Optics Prof Md Anisur Rahman

Properties of Electromagnetic waves
Electromagnetic waves share six properties with all
forms of wave motion:
1) Polarization 2)Interference
3) Diffraction 4) Dispersion
5) Reflection 6) Refraction
Here, 1 to 4 is physical optics and 5, 6 geometrical optic
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What is light?
Before discussing these properties we should have
clear idea about wave properties of light.
What is light?
Light may be defined as energy to which the human eye
is sensitive (Elkington. P: 1)
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There are seven domains of wavebands
1) Ultraviolet C (UV-C) 200 – 280 nanometer
2) Ultraviolet B (UV-B) 280 – 315 nanometer
3) Ultraviolet A (UV–A) 315 – 400 nanometer
4) Visible radiation 400-780 nanometer.
5) Infrared A (IR-A), 780-1400 nanometer.
6) Infrared B (IR-B), 1400-3000 nanometer.
7) Infrared C (IR-C), 3000-10000 nanometer.

• The shorter the wavelength, the greater the energy of
the individual quanta.

Absorption of light by eyeball
• The cornea and sclera of the eye absorb essentially all
the incident optical radiation at very short wavelength
in the ultraviolet (UV-B and UV-C) and long
wavelengths in the infrared (IR-B & IR-C)

• The incident UV-A is strongly absorbed by the
crystalline lens while wavelengths in the range 400-
1400(visible light and near infrared) pass through the
ocular media to fall on the retina.

• The visible wavelengths stimulate the retinal
photoreceptor giving the sensation of light while the
near infrared may give rise to thermal effects.
QUESTION.
• How eclipse burn causes retinal damage?

In the late 1600s, important questions were raised,
asking if light is made up of particles, or is it waves?

Sir Isaac Newton, held the theory that light was made
up of tiny particles.
By the help of Newton’s theory we can only prove
refraction and reflection of light

In 1678, Dutch physicist, Christiaan Huygens, believed
that light was made up of waves vibrating up and down
perpendicular to the direction of the light travels, and
therefore formulated a way of visualising wave
propagation. This became known as 'Huygens'
Principle'.

Waves have two important characteristics -
wavelength and frequency

Wavelength:
• Wavelength:
• This is the distance between peaks of a wave.
Wavelengths are measured in units of length - meters,
When dealing with light, wavelengths are in the order
of nanometres (1 x 10-9)

Frequency:
Frequency:
• This is the number of peaks that will travel past a
point in one second. Frequency is measured in cycles
per second. The term given to this is Hertz (Hz)
named after the 19th century discoverer of radio
waves - Heinrich Hertz. 1 Hz = 1 cycle per second

Theory of Light
 Light is based on three theory:
1. Newton: Particle theory
2. Huygens: Wave theory
3. Einstein: Quantum mechanics

Newton's theory came first, but the theory of Huygens,
better described early experiments. Huygens'
principle lets you predict where a given wavefront
will be in the future, if you have the knowledge of
where the given wavefront is in the present

In 1905 Albert Einstein light having characteristics of
both wave and particle theory. From work of Plank on
emission of light from hot bodies, Einstein suggested
that light is composed of tiny particles
called photons, and each photon has energy.

• Light theory branches in to the physics of quantum
mechanics, which was conceptualised in the twentieth
century. Quantum mechanics deals with behaviour of
nature on the atomic scale or smaller.
• As a result of quantum mechanics, this gave the proof
of the dual nature of light and therefore not a
contradiction.

The wave moves energy—without moving mass—from
one place to another at a speed independent of its
intensity or wavelength.
This wave nature of light is the basis of physical optics
and describes the interaction of light with media. Many
of these processes require calculus and quantum theory
to describe them rigorously.

So light is based on three theory:
1. Newton: Particle theory
2. Huygens: Wave theory
3. Einstein: Quantum mechanics

Picture of a light wave

light wave
 The maximum value of the wave displacement is
called the amplitude (A) of the wave.
 The cycle starts at zero and repeats after a distance.
This distance is called the wavelength (λ).
 Light can have different wavelengths. The inverse of
the wavelength (1/λ) is the wave number (ν), which
is expressed in cm–1.

light wave
 The wave propagates at a wave speed (v). This wave
speed in a vacuum is equal to c, and is less than c in a
medium.
 At a stationary point along the wave, the wave passes
by in a repeating cycle. The time to complete one
cycle is called the cycle time or period

light wave
 Another important measure of a wave is its
frequency (f). It is measured as the number of
waves that pass a given point in one second. The unit
for frequency is cycles per second, also called hertz
(Hz).

light wave
• As we can see, the frequency and the period are
reciprocals of one another. If the wave speed and
wavelength are known, the frequency can be
calculated.

Polarization: What to read?
1) What is Polarization?
2) How light is polarized?
3) Application of polarized light
4) Birefringence
5) Applications of Birefringence

What is Polarization?
Light waves are travelling may or may not be parallel
to each other. If directions are randomly related to
each other the light is UNPOLARIZED/ NONPOLARIZED.
If parallel to each other is called POLARIZED.

Non polarized light
NON
POLARIZED
LIGHT

Polarized light
POLARIZED
LIGHT

Polarized light

Polarization by Use of a Polaroid Filter
The most common method of polarization involves the
use of a Polaroid filter. Polaroid filters are made of a
special material that is capable of blocking one of the
two planes of vibration of an electromagnetic wave In
this sense, a Polaroid serves as a device that filters out
one-half of the vibrations upon transmission of the light
through the filter.

When unpolarized light is transmitted through a Polaroid
filter, it emerges with one-half the intensity and with
vibrations in a single plane; it emerges as polarized light.

How light is polarized?
Polarized light is produced from ordinary light by an
encounter with a polarizing substances or agent.
Polarizing substances, e,g. calcite crystal, only
transmit light rays which are vibrating in one
particular plane. Thus only a proportion of incident
light is transmitted onward and the emerging light is
polarized.

How light is polarized?
A polarizing medium reduces radiant intensity but
does not affect spectral composition.

Application of polarized light
Polarized sunglasses to exclude selectively the
reflected horizontal polarized light. Such glasses are
of great use in reducing glare from the sea or wet
roads.
Instruments: (to reduced reflected glare from the
cornea) example: Slit lamp Ophthalmoscope

Application of polarized light
 Binocular vision polarizing glass – May be used to
dissociate the eyes i,e in Titmus test
 Also used in pleoptic to produced Haidinger’s
brushes and in optical lens making to examine lens
for stress.

Birefringence
Some substances have double refractive index though
they transmit light into 2 direction and they are called
Birefringence
A widely used birefringent material is Calcite Its
birefringence is extremely large, with indices of
refraction for the o- and e-rays of 1.6584 and 1.4864
respectively.
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Calcite Crystal

Applications of Birefringence
Birefringence finds use in the following applications:
 Polarizing prisms and retarder plates
 Liquid crystal displays
 Medical Diagnostics

Interference: What to read?
1) Some basic concepts of light wave
2) Conditions for Interference
3) Superposition: constructive interference &
destructive interference.
4) What is coherent source?
5) Types of interference

2. Interference
• When two light waves from different coherent
sources meet together, then the distribution of energy
due to one wave is disturbed by the other. This
modification in the distribution of light energy due to
superposition of two light waves is called
"Interference of light"

Conditions for Interference
 The two sources of light should emit continuous
waves of same wavelength and same time period i.e.
the source should have phase coherence.
The two sources of light should be very close to each
other. The waves emitted by two sources should
either have zero phase difference or no phase
difference.

Coherent sources
Those sources of light which emit light waves
continuously of same wavelength, and time
period, frequency and amplitude and have
zero phase difference or constant phase
difference are coherent sources.

Types of interference
 There are two types of interference.
1) Constructive interference.
2) Destructive interference

Interference
constructive interference destructive interference

Interference
Resultant of constructive
interference
Resultant of destructive
interference
constructive interference destructive interference

constructive interference
 When two light waves superpose with each other in
such away that the crest of one wave falls on the crest
of the second wave, and trough of one wave falls on
the trough of the second wave, then the resultant
wave has larger amplitude and it is called constructive
interference

destructive interference
When two light waves superpose with each other in
such away that the crest of one wave coincides
the trough of the second wave, then the amplitude
of resultant wave becomes zero and it is
called destructive interference.

Diffraction
The term diffraction, from the Latin diffringere, 'to
break into pieces', referring to light breaking up

Concept of diffraction
Diffraction is the bending of waves around obstacles,
or the spreading of waves by passing them through an
aperture, or opening.
Any type of energy that travels in a wave is capable
of diffraction, and the diffraction of sound and light
waves produces a number of effects.

Concept of diffraction
Diffraction of light waves, is much more complicated,
and has a number of applications in science and
technology, including the use of diffraction gratings in
the production of holograms.

Diffraction of light

Observing Diffraction in Light
• Wavelength of light plays a role in diffraction; so,
too, does the size of the aperture relative to the
wavelength. Hence, most studies of diffraction in
light involve very small openings, as, for instance, in
the diffraction grating.
• But light does not only diffract when passing through
an aperture, it also diffracts around obstacles.

Observing Diffraction in Light
• When light passes through an aperture, most of the
beam goes straight through without disturbance, with
only the edges experiencing diffraction. If, however,
the size of the aperture is close to that of the
wavelength, the diffraction pattern will widen. when
light is passed through extremely narrow openings, its
diffraction is more noticeable.

Diffraction Grating
• A diffraction grating is an optical device that consists of not
one but many thousands of apertures: Rowland's machine used
a fine diamond point to rule glass gratings, with about 15,000
lines per in (2.2 cm). Diffraction gratings today can have as
many as 100,000 apertures per inch.

• The apertures in a diffraction grating are not mere
holes, but extremely narrow parallel slits that
transform a beam of light into a spectrum.
• Each of these openings diffracts the light beam, but
because they are evenly spaced and the same in
width, the diffracted waves experience constructive
interference.

• This constructive interference pattern makes it
possible to view components of the spectrum
separately, thus enabling a scientist to observe
characteristics ranging from the structure of atoms
and molecules to the chemical composition of stars.

• You may also notice that the light is alternately bright
and dark as you look through the curtain. This is
from interference. The bright places are where light
waves are adding together. The dark places are where
the waves cancel. With visible light, interference
always occurs with diffraction.

DISPERSION
 The separation of visible light into its different colors
is known as dispersion.
 The optical density is simply a measure of the
tendency of a material to slow down light as it travels
through it.

Optics Prof Md Anisur Rahman

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