Elecromagnetic Radiations.pdf

By- Mr. Shrikant Sir
Electromagnetic
Radiation and Spectrum

Electromagnetic radiation
 Electromagnetic radiation can be defined as a form of energy that is
produced by the movement of electrically charged particles travelling
through a matter or vacuum or by oscillating magnetic and electric
disturbances.
 Electromagnetic radiation is a form of energy that is present all around us
and takes various forms like microwaves, television waves, radio waves,
gamma rays, X-rays, etc

 The magnetic and the electric fields come at 90° to each other, and the
combined waves move perpendicular to both electric and magnetic
oscillating fields occurring during the disturbance.

 Clerk Maxwell, a Scottish physicist developed a unified theory of
electromagnetism. Its study deals with how the electrically charged
particles interact among themselves and with the magnetic field.

Properties of Electromagnetic Radiation
 When electromagnetic radiation occurs, it is released as photons.
 Electromagnetic radiation can travel through empty space.
 The speed of light is always a constant. (Speed of light : 2.99792458 x 10
8m s-1).
 Wavelengths are measured between the distances of either crests or
troughs. It is usually characterized by the Greek symbol λ.

Amplitude
 Amplitude is the distance from the maximum vertical displacement of the
wave to the middle of the wave. This measures the magnitude of
oscillation of a particular wave.
 In short, the amplitude is basically the height of the wave.
 Larger amplitude means higher energy and lower amplitude means lower
energy.
 Amplitude is important because it tells you the intensity or brightness of
a wave in comparison with other waves.

Wavelength
 Wavelength (λ) is the distance of one full cycle of the oscillation.
 Longer wavelength waves such as radio waves carry low energy; this is
why we can listen to the radio without any harmful consequences.
 Shorter wavelength waves such as x-rays carry higher energy that can be
hazardous to our health. c=λν
where
c is the speed of light,
λ is wavelength, and
ν is frequency.
It can be expressed in Angstrom
units or in millimicrons (mμ).
1 Å = 10–8 cm; 1 mμ = 10–7 cm.

Wave number
 It is the reciprocal of wavelength and it is expressed in per centimeter.

Frequency
 The number of cycles per second is defined as Frequency.
 It is defined as Hertz (Hz) or sec-1.
 If ‘E’ is the energy, ‘h’ is Planck’s constant which is equal to 6.62607 x 10-
34 and ‘ν‘ is the frequency we can derive the relation given below.
E = hν
Thus, we can see that frequency is directly proportional to the energy.
Frequency v = C/ 

 It is expressed as v (nu) in cycles per second or in Hertz (Hz) where 1 Hz =
1 cycle sec–1
 Frequency ⍺ 1/ Wavelength
 Greater the wavelength, smaller is the frequency.
Period
 Period is commonly characterized by the symbol ‘T’. It is the total time
that a wave takes to travel 1 wavelength.

Velocity
 In relation to electromagnetic radiation, the velocity is normally
expressed as:
Velocity = λν
[where, ν = frequency]
 The wave velocity in vacuum for electromagnetic wave is = 186,282
miles/second or 2.99×108 m/s.

Energy
 Energy of a wave of the particular radiation can also be calculated by
applying the relation :
E = h = h .c/ 

Electromagnetic Spectrum and Absorption of Radiations
 Electromagnetic radiation is then grouped into
categories based on its wavelength or frequency
into the electromagnetic spectrum.
 It consists of Gamma-rays, X-rays, ultraviolet rays,
infrared rays, radio waves and microwaves.
 All types of radiations travel as waves with the
same velocity, yet they differ from one another in
certain properties.

 Any wavelength of light that a particular molecule will absorb will be
due to the changes in the following
 Electronic
 Vibrational
 Rotational energy levels
 The wavelengths absorbed are measured with the help of a
spectrometer. If we plot the changes in absorb vs Wavelength.
 Get certain absorption bands which are highly characteristic of a
compound and the technique provides an excellent tool to ascertain
the molecular structure of an unknown substance.

EMR Wavelength Frequency
Hard gamma 1 × 10-9 nm -
Gamma Rays 1 × 10-6 nm -
X ray 0.001 nm to 10
nm, 1nm
-
UV 200 -400 nm -
Visible 400- 800nm -
IR 2.5 to 15 μ. -
Microwave 1 cm to 10 cm 30Ghz
Radio waves 100m to 100km 3 Mhz to 3K Hz
D
E
C
R
E
A
S
E
S
Energy, Frequency Wave Number Wavelength
I
N
C
R
E
A
S
E
S

Visible ultra-violet and IR radiations Subrange
Regions Range
Vacuum UV 10 to 200 nm
UV 200 to 400 nm
Visible 400 to 800 nm
Near IR 0.8 to 2.5 μ
Useful IR 2.5 to 15 μ
Far IR 15 to 25 μ

Visible Light is the only part of the electromagnetic spectrum that humans
can see with an unaided eye.
 Violet  400 - 420 nm.
 Indigo  420 - 440 nm.
 Blue  440 - 490 nm.
 Green  490 - 570 nm.
 Yellow  570 - 585 nm.
 Orange  585 - 620 nm.
 Red  620 - 780 nm.
Shortest wavelength
longest wavelength
V
I
B
G
Y
O
R

Important use in Spectroscopy
Radiation Absorbed Effect on the molecule & information
Ultra-violet(190–400 nm)
and visible 400–800 nm)
UV Visible Spectroscopy
Changes in electronic energy levels within the
molecule
Detection of conjugated unsaturation, conjugation
with non-bonding
electrons, extent of pi-electron system.
IR 2.5 to 15 Micrometer
667 – 4000 cm–1
IR Spectroscopy
Changes in the vibrational and rotational
movements of the molecule.
Detection of almost all functional groups

Radiation Absorbed Effect on the molecule & information
Radio-frequency
Frequency 60-300 MHz
NMR
Nuclear magnetic resonance induces changes in
the magnetic properties of certain atomic nuclei
Structure Determination
Electron beam impact
70 eV, 6000 kJ mol–1
Mass Spectroscopy
Ionisation and fragmentation of the molecule into
a spectrum of fragment ions (determination of
molecular weight and deduction of molecular
structure from the fragments obtained)

Q.1. EMR with highest wave number
(a) X Ray
(b) Gamma Rays
(c) UV
(d) Radio Wave
Q.2. unit of wavelength
(a) Meter
(b) Micrometer
(c) Nanometer
(d) All

Q.3. Choose correct equations
(a) E=hv
(b) E=hc/λ
(c) c=λv
(d) All
Q.4. Reciprocal of wavelength known as
(a) Frequency
(b) Wave counting
(c) Wavenumber
(d) All

Q.5. Possible changes in molecules after absorption of EMR
(a) Electronic change
(b) Vibrational
(c) Rotational
(d) All
Q.6. We Get signals from tower due to
(a) UV
(b) IR
(c) X ray
(d) Radio wave

Q.7. The radiation in the wavelength range 400-800 nm corresponds to
(a) ultra-violet
(b) Infra-red
(c) visible
(d) Far IR.
Q.8. Radio frequency radiations are useful in causing transition for
(a)Mass spectrum
(b) NMR spectra
(c) Raman spectra
(d) None

Q.9. The ionization and fragmentation of the molecule into spectrum of
fragment ions is caused by
(a)Microwaves
(b) lR radiations
(c) electron beam impact of 70 eV
(d) None of these
Q.10. Vacuum UV Regions
(a)Below 200nm
(b) 200 -400 nm
(c) 400-780 nm
(d) 250 nm

Q.11. IR region
(a) 0.8 to 25 micrometer
(b) 25 to 200 micrometer
(c) 200 to 300 nm
(d) 400 to 800 nm
Q.12. Near IR Region
(a) 0.8 to 2.5 micrometer
(b) 2.5 to 15 micrometer
(c) 15 to 25 micrometer
(d) All

Elecromagnetic Radiations.pdf

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