., varying like a sine function. According to the Maxwell’s
theory, an electromagnetic wave can be visualised as oscillatory electric and magnetic
fields travelling in the planes perpendicular to each other and also to the direction of
propagation (Fig. 1.1). This gives rise to its name viz., electromagnetic radiation.
However, since it is the electrical effect that is responsible for the phenomenon of
interest to a chemist, it is generally represented in terms of the electric field only.
Fig. 1.1: An EM radiation showing oscillatory electric and magnetic fields
Characteristic Parameters of Electromagnetic Radiation
The electromagnetic wave is characterised in terms of a number of parameters. These
are as follows:
8
, Amplitude: It refers to the maximum height to which the wave oscillates and equals Electromagnetic
the height of the crests or depths of the troughs. It is a measure of the radiant power of Radiation
the radiation. The radiant power refers to the energy of the radiation striking at a
given area per unit time. It is denoted as P and is related to the square of the
amplitude. It is important to remember that the radiant power is not related to the
wavelength. A closely related term is called intensity of the radiation which is
denoted as I and is defined as the radiant power per unit solid angle.
Wavelength: It is the linear distance between two consecutive wave-crests or wave-
1 cm = 10-2 m
troughs or the distance of complete cycle as shown in Fig. 1.1. It is represented by a
1 µm = 10-6 m
Greek letter lambda (λ) and expressed in terms of metre (m), centimetre (cm), 1 nm = 10-9 m
micrometre (µm), nanometre (nm) or Angstrom (Å) units. The wavelength of 1 Å = 10-10 m
electromagnetic radiation varies from a few Angstroms to several metres.
Frequency: It is defined as the number of wave crests or wave troughs that pass
through a given point per second. It is represented by a Greek letter nu (ν) and Different spectroscopic
techniques use different
expressed in terms of second inverse or per second (s −1 ) or Hertz (Hz). The units for wavelength.
relationship between wavelength and frequency is given as: Nanometre is popular for
c UV-visible regions while
ν= micrometre (µm) is
λ preferred for the IR
Where, the wavelength λ is in metres, the frequency ν is in reciprocal seconds (s −1 ) region.
and the velocity of light, c = 3 × l08 m s −1 .
Wave number: It equals the number of waves per centimetre or per unit distance. It
is denoted as nu bar (ν ) and is equal to the reciprocal of the wavelength expressed in
metres (m). The unit of ν is metre inverse (m −1 ) though cm‒1 is also commonly
employed; where the wavelength is expressed in the units of cm.
1
ν =
λ
Velocity: It is defined as the linear distance travelled by the wave in one second. The
velocity in metres per second can be obtained by multiplying frequency in second
inverse by wavelength in metres.
c = ν (s −1 ) λ
The velocity of EM radiation depends on the medium. It has maximum value in
vacuum and equals 3.00 × 108 m s −1 .
Energy: The energy of the electromagnetic radiation depends on its wavelength or
frequency. The relationship is as under.
hc
E = hν = ... (1.1)
λ
where, h is the Planck’s constant and has a value of = 6.626 × 10 −34 J s, ν is the
frequency c is the velocity and λ is the wavelength. As you can probably make out
that the energy is directly related to the frequency and inversely related to the
wavelength of the radiation. In other words a high frequency radiation will have a
higher energy while a longer wavelength radiation will be low in energy. Table 1.1
summarises the important characteristics of the EM radiation, their relationship with
wavelength and the common units of their measurement.
9
UNIT 1 ELECTROMAGNETIC RADIATION Radiation
Structure
1.1 Introduction
Objectives
1.2 What is Electromagnetic Radiation?
Wave Mechanical Model of Electromagnetic Radiation
Quantum Model of Electromagnetic Radiation
1.3 Consequences of Wave Nature of Electromagnetic Radiation
Interference
Diffraction
Transmission
Refraction
Reflection
Scattering
Polarisation
1.4 Interaction of EM Radiation with Matter
Absorption
Emission
Raman Scattering
1.5 Summary
1.6 Terminal Questions
1.7 Answers
1.1 INTRODUCTION
You would surely have seen a beautiful rainbow showing seven different colours
during the rainy season. You know that this colourful spectrum is due to the separation
or dispersion of the white light into its constituent parts by the rain drops. The rainbow
spectrum is just a minute part of a much larger continuum of the radiations that come
from the sun. These are called electromagnetic radiations and the continuum of the
electromagnetic radiations is called the electromagnetic spectrum. In the first unit of
this course you would learn about the electromagnetic radiation in terms of its nature,
characteristics and properties. Spectroscopy is the study of interaction of
electromagnetic radiation with matter. We would discuss the ways in which different
types of electromagnetic radiation interact with matter and also the types of spectra
that result as a consequence of the interaction.
In the next unit you would learn about ultraviolet-visible spectroscopy ‒ a
consequence of interaction of electromagnetic radiation in the ultraviolet-visible range
with the molecules constituting the matter.
Objectives
After studying this unit, you should be able to:
• describe the wave nature of electromagnetic radiation,
• define the parameters that characterise wave form of electromagnetic radiation,
• outline and explain the properties of the electromagnetic radiation that arise due
to its wave nature,
• describe the quantised or particle nature of electromagnetic radiation,
• explain the phenomena of absorption, emission and scattering of
electromagnetic radiation, and
• explain the origin of line spectrum and band spectrum.
7
,Molecular Spectroscopic
Methods-I 1.2 WHAT IS ELECTROMAGNETIC RADIATION?
You feel hot while sitting close to a fire. You would say it is obvious, what is so
special about it? Fine! But have you ever wondered what makes you feel hot or in
other words how does the energy as heat reaches you from the source? This is in the
form of electromagnetic (EM) radiation. Heat energy reaches us in the form of infra-
red radiations which is a type of electromagnetic radiation. Let us learn about EM
radiation in general.
An electromagnetic radiation may be defined as the radiant energy which is
transmitted through space at enormous velocities. It exists in various forms; the visible
light and radiant heat being the easily recognised forms which we experience in
daytoday life too. The EM radiation, unlike sound, does not require a medium for
transmission and pass readily through vacuum and in vacuum, the velocity of radiation
is 3 × 108 m s . The study of different ways in which EM radiation can interact with
‒1
matter is of great importance in Chemistry. In order to understand these interactions it
is therefore necessary to have knowledge of the properties of EM radiation. The
properties of an electromagnetic radiation could be explained in terms of both classical
wave model (wave mechanics) and particle model (quantum mechanics). Let us learn
about these models of EM radiation and the properties associated with them.
1.2.1 Wave Mechanical Model of Electromagnetic Radiation
In the wave mechanical model, the electromagnetic radiation is considered to be
sinusoidal in nature, i.e., varying like a sine function. According to the Maxwell’s
theory, an electromagnetic wave can be visualised as oscillatory electric and magnetic
fields travelling in the planes perpendicular to each other and also to the direction of
propagation (Fig. 1.1). This gives rise to its name viz., electromagnetic radiation.
However, since it is the electrical effect that is responsible for the phenomenon of
interest to a chemist, it is generally represented in terms of the electric field only.
Fig. 1.1: An EM radiation showing oscillatory electric and magnetic fields
Characteristic Parameters of Electromagnetic Radiation
The electromagnetic wave is characterised in terms of a number of parameters. These
are as follows:
8
, Amplitude: It refers to the maximum height to which the wave oscillates and equals Electromagnetic
the height of the crests or depths of the troughs. It is a measure of the radiant power of Radiation
the radiation. The radiant power refers to the energy of the radiation striking at a
given area per unit time. It is denoted as P and is related to the square of the
amplitude. It is important to remember that the radiant power is not related to the
wavelength. A closely related term is called intensity of the radiation which is
denoted as I and is defined as the radiant power per unit solid angle.
Wavelength: It is the linear distance between two consecutive wave-crests or wave-
1 cm = 10-2 m
troughs or the distance of complete cycle as shown in Fig. 1.1. It is represented by a
1 µm = 10-6 m
Greek letter lambda (λ) and expressed in terms of metre (m), centimetre (cm), 1 nm = 10-9 m
micrometre (µm), nanometre (nm) or Angstrom (Å) units. The wavelength of 1 Å = 10-10 m
electromagnetic radiation varies from a few Angstroms to several metres.
Frequency: It is defined as the number of wave crests or wave troughs that pass
through a given point per second. It is represented by a Greek letter nu (ν) and Different spectroscopic
techniques use different
expressed in terms of second inverse or per second (s −1 ) or Hertz (Hz). The units for wavelength.
relationship between wavelength and frequency is given as: Nanometre is popular for
c UV-visible regions while
ν= micrometre (µm) is
λ preferred for the IR
Where, the wavelength λ is in metres, the frequency ν is in reciprocal seconds (s −1 ) region.
and the velocity of light, c = 3 × l08 m s −1 .
Wave number: It equals the number of waves per centimetre or per unit distance. It
is denoted as nu bar (ν ) and is equal to the reciprocal of the wavelength expressed in
metres (m). The unit of ν is metre inverse (m −1 ) though cm‒1 is also commonly
employed; where the wavelength is expressed in the units of cm.
1
ν =
λ
Velocity: It is defined as the linear distance travelled by the wave in one second. The
velocity in metres per second can be obtained by multiplying frequency in second
inverse by wavelength in metres.
c = ν (s −1 ) λ
The velocity of EM radiation depends on the medium. It has maximum value in
vacuum and equals 3.00 × 108 m s −1 .
Energy: The energy of the electromagnetic radiation depends on its wavelength or
frequency. The relationship is as under.
hc
E = hν = ... (1.1)
λ
where, h is the Planck’s constant and has a value of = 6.626 × 10 −34 J s, ν is the
frequency c is the velocity and λ is the wavelength. As you can probably make out
that the energy is directly related to the frequency and inversely related to the
wavelength of the radiation. In other words a high frequency radiation will have a
higher energy while a longer wavelength radiation will be low in energy. Table 1.1
summarises the important characteristics of the EM radiation, their relationship with
wavelength and the common units of their measurement.
9
