01 properties of light

What is optical science?
AND
What is light?
In physics the term more broadly refers to
“the study of the behavior of light and its interactions
with matter”.
In short, “light can be defined as energy to which the
human eye is sensitive”. (A.R.Elkington. P: 01)

Theory of light
There are four common theories to describe nature of
light:
a) Newton’s corpuscular theory
b) Huygen’s wave theory:
c) Maxwell’s electromagnetic theory
d) Einstein quantum theory
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Newton’s corpuscular theory
It is based on the following points
1. Light consists of very tiny particles known as
“corpuscular”.
2. These corpuscles on emission from the source of
light travel in straight line with high velocity

Newton’s corpuscular theory
• 3. When these particles enter the eyes, they produce
image of the object or sensation of vision.
4. Corpuscles of different colour have different sizes.

Huygen’s wave theory of light
In 1679, Christian Huygens proposed the wave theory
of light.
According to Huygen’s wave theory:
1. each point in a source of light sends out waves in
all directions in hypothetical medium called "ETHER
• 2. Light is a form of energy

Huygen’s wave theory of light
• 3. Light travels in the form of waves
• 4. A medium is necessary for the propagation
of waves & the whole space is filled with an
imaginary medium called Ether
• 5. Light waves have very short wave length

Picture of a light wave
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Maxwell’s electromagnetic Theory
• In 1873, Maxwell proposed that light is not a mechanical
wave but electromagnetic in character i.e it consists of
electric and magnetic fields travelling freely through
vacuum.
• This theory explains most of the phenomena related to
light satisfactorily but has partial success in explaining
scattering and fails completely to explain the
photoelectric effect of light

Einstein quantum theory of light
In 1905 , Einstein created the quantum theory of light,
the idea that light exists as tiny packets, or particles,
which he called photons. ... Later in 1905 came an
extension of special relativity which Einstein proved
that energy and matter are linked in the most famous
relationship in physics: E=mc2.
E=mc2. (The energy content of a body is equal to the mass
of the body times the speed of light squared).

Three broad subfields of optics
1) Geometrical optics, the study of light as rays
2) Physical optics, the study of light as waves
3) Quantum optics, the study of light as particles

Geometrical optics
Light is postulated to travel along rays – line
segments which are straight in free space but may
change direction, or even curve, when encountering
matter.

Geometrical optics
Two laws dictate what happens when light encounters
a material surface. The law of reflection, evidently
first stated by Euclid around 300 BC, states that when
light encounters a flat reflecting surface the angle of
incidence of a ray is equal to the angle of reflection.

Geometrical optics
• The law of refraction, experimentally determined by
Willebrord Snell in 1621, explains the manner in
which a light ray changes direction when it passes
across a planar boundary from one material to
another.

 How images can be formed?
 Their relative orientation, and their magnification.
This is in fact the most important use of geometrical
optics to this day: the behavior of complicated optical
systems can, to a first approximation, be determined
by studying the paths of all rays through the system.

2. Physical optics
Looking again at the ray picture of focusing above, we
run into a problem: at the focal point, the rays all
intersect. The density of rays at this point is therefore
infinite, which according to geometrical optics
implies an infinitely bright focal spot. Obviously, this
cannot be true.

2. Physical optics
• If we put a black screen in the plane of the focal point
and look closely at the structure of the focal spot
projected on the plane, experimentally we would see
an image as simulated below:

Picture of a 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.

 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

 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).

• 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.

Wave like model of Light
• The particle-like model of light describes large-scale effects
such as light passing through lenses or bouncing off
mirrors.
• However, a wavelike model must be used to describe fine-
scale effects such as interference and diffraction that occur
when light passes through small openings or by sharp edges.
• The propagation of light or electromagnetic energy through
space can be described in terms of a traveling wave motion.

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.

Characteristics of light waves
• To understand light waves, it is important to understand basic
wave motion itself. Water waves are sequences of crests (high
points) and troughs (low points) that “move” along the surface
of the water. When ocean waves roll in toward the beach, the
line of crests and troughs is seen as profiles parallel to the
beach. An electromagnetic wave is made of an electric field
and a magnetic field that alternately get weaker and stronger.

Characteristics of light waves
• The directions of the fields are at right angles to the direction
the wave is moving, just as the motion of the water is up and
down while a water wave moves horizontally.

2. Physical optics
• There is a very small central bright spot, but also
much fainter (augmented in this image) rings
surrounding the central spot. These rings cannot be
explained by the use of geometrical optics alone, and
result from the wave nature of light.

2. Physical optics
• Physical optics is the study of the wave properties of
light, which may be roughly grouped into three
categories:
1) Interference,
2) Diffraction, and
3) Polarization.
4) Dispersion

So what are the properties of light?
1) Reflection
2) Refraction
3) Interference
4) Polarisation
5) Diffraction
6) Dispersion

Electromagnetic spectrum: optical
radiation: colour

Electromagnetic spectrum: optical radiation: colour
Optical radiation lies between X- rays and
microwaves in the electromagnetic spectrum, and is
divisible into 7 wavebands.
Each of these 7 wavebands group together
wavelength which elicit similar biological reactions
 The 7 domains are:

The 7 domains are
i. Ultraviolet C (UV – C): 200 – 280 nanometers.
ii. Ultraviolet B (UV – B): 280 – 315 nanometers.
iii. Ultraviolet A (UV – A): 315 – 400 nanometers.
iv. Visible radiation: 400 – 780 nm
v. Infrared A (IRA): 780 – 1400 nm
vi. Infrared B (IRB) 1400 – 3000 nm
vii. Infrared C (IRC) 3000 – 10000 nm

The normal eye is able to discriminate between light
of shorter or longer wave length within the visible
spectrum by means of colour sense originating from
three different classes of cone cells into retina.
Shorter the wavelength greater the energy

Cornea & Sclera absorb essentially all the incident
optical radiation at very short wavelengths in the UV
– B & C. And long wavelength in the infrared (IR –
B, & IR – C).
LENS absorb the UV- A

The wavelength 400 – 1400 passes through the ocular
media to fall on the RETINA.
The visible wavelengths stimulate the retinal
photoreceptor & giving sensation of light. Near IR
causes thermal effect to retina.

The visible wavelengths of the electromagnetic
spectrum are between 400 nm & 780 nm,
The colour of any object is determined by the
wavelength emitted or reflected from the surface.
White colour is the mixture of wavelengths of the
visible spectrum.

Colour is perceived by three population of cone
photoreceptor in the retina which are sensitive to light
of Short (BLUE). Middle (GREEN). Long (RED)
wavelengths of the visible spectrum.

Explain: Visual field loss in glaucoma is detected
earlier if perimetry is performed using a blue light
stimulus on a yellow background.
Explain: Why there is macular burn during solar
eclipse?

• Acquired optic nerve disease tends to cause red –
green defects. An exception occurs in glaucoma and
in AD optic neuropathy which initially causes a
predominantly blue – yellow defect; it has been
recently found that visual field loss in glaucoma is
detected earlier if perimetry is performed using a blue
light stimulus on a yellow background.

• Acquired retinal disease tends to cause blue – yellow
defects (except in cone dystrophy and Stargardt’s
disease, which cause a predominantly red – green
defect,)

Different properties of light
1. INTERFERENCE

Interference: What to read?
a) Some basic definitions:
 Coherent sources.
 Conditions for coherent sources.
 Interference.
b) Conditions for Interference
c) Types of Interference
d) Practical application of interference

What is coherent sources?
Coherent sources two waves are said to be
coherent , if they emit same frequency or
wave length and are in phase or constant
phase difference.

Conditions for obtaining coherent
source:
 Coherent sources are obtained from single
source.
 The source must emit mono chromatic
light.
 The path difference between light sources
must be very small.

Sunlight (Many different colours)
LED One colour (Monochromatic) and
waves not in phase (non-coherent)
LASER: One colour (Monochromatic) and
waves in phase (coherent)

Why can’t two sources behave as
coherent sources?
Two different sources can never produce
waves of same phase because each source of
light contains infinite number of atoms and
the waves which are emitted by them will
not be in phase. The atoms after absorbing
energy go to excited states and emit
radiations when fall back to ground state.

Interference: Definition
• Interference The phenomenon of
redistribution of energy due to super
position of light waves from two coherent
sources is called interference.

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.

Types of interference
• CONSTRUCTIVE INTERFERENCE: In
constructive interference the amplitude of
the resultant wave is greater than that of
either individual wave.
• DESTRUCTIVE INTERFERENCE: In
destructive interference the amplitude of the
resultant wave is less than that of either
individual wave.

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.

Practical application of Interference
 Young’s double slit experiment.
 Expression for fringe width.
 Sustained interference
 Conditions for sustained interference

Practical application of Interference
• Young’s Double Slit Experiment Thomas
Young first demonstrated interference in
light waves from two sources in 1801. The
narrow slits S1 and S2 act as sources of
waves. The waves emerging from the slits
originate from the same wave front and
therefore are always in phase.

Resulting Interference pattern
 The light from the two slits forms a visible
pattern on a screen.
 The pattern consists of a series of bright and
dark parallel bands called fringes.
 Constructive interference occurs where a bright
fringe occurs.
 Destructive interference results in a dark fringe.

2. Polarization

Polarization
Since a light wave’s electric field vibrates in
a direction perpendicular to its
propagation motion, it is called a
transverse wave and is polarizable.
• A sound wave, by contrast, vibrates back
and forth along its propagation direction
and thus is not polarizable.

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.
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Non polarized light
NON
POLARIZED
LIGHT

Polarized light
POLARIZED
LIGHT

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.
In nature, light is polarized on reflection from a plane
surface. Such as water, if the angle of incidence is
equal to the polarizing angle for the substances. The
polarizing angle is dependent on the refractive index
of the substance.

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)
 Slit lamp
 Ophthalmoscope

 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.

3. Diffraction

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.

Hologram:
a special type of photograph or image
made with a laser in which
the objects shown look solid, as if they
are real, rather than flat. In short, it is 3
dimensional image.

Properties of light
4. Dispersion of light

Physical properties of light
4. Dispersion

Dispersion of Light?
The light rays from the sun consist of seven different
colors – red, orange, yellow, green, blue, indigo and
violet (ROYGBIV). We see seven different colors when
these rays are passed through a glass prism. The
splitting of a ray into its component colors is known as
dispersion of light. The band of colors into which the
light splits is known as a spectrum.

Photons of different wavelength travel with
different speed. The red photon has the longest
wavelength and the violet photon has the
shortest wavelength. Thus when a white light
passes through a prism, different photons cross
the medium at different speeds and deviating by
different angles.

The red color appears at the top of the spectrum because
it is bent the least or it is refracted the least.. On the
other hand, the violet end of the spectrum is bent the
most or refracted most, as it takes longer to traverse
the glass medium.

01 properties of light

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