Spectroscopy is the science that deals with interactions between electromagnetic radiation and matter. It involves transitions between energy levels of atoms and molecules when they absorb or emit photons. Different regions of the electromagnetic spectrum, such as visible light, ultraviolet light, infrared light, and radio waves, can be used in spectroscopy. Absorption spectroscopy measures the absorption of radiation to identify substances based on their unique absorption patterns. Beer's law describes the relationship between absorbance of a sample and its concentration.

1. Introduction to spectroscopy
By: Analytical Chemist
March, 2020
Injibara University

2.  The science that deals with light and its absorption and emission by
solutions and other material substances is called spectroscopy or
spectrometry. OR
 Spectroscopy is the science that deals with the interactions of
radiation with matter (atomic and molecular).
 Spectrometric methods are a large group of analytical methods
 The most widely used spectrometric methods are based on
electromagnetic radiation (light, gamma rays, X-rays, UV,
microwave, and radio-frequency).
 The most interesting types of interactions in spectroscopy involve
transitions between different energy levels of chemical species.
 Other types of interactions, such as reflection. refraction. elastic
scattering, interference, and diffraction, are often related to the bulk
properties of materials rather than to energy levels of specific
molecules or atoms.

3.  The specific types of interactions that we observe depend strongly
on the energy of the radiation used and the mode of detection.
 In spectrochemical analysis procedures, the degree to which light is
absorbed, or the intensity of light that is emitted, is related to the
amount of an analyte present in the sample tested.
The Electromagnetic Spectrum
 Wavelengths can vary in distance from as little as fractions of
atomic diameters to as long as several miles.
 This suggests the existence of an extremely broad spectrum of
wavelengths(Fig.1.1)
 This electromagnetic spectrum of light is so broad that we break it
down into regions.
 The region of wavelengths that we see with our eyes is called the
visible region
 EMR is a form of energy whose behavior is described by the
properties of both waves and particles.

4. Fig.1.1, Electromagnetic Spectrum. E increase from right to left.

5.  EMR consists of oscillating electric and magnetic fields that
propagate through space along a linear path and with a constant
velocity (Fig.1.2).
 In a vacuum, EMR travels at the speed of light, c, which is 2.99792
x108 m/s.
 In a medium containing matter, EMR travels with a velocity v, less
than c because of interaction b/n the EM field and e-s in the atoms
or molecules of the medium.
 The difference between v and c is small enough (< 0.1%) that the
speed of light to 3 significant figures, 3.00x108 m/s, is sufficiently
accurate for most purposes.
 Oscillations in the electric and magnetic fields are perpendicular to
each other, and to the direction of the wave’s propagation.
5

6. Fig.1.2.Plane-polarized EMR of wavelength λ, propagating
along the x-axis. The electric field of plane-polarized light is
confined to a single plane. Ordinary, unpolarized light has
electric field components in all planes.

7.  With regard to energy, it is more convenient to think of light as
particles called photons.
 Each photon carries the energy, E, which is given by E= hv
 When a sample absorbs EMR it undergoes a change in energy.
 The interaction between the sample and the EMR is easiest to
understand if we assume that EMR consists of a beam of energetic
particles called photons.
 When a photon is absorbed by a sample, it is “destroyed,” and its
energy acquired by the sample.
 The energy of a photon, in joules, is related to its frequency,
wavelength, or wave number by E = hv = hc/λ = hcṼ
where h is Planck's constant, which has a value of 6.62618x10-34 J·s.
C is velocity of Light on vacuum and its value is 2.99792 x 108 m/s
λ is wavelength , Ṽ is wave number and v is frequency

8.  The length of an electromagnetic wave is called its
wavelength (λ). In a set of repeating waves, λ is the
physical distance from a point on one wave, such as the
crest of the wave, to the crest on the next wave.
Unit: Angstrom, nm, µm
Frequency () The number of flips, or oscillations, that
occur in one second.
The relationship between the speed of light c , wavelength,
and frequency is :
C=

9. The energy, E, of one photon depends on its frequency of
oscillation :
where h is Planck's constant (6.62618x10-34 J·s)
Velocity Of Light on vacuum = 2.99792 x 108 m/s
When light passes through other media, the velocity of light
. Since the energy of a photon is fixed, the frequency of
a photon does not change.
Thus for a given frequency of light, the wavelength must 
as the velocity decreases.
What is the frequency of a light that has a wavelength of
537 nm? Ans = 5.59 x1014 sec-1
What is the λ of light that has a frequency of 7.89 1014sec-1?
Express the answer in both cm and nm.
Ans 0.0000380 cm = 380 nm
E = h = hc / 

10.  Light is a form of energy & each wavelength or frequency
has a certain amount of energy associated with it.
 This energy is considered to be the energy associated with
a single photon of the light. Thus, the particle theory and
the wave theory are linked via energy.
 The relationship between energy & frequency is as
follows E = hv
 where E is energy, v is the frequency, and h is a
proportionality constant known as Planck’s constant, after
the famous physicist Max Planck (6.63x10-34J sec)

11.  A line spectrum, meaning that individual absorption lines
are observed, rather than a continuous, unbroken band,
like that observed for the copper sulfate solution.
 Continuous spectrum, the spectrum is an unbroken
pattern, left to right. It does not display any breaks or
sharp peaks of absorption at particular wavelengths, but
rather shows that a smooth band of wavelengths in a
given region, such as the red region, is absorbed.
 If a solution displays a blue color, which means that the
blue portion of the visible region is not absorbed, but
transmitted to our eyes, while the red portion is absorbed.
 The absorption spectrum of this solution in the visible
region is shown in Fig.1.3.

12.  Atoms in which no electrons are in the higher levels are
said to be in the ground state. This state is designated in
energy level diagrams as E0.
 Atoms in which there is an electron in the higher level
are said to be in an excited state.
 Excited states are designated in energy level diagrams as
E1, E2, E3, etc. An energy level diagram consists of short
horizontal lines representing the levels or states, with
each line labeled as E0, E1, etc.
 Often, an energy level diagram shows the movement of
electrons between levels with longer vertical arrows.
 The movement of an electron between electron energy
levels is called an electronic energy transition.

13. This spectrum clearly shows that wavelengths in the blue
& violet regions (350-500nm) are not absorbed, while
wavelengths in the red region (650-750nm) are absorbed.
a. b.
Fig.1.3.The absorption spectrum of a)visible region of a
copper sulfate solution. b) ultraviolet region of a gaseous
copper atoms.

14.  When a molecule absorbs a photon, the energy of the
molecule increases & the molecule is promoted to an
excited state. If a molecule emits a photon, the energy of
the molecule is lowered.
 The lowest energy state of a molecule is called the ground
state.
 The amount of light absorbed is called the absorbance(A).
 When radiation passes through a layer of solid, liquid/gas,
certain frequencies may be absorbed, a process in which
EM energy is transferred to the sample.
 It is important to keep in mind that the light coming in
must be exactly the same energy as the energy difference
between the two electronic levels; otherwise, it will not be
absorbed at all.

15.  An energy level diagram of an atom showing the fact
that some wavelengths possess too much or too little
energy to be absorbed, while another possesses the exact
energy required and is therefore absorbed

16. Absorption is produced when electron absorbs incoming
photon and jumps from a lower orbit to a higher orbit
Emission is produced when electron jumps from a higher
orbit to a lower orbit and emits a photon of the same
energy

17. Continuous spectrum
Emission line spectrum
Absorption line spectrum

18.  Many cpds absorb radiation. The diagram below shows a beam of
monochromatic radiation of radiant power I0 directed at a sample
solution.
 Absorption takes place and the beam of radiation leaving the
sample has radiant power I.
 Transmittance, T, is defined as the fraction of the original light
that passes through the sample.
 Transmittance: T= I/ Io Therefore, T has the range 0 to l. The
percent transmittance is simply l00T & ranges between 0 & 100%.
 Absorption promotes these particles from their ground state to
more higher-energy excited state.

19. The amount of radiation absorbed may be measured in a number of
ways: Transmittance, T = I / I0 % Transmittance, %T = 100 T
Absorbance, A = log10 1 / T A = log10 I0 / I
A = log10 100 / %T A = 2 - log10 %T

20.  The last equation, A = 2 - log10 %T , is worth remembering
because it allows you to easily calculate absorbance from
percentage transmittance data.
 The relationship between absorbance and transmittance is
illustrated in the following diagram:
 The equation representing the Beer’s law: A = ε b c
Where :- A is absorbance (no units, A = log10 I0 / I )
 ε is the molar absorbtivity that measure the amount of light
absorbed per unit conc. with units of L mol-1
cm-1
.
 b is the path length of the sample i.e. the path length of the cuvette
in which the sample is contained.

21.  We will express this measurement in cm and c is the conc. of the
cpd in solution, expressed in mol L-1.
 Beer’s law tells us that absorbance depends on the total quantity of
the absorbing cpd in the light path through the cuvette.
 If we plot absorbance against concentration, we get a straight line
passing through the origin (0,0).