1.

Electromagnetic Waves

Electromagnetic waves are a propagating couple of an electric and magnetic field. They are hence known as
'electromagnetic' waves.
The electric and magnetic field vectors are at angle of 90 degree, and both are perpendicular to direction of
propagation of wave.
An electromagnetic wave can be created by accelerating charges. Moving charges back and forth will produce
oscillating electric and magnetic fields, and this travel at the speed of light.
Properties of EM Waves
They are transverse in nature.

They consist of oscillating electric (E) and magnetic (B) field vectors at right angles to each other and at right
angles to the direction of propagation of the wave.

They can travel through a vacuum.

They travel at the speed of light in vacuum, c=3×108m/s.

Electromagnetic wave follows the principle of superposition.
They obey the wave equation c = f λ.
Their frequencies remain unchanged but its wavelength changes when the wave travels from one medium to
another.
 2. Electromagnetic Spectrum

The electromagnetic spectrum is the categorization of electromagnetic wave according to their wavelengths and
frequencies.
The behavior of an electromagnetic wave in a substance depends on its frequency or wavelength.
These waves can be transmitted, reflected, and absorbed
The Visible light is the type of electromagnetic waves to which our eyes responds.
Human body is opaque to visible light—we cannot see through human body—but transparent to X-rays. X-rays are
electromagnetic waves.
Another example is our car window glass is transparent to visible light but largely opaque to ultraviolet radiation.
Ultraviolet radiation is electromagnetic waves.

Entire range of light that exists, from radio waves to gamma rays, most of the light in the universe visible or
invisible to us is represented on electromagnetic spectrum.

Gadi XUV In My Range

The behavior of an electromagnetic wave in a substance depends on its frequency or wavelength.
The differing behaviors of different groups in the electromagnetic spectrum make them suitable for a range of
uses.
Radio Waves
• The longest wavelength and lowest frequency.

• Radio waves are longer than 1 mm.

• Radio wavelengths are found everywhere: in the

background radiation of the universe, in interstellar
clouds, and in the cool remnants of supernova
explosions.
Radio Waves
• Radio stations use radio wavelengths (10 cm – 1000 m)
of electromagnetic radiation to send signals that our
radios then translate into sound.

• These wavelengths are typically 1 m long in the FM

band.
Radio Waves

• Radio stations transmit electromagnetic radiation, not

sound. The radio station encodes a pattern on the em
radiation it transmits, and then our radios receive the
em radiation, decode the pattern and translate the
pattern into sound.
Radio waves
Radio waves are used for communication such as television and radio.

Radio waves are transmitted easily through the air.

They do not cause damage if absorbed by the human body, and they can be reflected to change their direction.

These properties make them ideal for communications.

Microwaves - 0.1 – 10 cm

Basis of almost all space

communications.
Microwaves
Microwaves are used for cooking food and for satellite communications.

High frequency microwaves have frequencies which are easily absorbed by molecules in food.

The internal energy of the molecules increases when they absorb microwaves, which causes heating.

Microwaves pass easily through the atmosphere, so they can pass between stations on Earth and satellites in
orbit.
Infrared 0.1 cm to 0.00007 cm

Heat!
Infrared wavelengths
are about same size as
a single bacteria.
 Infrared radiations

Infrared light is used by electrical heaters, cookers for cooking food, and by infrared cameras which detect
people in the dark.
Infrared light has frequencies which are absorbed by some chemical bonds. The internal energy of the bonds
increases when they absorb infrared light, which causes heating.
This makes infrared light useful for electrical heaters and for cooking food.
All objects emit infrared light. The human eye cannot see this light, but infrared cameras can detect it.
This 'thermal imaging' is useful for detecting people in the dark.
 Visible Light
• Visible light covers the range of wavelengths from 400 to 700 nm.

• Our eyes are sensitive only to this small portion of the electromagnetic
spectrum.

• The Sun emits most of its radiation in the visible range, which our
eyes perceive as the colours of the rainbow.
Visible light
Visible light is the light we can see.

It is used in fiber optic communications, where coded pulses of light travel through glass fibers from a source to
a receiver.
Ionizing radiation
Ultraviolet waves, X-rays and gamma rays are types of ionizing radiation.
This means that they can knock electrons from the shells of atoms, turning them into ions.
This process of ionization can lead to mutations in cells, which can lead to cancer.
Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer.
Gamma ray scan also damage or kill the cells in a person's body.
To be safe, exposure to ionizing radiation needs to be kept as low as possible, especially for people who work
with this type of radiation every day in hospitals.
A radiographer using X-rays in a hospital must stand behind a lead shield or be in another room when the X-ray
machine is being operated.
Ultraviolet 100 to 315 nm

(315-400 nm) UVB (280-315 nm) UVC (100-280

 X-rays
• X-rays range in wavelength from 0.01 to 10 nm (about the size
of an atom).

• Generation:
• Deceleration of the electrons

• Knocking of electrons from the inner shell orbits

• They are also generated, for example, by super heated gas

from exploding stars and quasars, where temperatures are
near a million to ten million degrees.
 Gamma rays
• Gamma rays have the shortest wavelengths, of less than 0.01 nm (about the size
of an atomic nucleus).

• This is the highest frequency and most energetic region of the em spectrum.

• Gamma rays can result from nuclear reactions taking place in objects such as
pulsars, quasars, and black holes.
Geometrical and Optical path
 Path difference and Phase difference
Path Difference

The light ray travel along different paths in same or different medium, and they meet at a point .The
difference between optical path of two rays travelling in different direction Is known as the optical path
difference.
Phase Difference (
The phase of a wave arriving at a point depends on the optical path length it travelled. We know that if a
wave covers in air a distance of one wavelength, 1λ, and its phase changes by 2π radians. So we can
calculate if wave travels a distance L in air, its phase change is,

if the wave travels the distance L in a medium, then

 3. Interference of Light
Interference is an optical phenomenon. In nature many times we can see the interference.
Interference is due to the superposition principle.
“Two or more waves with constant phase difference, same intensity and same amplitude i.e. coherent waves ,
travelling through the medium each wave produces its own displacement irrespective of each other. The resultant
of these waves is the vector sum of the amplitude of each wave”.
“The modification or the retribution of intensity of resultant wave due to superposition principle is known as
interference”.
The bright colors seen in an oil slick floating on water or in a sunlit soap bubble are caused by interference. The
brightest colors are those that interfere constructively.
For constructive interference (maxima), Phase difference = 0, 2π, 4π……

The phase difference of 2π corresponds to the path difference of λ.

Path difference = 0, λ, 2λ, 3λ…

= nλ

Thus, if the path difference between two waves is an integral multiple of the wavelength, then it produces the
constructive interference or maxima.
Destructive Interference

At certain points waves superimpose in such a way that resultant intensity is less than the intensities due to
individual waves. The interference produced at these points is known as destructive interference.

When the crest of one wave coincides with trough of another wave then resultant intensity become minimum and
this is destructive interference. For destructive interference, the two waves must be out phase or having different
phase difference.
For destructive interference (minima), Phase difference = 0, π, 3π, 5π ……

The phase difference of π corresponds to the path difference of 𝜆

2
Path difference = (2n+1)λ/2 where n= 0, 1 , 2 ……
Or (2n-1)λ/2 where n= 1 , 2 ……

Thus, if the path difference between two waves is an odd integral multiple of half of the wavelength, then it
produces the destructive interference or minima.
 Condition for Sustained Interference of Light

To obtain well defined interference patterns, the intensity at points corresponding to destructive interference
must be zero, while intensity at the point corresponding to constructive interference must be maximum.
To accomplish this the following conditions must be satisfied-
1. The two interfering sources must be coherent, that is, they must keep a constant phase difference.
2. The two interfering sources must be monochromatic. The two interfering sources must emit the light of the
same wavelength and time-period. This condition can be achieved by using a monochromatic common original
source, that is, the common source emits light of a single wavelength.
3. The two interfering sources must be of same amplitude, means the amplitudes or intensities of the interfering
waves must be equal or very nearly equal so that the minimum intensity would be zero.

4. The two interfering sources must be close to each other, means the separation between the two coherent
sources must be as small as possible so that the width of the fringes is large and are separately visible.

5. The two sources must be narrow, or they must be extremely small. A broad source is equivalent to many fine
sources. Each pair of fine sources will give its own pattern. The fringes of different interference patterns will
overlap.
Methods to Produce Coherent Waves
Division of Wave front
When light from the source is allowed to pass through two different slits, original wavefront divided into two
wavefronts, travel through different paths and when they united, they interfere.
Examples: Fresnel’s bi-prism, Lloyd’s mirror, Youngs double slit experiment
Division of amplitude
The incident beam is divided into two or more beams by partial reflection at the surface of thin film.
The amplitude, and therefore the intensity of the original wavefront, gets divided.
Examples: Interference in thin film, Newton’s rings.
 4. Stokes Law

Phase change of π or path difference (λ/2) occurs when light waves are reflected at the surface of the denser
medium

No change of phase occurs when light waves are reflected at the surface of a rarer medium.
 5. Interference in Thin Film of Uniform Thickness

Thin layer of oil on water surface and a soap bubble are example of thin film.

If the film thickness is of the order 1 micrometer or nano meter, then it is considered as a thin film. A film with
thickness of few micrometers is considered as thick film.

When a thin film of oil spreads on the surface of water and is exposed to white light beautiful colors are seen. The
brightest colors are those that interfere constructively. This interference is between light reflected from different
surfaces of a thin film. Thus, effect is known as thin film interference.
This phenomenon is also observed when the soap film is illuminated by white light. And can be explained on the
basis of interference between light reflected from upper and lower surfaces of thin films.

Interference due to thin film is due to division of amplitude. When light falls on a thin film some light rays get
reflected, refracted, and transmitted.

Thus, to study the interference due to thin film there are two systems
I. Reflected System
II. Transmitted System
Interference due thin film in reflected system
Ray AB of monochromatic light having a wavelength (λ) incident on the upper surface of a transparent film of thickness
(t) and R.I. (µ) at an angle (i).
Ray AB is partly reflected along BR1 and partly refracted along BC at an angle(r).
The refracted ray BC reflected along CD and finally emerges out along DR2.
BR1 and DR2 are derived from the same incident ray, so they are coherent.
To calculate the path difference between BR1 and DR2, construct a perpendicular DN on BR1 and CM on BD.
Path of BR1 and DR2 beyond DN are same, so the path difference between these two rays is given by,

Path difference= ∆ = Path (BC+CD) in film – Path (BN) in air

………………………………(1)……..(µair=1)

So let’s calculate this path difference

Here we want to calculate the Path (BC+CD)
i
Ray BR1 is reflected from a denser medium to rarer medium, so according to stokes law, additional path
difference of /2 or phase difference (π) is introduced.

Total path difference

Condition for constructive interference or maxima
For bright point path difference is integral multiple of λ,

= n
 Condition for destructive interference or minima
For dark point path difference is odd multiple of
= (2n1)/2

From (a) and (b), maxima and minima in the interference pattern depends upon two factors,
1. Thickness of film
2. The cosine of angle r
When t = 0, the film will appear dark and as the thickness is increased, maxima and minima occur alternatively.
For Example: Newtons
Ring Experiment
 6. Interference in wedge shaped films

An arrangement of two surfaces in contact with

each other at one point and gradually
increasing the thickness at other is known as
wedge shaped thin film as shown in figure.
Condition for constructive interference or maxima
Condition for destructive interference or minima

Fringe width
Fringe width is defined as “The separation between two successive bright or dark fringes”.
7. Formation of Colors In Thin Film
According to interference phenomenon, when thin film of soapbubble, or an oil film on water or wedge-shaped
air film between two glass plates interacted with light beautiful colors spectrum is seen in reflected light.
When white light interacts with film, light reflected from top and bottom surfaces of the film, then these rays
interfere with each other and produce interference pattern of colored fringes.
The path difference between these rays depends upon thickness(t) of film and angle of refraction(r) of the film.
Due to constructive interference, some colors satisfied the condition of maxima

and will be visible with maximum intensity.

While other colors satisfy condition of minima i.e will be absent from the reflected system.

Similarly, if a point is observed at a different angle by keeping the same thickness or different points at
different thickness, a different set of colors is observed at each time.
The colors visible in reflected system will be complementary to the colors visible in transmitted system.
 Engineering Application of Interference

Non-reflecting /Anti-reflecting coating (AR)

When light falls on camera then some light gets reflected back it decreases the quality of image.

Thus, it is necessary to reduce the reflection to improve quality of an image.

The anti-reflection coating is used in cameras, projector lens, telescopes etc., to reduce loss of light by reflection.
Anti-Reflecting Coating On Camera Lenses
When light falls on camera it gets reflected from upper and lower surfaces of an anti-reflecting coating as shown
in above figure.
Ray BC (ray 1) is reflected from surface of coating ray and ray EF (ray 2) reflected from the surface of the lens.
To reduce the reflection the ray 1 and ray 2 must produce the destructive inference.
So, the thickness of the anti-reflecting coating is chosen such that after reflection the ray 1 and ray 2 are in out of
phase to produce destructive interference.

So, the path difference between reflected rays is or phase difference π (destructive interference).

Due to destructive interference the intensity of reflected rays reduced and thus reflections can be minimized.
Thus, the thickness of anti-reflecting coating can be determined by the above formula.
There are different materials are available for anti-reflecting coating. But for by considering the wavelength of
light (5500A0) the most common AR coating used are magnesium fluoride and cryolite. The Refractive index of
MgF2 i.e.
while for cryolite it is