Optics basics concepts

OPTICS: BASICS CONCEPTS

Md Anisur Rahman (Anjum)Professor & Head of the

department (Ophthalmology)Dhaka Medical College, Dhaka

What is optical science.

Optical science. Though most people associate the word

‘optics’ with the engineering of lenses for eyeglasses,

telescopes, and microscopes,

In physics the term more broadly refers to the study of

the behavior of light and its interactions with matter.

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.

1. 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.

From the laws of reflection and refraction:

One can determine the behavior of optical devices

such as telescopes and microscopes.

One can trace the paths of different rays (known as

‘ray tracing’) through the optical system

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.

• 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:

• 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.

• 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.

Interference

Interference is the ability of a wave to interfere with

itself, creating localized regions where the field is

alternately extremely bright and extremely dark.

Diffraction

Diffraction is the ability of waves to ‘bend’ around

corners and spread after passing through an aperture.

Polarization

Polarization refers to properties of light related to its

transverse nature. We will cover all these terms in

more detail in subsequent posts.

Quantum optics

We return to the picture of the focal spot illustrated

above and now imagine that the light source which

produces the focal spot is on a very precise dimmer

switch. What happens as we slowly turn the dimmer

switch down to the off position?

• Physical optics predicts that the shape of the focal

spot will remain unchanged; it will just grow less

bright. When the dimmer switch is turned below some

critical threshold, however, something different and

rather unexpected happens: we detect light in little

localized ‘squirts’ of energy, and do not see our ring

pattern at all.

If we keep a running tally of how many squirts hit at each location, we can slowly build up an average picture of where light energy is being

deposited in above figure.

May 2, 2023 anjumk38dmc@gmail.com 21

Geometric Optics

Geometric Optics deals with the formation of images by using

such optical devices as lenses, prisms and mirrors and with the

laws governing the characteristics of these images, such as their

size, shape, position and clarity.

Rays of light

Pencil of light

Beam of light

• (M.A MATIN P=19)

Reflection

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

Reflection of light

• When light meets an interface between two media, its

behavior depends on the nature of the two media

involved. Light may be absorbed by the new medium

or transmitted onward through it or it may bounch

back into first medium. This bouncing of light at an

interface is called Reflection.

(M.A MATIN = 21)

Q. What happened to the light when it strikes a surface?

Ans) 3 things may happen. It may be:

Absorbed

Reflected

Or Refracted

Defination of Reflection

Reflection is defined as the change of path of light

without any change in the medium.

All the reflections end up in producing images of the

object kept in front of the reflecting surface.

Laws of Reflection

1) The incidence ray and the reflected ray lie in the same plane which is perpendicular to the mirror surface at the point of incidence.

2) When light is reflected off any surface, the angle of incidence is always equal to the angle of reflection,

Mirror

• A mirror is optical media which reflects light

backwards when fall on it. It may be:

1) Plane mirrors or

2) Spherical mirrors.

Mirror: Rules for rays tracing through a mirror

1) The ray which pass through the pole shall pass undeviated.

2) The ray which is parallel with the axis shall pass through the focal point after convergence or divergence.

3) The ray passing through the focal point & falling on the mirror surface shall pass parallel to the optical axis.

4) The ray passing through the centre of curvature of a mirror shall also pass undeviated.

5) Path of light rays are also reversible.

Reflection at a plane surface

Spherical Mirrors

• Silvering a piece of glass which would form part of

the shell of a hollow sphere. Silvering the glass on

the outside gives a concave or converging mirror,

while silvering on the inside gives a convex or

diverging mirror.

Types of images

There are two types of images formed mirrors. They

• 1) Virtual image.

• 2) Real image.

Virtual image

1) Virtual image can not be focused on a screen.

2) It is always upright.

3) No light is really passing through the apparent

location of the image.

4) The virtual image formed by plane mirror is laterally

inverted

Real image

1) Real image can be focus on a screen.

2) It is always inverted.

3) The light passes through the location of the image.

Nomenclature

1) Light rays falling on the surface are called incident

2) Light rays travelling back are called reflected rays.

3) A line at right angle to the reflecting surface is called

normal

4) Light travelling along the normal is reflected back

along the normal

angle of incident.

angle of reflection.

Nomenclature

5) The angle formed by the incident ray and the normal

is called angle of incident.

6) The angle formed by the reflected ray and the normal

is called angle of reflection.

7) The angle of incident and the angle of reflection are

equal.

Nomenclature

8) The incident ray, the reflected ray and the normal are

in the same plane.

9) The line joining the centre of curvature to any point

on the curved mirror is the normal of that mirror.

10) The focal length of the plane mirror is infinity.

Image formation by plain mirror

If the reflecting surface of the mirror is flat then we

call this type of mirror as plane mirrors. Light always

has regular reflection on plane mirrors.

Given picture below shows how we can find the

image of a point in plane mirrors.

Characteristics of image formed by a plane mirror.

1) Image is virtual and erect.

2) It is of same size as the object.

3) It has the same distance as object to the mirror.

4) It is laterally reversed.

5) The minimum length of the mirror required to form full

size image of the object is half the size of the object.

Number of images

How many images can you form by two plane

mirror?

It depends upon the inclination of two mirrors with

each other.

• The number of images formed by two plane mirrors

inclined to each other is calculated by the formula:

Number of images

• N=360/ ᴓ - 1 (Here, N = number of images form, ᴓ is

the angle between two mirrors)

• Less the angle between two mirrors, more the number

of images.

Number of images

N = 360/90 – 1 = 4 – 1 = 3.

N = 360/60 – 1 = 6 – 1 = 5

N= 360/45 – 1 = 8 – 1 = 7.

An object placed between two parallel plane mirrors will

form infinite number of images.

This is true only for mirrors kept at right angles or less

than that.

Uses of plane mirror in ophthalmology

1) A plane mirror is used at a distance of 3 m with a

reverse Snellen’s chart kept at little higher position

than patient’s head.

2) Used in plane mirror retinoscope.

3) Used in both direct & indirect ophthalmoscope.

4) Used in slit lamp, synaptophore, stereoscope, to

change the direction of rays & save space.

Spherical mirror

Pole Center of curvature

Nomenclature in spherical mirror image

1) Pole: It is the vertex of the mirror.

2) Center of curvature: It is the center of curvature of the

sphere out of which the mirror is fashioned.

3) Radius of curvature: It is the line joining the center of

curvature to the pole.

4) Principal axis: It is the ling joining center of curvature

and the vertex.

5) Normal in a spherical mirror: It is a line that joins

any point of the mirror to the center of curvature.

6) All the measurements are valid from the pole of the

center.

7) By convention, all the incident rays are taken to

travel from the left to right.

• 8) Focal length of a concave mirror is taken as

negative and positive in convex lens

The principal axis of a spherical mirror is the line joining the pole P or centre of the mirror to the centre of curvature C which is the centre of the sphere of which the mirror forms a part.

radius of curvature r

• The radius of curvature r is the distance CP. In the

case of a concave mirror the centre of curvature is in

front of the mirror ; in a convex mirror it is behind.

Principal Focus

• When a parallel beam of light falls on a plane mirror it is

reflected as a parallel beam ; but in the case of a concave

mirror the rays in a parallel beam are all reflected so as to

converge to a point called a focus.

• If the incident rays are parallel to the principal axis the point

through which all the reflected rays pass is on the principal axis

just midway between the pole and the centre of curvature and

is called the principal focus F.

• What happens when a beam of light parallel to the

principal axis falls on a convex mirror?

• In this case the rays are reflected so that they all

appear to be coming from a principal focus midway

between the pole and centre of curvature behind the

mirror.

• A concave mirror, therefore has a real principal focus,

while the convex mirror has a virtual one.

• The focal length of a spherical mirror is half its radius

of curvature.

Construction of ray diagrams

• Since a point on an image can be located by the point of

intersection of two reflected rays, we have to consider which

are the most convenient rays to use for this purpose.

• Remembering that, by geometry, the normal to a curved

surface at any point is the radius of curvature at that point, one

very useful ray to draw will be one which is incident along a

radius of curvature. Since this is incident normally on the

mirror, it will be reflected back along its own path.

• Another useful ray is one which falls on the mirror parallel to

the principal axis. By definition, this will be reflected through

the principal focus. Conversely, any incident ray passing

through the principal focus will be reflected back parallel to

the principal axis. The same observations also apply to the

convex mirrors, so we may briefly sum them up into a set of

rules for constructing images formed by spherical mirrors.

1) Rays passing through the centre of curvature are reflected back

along their own paths.

2) Rays parallel to the principal axis are reflected through the

principal focus.

3) Rays through the principal focus are reflected parallel to the

principal axis.

4) (Useful when using squared paper) Rays incident at the pole

are reflected, making the same angle with the principal axis.

Images formed by a concave mirror

• . We wish to describe the characteristics of the image for any given object location. The L of L•O•S•T represents the relative location. The O of L•O•S•T represents the orientation (either upright or inverted). The S of L•O•S•T represents the relative size (either magnified, reduced or the same size as the object). And the T of L•O•S•T represents the type of image (either real or virtual). The best means of summarizing this relationship between object location and image characteristics is to divide the possible object locations into five general areas or points:

Images formed by a concave mirror

Case 1: the object is located beyond the center of curvature (C)

Case 2: the object is located at the center of curvature (C)

Case 3: the object is located between the center of curvature

(C) and the focal point (F)

Case 4: the object is located at the focal point (F)

Case 5: the object is located in front of the focal point (F)

Case 1: The object is located beyond C

When the object is located at a location beyond the

center of curvature, the image will always be located

somewhere in between the center of curvature and the

focal point.

• In this case, the image will be an inverted image.

reduced in size;

The object is located beyond C (contd)

• Finally, the image is a real image. Light rays actually

converge at the image location. If a sheet of paper

were placed at the image location, the actual replica

of the object would appear projected upon the sheet

of paper.

Case 1: The object is located beyond C

Case 2: The object is located at C

When the object is located at the center of curvature,

the image will also be located at the center of

curvature.

In this case, the image will be inverted. The image

dimensions are equal to the object dimensions.

Finally, the image is a real image.

Case 2: The object is located at CLight rays actually converge at the image

location. As such, the image of the object could be projected upon a sheet of paper.

Case 3: The object is located between C and F

When the object is located in front of the center of

curvature, the image will be located beyond the

center of curvature.

In this case, the image will be inverted.

The image dimensions are larger than the object

dimensions.

Case 3: The object is located between C and FFinally, the image is a real image. Light rays

actually converge at the image location. As such, the image of the object could be projected upon a sheet

of paper.

Case 4: The object is located at F

• When the object is located at the focal point, no

image is formed. Light rays from the same point on

the object will reflect off the mirror and neither

converge nor diverge. After reflecting, the light rays

are traveling parallel to each other and do not result in

the formation of an image.

Case 4: The object is located at F

Case 5: The object is located in front of F

When the object is located at a location beyond the focal point,

the image will always be located somewhere on the opposite

side of the mirror. Regardless of exactly where in front of F

the object is located, the image will always be located behind

the mirror.

In this case, the image will be an upright image, magnified and

virtual

• This type of image is formed by a shaving or make-up mirror

and also by small concave mirror used by dentists for

examining teeth.

Light rays from the same point on the object reflect off the mirror

and diverge upon reflection. For this reason, the image

location can only be found by extending the reflected rays

backwards beyond the mirror. The point of their intersection is

the virtual image location. It would appear to any observer as

though light from the object were diverging from this location.

Any attempt to project such an image upon a sheet of paper

would fail since light does not actually pass through the image

location.

• It might be noted from the above descriptions that there is a

relationship between the object distance and object size and

the image distance and image size. Starting from a large value,

as the object distance decreases (i.e., the object is moved

closer to the mirror), the image distance increases; meanwhile,

the image height increases.

• At the center of curvature, the object distance equals the

image distance and the object height equals the image height.

• As the object distance approaches one focal length, the image

distance and image height approaches infinity.

• Finally, when the object distance is equal to exactly one focal

length, there is no image.

• Then altering the object distance to values less than one focal

length produces images that are upright, virtual and located on

the opposite side of the mirror.

• Finally, if the object distance approaches 0, the image distance

approaches 0 and the image height ultimately becomes equal

to the object height.

• Nine different object locations are drawn and labeled with a

number; the corresponding image locations are drawn in blue

and labeled with the identical number.

IMAGE FORM BY CONVEX MIRROR

The diagrams above show that in each case, the image is

located behind the convex mirrora virtual imagean upright imagereduced in size (i.e., smaller than the object)

Unlike concave mirrors, convex mirrors always produce images that share these characteristics. The location of the object does not affect the characteristics of the image. As such, the characteristics of the images formed by convex mirrors are easily predictable.

• Another characteristic of the images of objects formed by convex mirrors pertains to how a variation in object distance affects the image distance and size. The diagram below shows seven different object locations (drawn and labeled in red) and their corresponding image locations (drawn and labeled in blue).

• The diagram shows that as the object distance is decreased, the image distance is decreased and the image size is increased. So as an object approaches the mirror, its virtual image on the opposite side of the mirror approaches the mirror as well; and at the same time, the image is becoming larger.

Image formed by concave mirror

Position of the object

Position of the image

Nature of the image

Inverted/ Erect

Between focus & pole

Behind the mirror

Virtual Erect Magnified

At focus Infinity Real Inverted Highly Magnified

Between focus & curvature

Beyond center of curvature

Real Inverted Little Magnified

Image formed by concave mirror

Position of the object

Position of the image

Nature of the image

Inverted/ Erect

Center of curvature Same place Real Inverted

Same size

Beyond the center of curvature

Between focus & center of curvature

Real Inverted

Diminished

At infinity Real Inverted

Very small

Image formed by convex mirror

The image of an object kept in front of the mirror is

formed behind the mirror.

It is smaller than the object , erect and virtual.

The distance between the image and the mirror is less

than between the object and the mirror.

Behavior of images in relation to position of the object

The image formed by CONVEX and PLANE mirrors are virtual

The image formed by CONCAVE mirrors can be real or virtual

The distance between mirror and the image is least in CONVEX mirror, most in CONCAVE mirror and

equal in PLANE mirror

specular reflection & diffuse reflection

Reflection of smooth surfaces such as mirrors or a

calm body of water leads to a type of reflection

known as specular reflection.

Reflection of rough surfaces such as clothing, paper,

and the asphalt roadway leads to a type of reflection

known as diffuse reflection.

• Whether the surface is microscopically rough or

smooth has a tremendous impact upon the subsequent

reflection of a beam of light.

specular reflection & diffuse reflection

The diagram depicts two beams of light incident upon a rough and a smooth surface.

Applications of Specular and Diffuse Reflection

There are several interesting applications of this

distinction between specular and diffuse reflection.

One application pertains to the relative difficulty of

night driving on a wet asphalt roadway compared to a

dry asphalt roadway. Most drivers are aware of the

fact that driving at night on a wet roadway results in

an annoying glare from oncoming headlights.

The glare is the result of the specular reflection of the

beam of light from an oncoming car. Normally a

roadway would cause diffuse reflection due to its

rough surface. But if the surface is wet, water can fill

in the crevices and smooth out the surface.

• Rays of light from the beam of an oncoming car hit

this smooth surface, undergo specular reflection and

remain concentrated in a beam. The driver perceives

an annoying glare caused by this concentrated beam

of reflected light.

A second application of the distinction between

diffuse and specular reflection pertains to the field of

photography. Many people have witnessed in person

or have seen a photograph of a beautiful nature scene

captured by a photographer who set up the shot with a

calm body of water in the foreground.

The water (if calm) provides for the specular

reflection of light from the subject of the photograph.

Light from the subject can reach the camera lens

directly or it can take a longer path in which it

reflects off the water before traveling to the lens.

• Since the light reflecting off the water undergoes

specular reflection, the incident rays remain

concentrated (instead of diffusing).

The light is thus able to travel together to the lens of the camera and produce an image (an exact replica) of the subject which is strong enough to perceive in the photograph. An example of such a photograph is shown.

Question

If a bundle of parallel incident rays undergoing

diffuse reflection follow the law of reflection, then

why do they scatter in many different directions after

reflecting off a surface?

Answer

Each individual ray strikes a surface which has a

different orientation. Since the normal is different for

each ray of light, the direction of the reflected ray

will also be different.

Question

Perhaps you have observed magazines which have

glossy pages. The usual microscopically rough

surface of paper has been filled in with a glossy

substance to give the pages of the magazine a smooth

surface. Do you suppose that it would be easier to

read from rough pages or glossy pages? Explain your

answer.

It is much easier to read from rough pages which provide

for diffuse reflection. Glossy pages result in specular

reflection and cause a glare. The reader typically sees an

image of the light bulb which illuminates the page. If

you think about, most magazines which use glossy pages

are usually the type which people spend more time

viewing pictures than they do reading articles.

Refraction

Luminous versus Illuminated Objects

The objects that we see can be placed into one of two

categories: luminous objects and illuminated objects.

Luminous objects are objects that generate their own

Illuminated objects are objects that are capable of

reflecting light to our eyes.

The sun is an example of a luminous object, while the

moon is an illuminated object.

Refraction

Q) What happened to the light when it strikes a surface?

Ans) 3 things may happen. It may be:

Absorbed

Reflected

Or Refracted

Refraction

Q) What is refraction?

Ans) Refraction of light is a phenomenon of change in

the path of light when it passes from one medium to

another due to change in velocity.

Terms used in refraction

1) NORMAL: This is a line right angles to the interface

2) INCIDENCE RAY: The ray that strikes the interface

at the base of the normal in an angular fashion.

3) REFRACTED RAY: This is the deviated ray in the

second medium.

4) ANGLE OF INCIDENCE: Angle between the

normal and the incident ray

5) ANGLE OF REFRACTION: The angle between the

refracted ray & the normal is called ANGLE OF

REFRACTION

6) The two angles are never equal.

Snell’s Law

Total Internal Reflection (TIR)

Critical Angle

Critical angle is the angle of incidence above which total internal reflection occurs.

It is defined as the angle when the incidence ray is of such an angle that the refracted ray is at right angles to the normal

Critical Angle

• Critical angle of glass is 48.60, diamond is 240 (refractive

index is 2.42) and water is 48.750. An incident ray when

passing through a slab of glass with air on either side will exit

the slab as refracted ray and will be parallel to incident ray.

• The complete reflection of a light ray reaching an

interface with a less dense medium when the angle of

incidence exceeds the critical angle.

Different uses of TIR

1) Gonioscopy employs total internal reflection to view

the anatomical angle formed between the

eye's cornea and iris.

2) Total internal reflection is the operating principle

of optical fibers, which are used in endoscopes and

telecommunications.

Different uses of TIR

3) Total internal reflection is the operating principle of

automotive rain sensors, which control automatic

windscreen/windshield wipers

Lenses

A lens is defined as a portion of a refracting medium

bordered by two curved surfaces which have a

common axis.

When each surface forms part of a sphere the lens is

called a spherical lens.

Sometimes, a spherical lens has a one plane surface, it

is acceptable because a plane surface can be thought

of as part of a sphere of infinite radius.

Spherical Lens

Lens may be spherical (when each surface forms part

of sphere, the lens is called a Spherical lens) where

the concavity or convexity two different meridians

are equal.

Cylindrical Lens

It may be cylindrical where there is unequal

concavity in two meridians. The two meridians

usually remains at right angels to each other and the

less curved meridian being designed as axis of the

Lenses: (A–F), Spherical lenses: (A), biconvex; (B), biconcave; (C), planoconvex; (D), planoconcave; (E), concavoconvex, periscopic convex, converging meniscus; (F), convexoconcave, periscopic concave, diverging meniscus; (G, H), cylindrical lenses, concave and convex.

Spherical Aberration

The prismatic effect of the peripheral parts of the

spherical lens causes spherical aberration.

It was seen that the prismatic effect of a spherical lens

is least in the paraxial zone and increases towards the

periphery of the lens.

Spherical Aberration

Thus, rays passing through the periphery of the lens

are deviated more than those passing through the

paraxial zone of the lens.

Correction of Spherical Aberration

Spherical aberration may be reduced by occluding the

periphery of the lens by the use of “stops” so that

only the paraxial zone is used.

Lens form may also be adjusted to reduced spherical

aberration, e,g plano-convex is better than biconvex.

To achieve the best results, spherical surface must be

abandoned and the lenses ground with aplantic surface,

that the peripheral curvature is less than the central

curvature.

Another technique of reducing spherical aberration is

to employ a doublet. This consists of a principal lens

and a somewhat weaker lens of different R.I

cemented together.

The weaker lens must be of opposite power, and

because it too has spherical aberration, it will reduce

the power of the periphery of the principal lens more

than the central zone. Usually, such doublets are

designed to be both aspheric and achromatic.

• A convex lens is thicker at the centre than at the edges.

Image form by lens

• Unlike the mirrors, lenses have got two principal foci

one on each side of the lens and the nodal point is

situated within the substance of the lens just at the

centre. If the image is situated on the other side of the

object, it is called a Real Image and if it is on the

same side it is called a Virtual Image.

The point at which the principal plane and principal axis intersect is

called the principal point or nodal point. Rays of light passing through

the nodal point are undeviated.

Light parallel to the principal axis is converged or diverged from the

point F, the principal focus.

Image form by lens

• For, an object in any position, the image can be

constructed using two rays:

1) A ray from the top of the object which passes through

the principal point/nodal point.

2) A ray parallel to the principal axis, which after

refraction passes through (convex) or away from

(concave) the second principal focus.

• Convex lenses are thicker at the middle. Rays of light that pass

through the lens are brought closer together (they converge). A

convex lens is a converging lens.

• When parallel rays of light pass through a convex lens the

refracted rays converge at one point called the principal focus.

• The distance between the principal focus and the centre of the

lens is called the focal length.

Use of Convex Lenses

Use of Convex Lenses – The Camera

A camera consists of three main parts.

I. The body which is light tight and contains all the mechanical

parts.

II. The lens which is a convex (converging) lens.

III. The film or a charged couple device in the case of a digital

camera.

• The rays of light from the person are converged by the convex lens forming an image on the film or charged couple device in the case of a digital camera.

• The angle at which the light enters the lens depends on the distance of the object from the lens. If the object is close to the lens the light rays enter at a sharper angled. This results in the rays converging away from the lens. As the lens can only bend the light to a certain degree the image needs to be focussed in order to form on the film. This is achieved by moving the lens away from the film.

• Similarly, if the object is away from the lens the rays enter at a wider angle. This results in the rays being refracted at a sharper angle and the image forming closer to the lens. In this case the lens needs to be positioned closer to the film to get a focused image.

• Thus the real image of a closer object forms further away from the lens than the real image of a distant object and the action of focusing is the moving of the lens to get the real image to fall on the film.

• The image formed is said to be real because the rays of lighted from the object pass through the film and inverted (upside down).

The Magnifying Glass

A magnifying glass is a convex lens which produces a magnified

(larger) image of an object.

• A magnifying glass produces an upright, magnified virtual

image. The virtual image produced is on the same side of the

lens as the object. For a magnified image to be observed the

distance between the object and the lens must be shorter than

the focal length of the lens.

For a magnified image to be observed the distance

between the object and the lens has to be shorter than

the focal length of the lens. The image formed is

upright, magnified and virtual.

Magnification :The magnification of a lens can be

calculated using the following formula;

Aspheric lens

• An aspheric lens or asphere is a Lens whose surface

profiles are not portions of a sphere or cylinder.

• The asphere's more complex surface profile can

reduce or eliminate spherical aberration and also

reduce other optical aberration compared to a simple

PHYSICAL PROPERTIES OF LIGHT

1) Polarization 2) Interference

3) Diffraction 4) Superimposition

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.

Non polarized light

NON POLARIZED

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) 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.

Calcite Crystal

Applications of Birefringence

Birefringence finds use in the following applications:

Polarizing prisms and retarder plates

Liquid crystal displays

Medical Diagnostics

2. Interference

Before discussing interference we should have clear

idea about wave properties of light.

05/02/2023 anjumk38dmc@gmail.com 161

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

• 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. 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

super- position 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.

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