Light is a type of energy which propagates as electromagnetic waves. In the spectrum of electromagnetic waves, light has place between ultraviolet and infrared region.

Some Facts About Light

•    Electromagnetic waves are transverse waves, therefore, light is also transverse wave.
•    Wave nature of light explains; rectilinear propagation of light, reflection of light, refraction of light, interference of light, diffraction of light and polarization of light.
•    The happenings in physics like Photoelectric Effect and Compton Effect can not be explained on the basis of wave nature of light. These phenomenons are explained on the basis of quantum theory of light, explained by Albert Einstein.
•    The quantum theory of light, considers light as a packet or bundle of energy, these energy packets are called photons. Photon associates with it as Energy E; where E = hv
•    Light has dual nature and behaves as a particle as well as wave.
•    Speed of Light was first calculated by Roemer in 1678 AD.
•    Speed of light is maximum in vacuum, which is equivalent to 3x108 m/s.

Refractive Index

•    Refractive Index of a medium is defined as the ratio of speed of light in vacuum to the speed of light in the medium.

•    ᘈ=c/v= speed of light in vacuum/speed of light in medium

•    Speed of light is different in media. Velocity of light is greater in a medium which has smaller refractive index.

Speed of Light in Different Mediums

Speed of light in m/s
Rock Salt
Terpentine Oil

•    Light takes 8 minute 19 second to reach from sun to earth.
•    The light refracted from moon takes 1.28 seconds to reach to earth.

Luminous Bodies- The objects which emit light of their own are called luminous bodies. Examples of luminous bodies are – sun, stars, electric bulb etc.

Non Luminous Bodies- Those objects which do not emit light by themselves but visible due to light incident on them are called non luminous bodies.

On the basis of optical property materials can be classified into following types-

Transparent- The objects which allow most of the incident light passes through them are called transparent material. For example: Glass, Water etc.

Translucent- The materials which allow incident light partially pass through them are called Translucent. For example: oiled paper.

Opaque- The materials which do not allow incident light to pass through them are called Opaque. For example: mirror, metal, wood etc.

Reflection of Light- Light traveling in one medium, when falls at the surface of other medium, some light returns back to the same medium. This behavior of light of returning back to the same medium is called Reflection of Light.

Laws of Reflection
If the reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows:

•    The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.
•    The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal.
•    The reflected ray and the incident ray are on the opposite sides of the normal.

Reflection from a Plane Mirror

•    The image is virtual, laterally inverted.
•    The size of image is equal to the size of the object.
•    The distance of image from the mirror is equal to the distance of object from the mirror.
•    If an object moves towards or away from a plane mirror with speed v, relative to the object the image moves towards or away in either case with speed 2v.
•    If a plane mirror is rotated by an angle Θ, keeping the incident ray fixed, the reflected ray is rotated by an angle 2Θ.
•    To see full image of oneself in a plane mirror, a person needs a mirror of atleast half of the height of the self.
•    If two plane mirrors are inclined to each other at an angle Θ, the number of image n of a point object formed are determined as follows-
•    If 360/Θ is an even integer then n = 360/Θ -1
•    If 360/Θ is odd integer then n = 360/Θ -1 for the object is symmetrically placed n = 360 / Θ for the object is not symmetrically placed.
•    If 360/Θ is a fraction then n is equal to the integral part.

Reflection from the Spherical Mirror

Spherical mirror are of two types- 1. Concave Mirror and 2. Convex Mirror

Position and Nature of Image Formed By Spherical Mirror

Concave Mirror

Position of Object
Position of Image
Size of Image in Comparison to Object
Nature of Image
At Infinity
At Focus
Highly Diminished
Real, Inverted
Between Infinity and Centre of Curvature
Between Focus and Centre of Curvature
Real, Inverted
At Centre of Curvature
At Centre of Curvature
Of same size
Real, Inverted
Between Focus and Centre of Curvature
Between Centre of Curvature and Infinity
Real, Inverted
At Focus
At Infinity
Highly Enlarged
Real, Inverted
Between Focus and Pole
Behind the Mirror
Virtual, Erect

Convex Mirror

At Infinity
At focus
Highly Diminished
Virtual, Erect
In front of mirror
Between pole and focus
Virtual, Erect

Note- Image formed by Convex Mirror is always virtual, erect and diminished.

Uses of Concave Mirror
•    Concave mirrors are often used as shaving mirrors and makeup mirrors. Objects held close are reflected in a concave mirror as a magnified image. When the mirror is held close to the face, an enlarged image of the skin can be seen. For shaving purposes, this allows you to see if any hair has been missed and to make sure that all hairs have been cut to the same length. For makeup purposes, it allows you to make sure all the skin on the face is covered and blended evenly.

•    Concave mirrors are used in motor vehicle headlights to send out strong beams of light. Instead of reflecting an image, they are used to focus the light from the bulb. Rays of light from the bulb are reflected off the concave mirror, creating a strong beam that shines on the road.

•    Concave mirrors are used in microscopes to collect light from a lamp, shining it up onto a slide containing a specimen so it can be viewed through a magnification lens. It is important to never point the mirror toward the sun to collect light; the sunlight would be focused and could blind the person looking through the lens of the microscope.

•    Large telescopes traditionally have a concave mirror at one end. Similar to how a concave mirror works in a microscope, the concave mirror in the telescope collects light. Instead of shining the light up to a specimen, it shines the light from distant stars onto a flat mirror. The viewer looks through the lens on the eyepiece of the telescope and sees the reflection on the mirror, allowing a view of stars that the naked eye is unable to see.

Uses of Convex Mirror

•    Large hospitals, stores and office buildings often use convex mirrors to allow people to see what is around a corner to help keep people from running into one another.

•    Convex mirrors are used to make sunglass lenses. These mirrors help reflect some of the sunlight away from the wearer's eyes.

•    Convex mirrors are often found on the passenger sides of motor vehicles. These mirrors make objects appear smaller than they really are. Due to this compression, these mirrors to reflect a wider image area, or field of vision.

•    Convex mirrors are often placed near ATMs to allow bank customers to see if someone is behind them. This is a security measure that helps keep ATM users safe from robbery of any cash withdrawals and helps keep ATM users' identity more secure.

•    Two convex mirrors placed back to back are used to make a magnifying glass.

Reflection of light

Refraction is the bending of a wave when it enters a medium where it's speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell's Law.

As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes.

Refraction is responsible for image formation by lenses and the eye.

Snell's law    

Snell's law gives the relationship between angles of incidence and refraction for a wave impinging on an interface between two media with different indices of refraction. The law follows from the boundary condition that a wave be continuous across a boundary, which requires that the phase of the wave be constant on any given plane, resulting in
n1 Sinϴ 1 = n2 Sinϴ 2

where ϴ1 and ϴ2 are the angles from the normal of the incident and refracted waves, respectively.

The absolute refractive index of a medium is defined as the ratio of speed of light the ratio of speed of light in free space i.e. vacuum to that in the given medium.

The refractive index of a medium is different for different colors. The refractive index of a medium decreases with the increase in wavelength of light. Therefore refractive index of a medium is maximum for violet color of light and minimum for red color of light.

When a ray of light enters from one medium to other medium, its frequency and phase do not change but wavelength and velocity change.

Some of the examples of refraction

•    Bending of a linear object when it is partially dipped in a liquid inclined to the surface of the liquid.
•    Twinkling of stars
•    Oval shape of sun during dawn and dusk.
•    An object in a denser medium when viewed from a rarer medium appears to be at a shorter distance.
•    A fish in pond when viewed from air appears to be at smaller depth
•    A coin lying at the base of a water tank appears raised.

Critical Angle- In case of propagation of light from denser to rarer medium through a plane boundary, critical angle is angle of incidence for which angle of refraction is 900 .

Total Internal Reflection

When light is incident upon a medium of lesser index of refraction, the ray is bent away from the normal, so the exit angle is greater than the incident angle. Such reflection is commonly called "internal reflection". The exit angle will then approach 90° for some critical incident angle θc , and for incident angles greater than the critical angle there will be total internal reflection.

Conditions for Total Internal Reflection

(a) The ray must travel from denser medium to rarer medium.

(b) The angle of incidence i must be greater than critical angle C

Examples of Total Internal Reflection

•    Sparkling of diamond
•    Mirage and looming
•    Shining of air bubble in water
•    Increase in duration of Sun’s visibility- when appears before sunrise and / or appears till after sunset.
•    Shining of a smoked ball or metal ball on which lamp soot is deposited when dipped in water.
•    Optical Fibre- Optical Fibre consists of thousands of strands of very fine quality glass or quartz of refractive index 1.7, each strand coated with a layer of material of lower refractive index 1.5. In this light is propagated along the axis of fibre through multiple total internal reflection, even though the fibre is curved, without loss of energy.

Applications of Optical Fibre

Communication - Telephone transmission method uses fibre-optic cables. Optical fibres transmit energy in the form of light pulses. The technology is similar to that of the coaxial cable, except that the optical fibres can handle tens of thousands of conversations simultaneously.

Medical uses - Optical fibres are well suited for medical use. They can be made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibres are used in a number of instruments that enable doctors to view internal body parts without having to perform surgery.

Simple uses - The simplest application of optical fibres is the transmission of light to locations otherwise hard to reach. Also, bundles of several thousand very thin fibres assembled precisely side by side and optically polished at their ends, can be used to transmit images.

Refraction of Light Through Lens

Refraction - When a ray of light passes from one transparent medium to another, it may change direction as it does so, depending on its speed in each medium and its angle to the surface. This phenomenon is called refraction.

Lens - Lens is a cross section of transparent refractive material of two surfaces of definite geometrical shape of which one surface must be spherical.

Types of Lenses – 1. Concave Lens and 2. Convex Lens

Convex Lens – When a lens is thicker at the middle than at the edges, it is called a convex lens or converging lens.

Concave Lens – When a lens is thicker at the edges than at the middle, it is called a concave lens or diverging lens.

Terms Used In The Study of Lens

Principal axis: A line which passes through the center of the lens, perpendicular to the lens surface. (Lines X-Y in the diagrams on the left illustrate the principal axes of the lenses.)

Optical centre: This is a point on the principal axis of a lens through which light passes without undergoing any deviation. In other words, a ray of light passing through the optical center will not change its direction. For thin lenses whose faces have the same curvature, this point, marked O in the diagram, is in the center of the lens.

Principal focus or focal point: This is a point, marked F in the diagram, to which all rays parallel to the principal axis converge (in the case of a convex lens), or (in the case of a concave lens) from which the rays appear to diverge.

Focal length: This is the distance between the optical centre and the principal focus. In the diagram, it is the distance OF.

Principal focal plane: An imaginary plane located at the principal focus, perpendicular to the principal axis.

Power of a Lens – The Power of a lens is its ability to deviate a ray from its original path. The power of lens is measured as the reciprocal of focal length in meters. Mathematically, P = 1/f; where f = focal length, P = Power

The SI Unit of Power of Lens is Dioptre, symbol ‘D’.
Power of concave lens is negative and convex lens is positive.

If two lenses are placed in contact, then the power of combination is equal to the sum of powers of individual lenses.

Change of Power of a Lens- If a lens is dipped in a liquid, its focal length and power both change. This change depends upon the refractive indices of lens and the liquid. If a lens of refractive index ᘈ is dipped in a liquid of refractive index ᘈ’ then the following three conditions are possible –

1.    ᘈ > ᘈ’ i.e. lens is dipped in a liquid of smaller refractive index like a lens of glass, refractive index, ᘈ = 1.5 is dipped in water of refractive index ᘈ’ = 1.33, then the focal length of lens increases and the power of lens decreases.
2.    ᘈ = ᘈ’ i.e. lens is dipped in a liquid of equal refractive index then the focal length of the lens becomes infinite i.e. its power becomes zero. The lens and the liquid behaves as a single medium.
3.    ᘈ<ᘈ’ i.e. lens is dipped in a liquid of higher refractive index the focal length increases i.e. power decreases as well as the nature of the lens also changes, i.e convex lens behaves as concave lens and vice versa. For example, an air bubble trapped in water or glass appears as convex but behaves as concave lens. Similarly a convex lens of glass, ᘈ = 1.5 when dipped in carbon disulphide ᘈ’ = 1.68, it behaves as a concave lens.

Formation of image by lenses

Convex Lens

Position of Object
Position of Image
Size of Image
Nature of Image
At Infinity
At Focus
Highly Diminished
Real, Inverted
Beyond 2F
Between F and 2F
Real, Inverted
At 2F
At 2F
Of same size
Real, Inverted
Between F and 2F
Beyond 2F
Real, Inverted
At F
At Infinity
Highly Enlarged
Virtual, Erect

Concave Lens

Position of Object
Position of Image
Size of Image
Nature of Image
At Infinity
At Focus
Highly Diminished
Virtual and Erect
Between Lens and Infinity
Between Lens and F on the same side
Virtual and Erect

Dispersion of Light

Newton had shown that light rays that we obtain from the sun consist of seven different colours – red, orange, yellow, green, blue, indigo and violet. When rays of the sun are made to pass through a glass prism, we will see the seven different colours.

The splitting of a ray into its component colours is known as dispersion of light and the band of colours is known as a spectrum. A spectrum of yellow light from a sodium lamp will remain yellow itself. This means that light rays from a sodium lamp do not show dispersion.

The rainbow :  A rainbow is seen when the sun appears in the sky after the rains. The moisture in the atmosphere behaves like tiny prisms, dispersing the sun’s rays into seven colours (VIBGYOR). The red colour appears on the top of the rainbow and the violet colour appears at the bottom.

Colours of objects :  Visually we see colours because of the reflection of that particular colour only. For example a leaf will look green in the sun, because it absorbs all the other six colours from the sun’s rays and will not absorb the green photons of the spectrum. These green photons reach our eyes and give us a sensation that the leaf is green in colour. If you observe a leaf under a sodium lamp, which gives off yellow light, the leaf will appear black in colour. Similaraly, if an object looks black, the entire spectrum of light is absorbed by the black object, hence it looks black.When an object seems white in colour, it means that it is reflecting all the light back and is not absorbing any photons of the seven colours.  Any colour that we see other than the seven colours are actually a combinations of two or more of the seven colours of the spectrum.

It is because of this reason that we wear dark colours in winter. The dark colours absorb all the light falling on it and hence keep the wearer warm.  In summer season, when we do not want to wear clothes that absorb sun light, we tend to wear white or light coloured clothes.

Primary Colours - Red, yellow, and blue are primary colors. They are the three pigment colors that cannot be made by mixing any other colors. These three colors are mixed to create all other colors and can be combined with white or black to create tints (lighter tones) and shades (darker hues) of these colors.

Secondary Colours -  Orange, green, and purple are secondary colors. They are created by mixing two of the three primary colors together.

Monochromatic- A monochromatic color scheme uses a single color on most every room surface. In this type of scheme, various darker shades, grayer tones, and paler tints of the main color may be included in the palette. In addition, the one color is often paired with white or another neutral.

For example, a monochromatic room in blue might use single shade of blue paired with white. Yet it might also include dark blue upholstery fabric, pale blue walls, medium blue draperies, and a patterned area rug that includes both blue anjd white. The window and door trim as well as the ceiling might be painted in white.

Tint - A tint is a mixture of pure hue and white. Think of a color like red saturated with lots of white. As more white is added the color becomes a lighter and lighter tint of red, until it turns to pale pink.

Colour of Bodies-  The colour of a body is the colour of light which it reflects or transmits. An object is white if it reflects all the components of white light and it is black if it absorbs all the light falling on it. This is why red rose appears red when viewed in white or red light but appears black when viewed in blue or green light.

How a body will appear in light of different colours is given in the table below-

Name of Object
In White Light
In Red Light
In Green Light
In Yellow Light
In Blue Light
White Paper
Red Paper
Green Paper
Yellow Paper
Blue Paper

Scattering of Light- In the air, part of the sunlight is scattered. The small particles (molecules, tiny water droplets and dust particles) scatter photons the more, the shorter their wavelength is. Therefore, in the scattered light, the short wavelengths predominate, the sky appears blue, while direct sunlight is somewhat yellowish, or even reddish when the sun is very low. Goethe believed this to be the basic phenomenon to generate colours.

The fraction of the light which is deviated by scattering increases with increasing path length, so that at sunset the shorter wavelengths are depleted in direct sunlight and the sun appears orange or red, depending on the amount of haze or dust in the air.

Interference of Light - Interference is the interaction between waves traveling in the same medium. When two waves come into contact, depending on the phase differences along the waves, constructive and destructive interferences will occur.

Constructive Interference - In constructive interference, the amplitude of the wave is amplified. This happens when the two waves are in phase -- if the crests and troughs of the waves coincide with each other. Consider two waves, one with a crest of +1 units and coinciding with a wave of +2 units in amplitude at that point, traveling in opposite directions on the same medium. When these troughs come into contact, the resulting amplitude will be the sum of the two waves, which is +3 in this case. Once the waves pass each other, however, they will resume their original course with their original amplitude -- as if they have not been disturbed at all.

Destructive Interference - Destructive interference is very much like constructive interference except that the two waves cancels out each other. This happens when the waves are out of phase -- when the crests of one wave coincide with the troughs of the other. Consider two waves, one with a crest of +1 coinciding with a wave of -2 units in amplitude at that point, traveling in opposite directions on the same medium. At the point of contact, the resulting amplitude will be the difference of the two waves, which is -1 in this case. Just like constructive interference, once the waves pass each other, they will resume their original course with their original amplitude -- as if they have not been disturbed at all.

Diffraction of Light – The bending and spreading of light waves around sharp edges or corner or through small openings is called   Diffraction of Light.

Conditions For Diffraction- Diffraction effect depends upon the size of obstacle. Diffraction of light takes place if the size of obstacle is comparable to the wavelength of light.

Light waves are very small in wavelength, i.e. from 4 x 10-7 m to 7 x 10-7 m. If the size of opening or    obstacle is near to this limit, only then we can observe the phenomenon of diffraction.

Types Of Diffraction- Diffraction of light can be divided into two classes:

•    Fraunhoffer diffraction.
•    Fresnel diffraction.

Fraunhoffer Diffraction- In Fraunhoffer diffraction

•    Source and the screen are far away from each other.
•    Incident wave fronts on the diffracting obstacle are plane.
•    Diffracting obstacle give rise to wave fronts which are also plane.
•    Plane diffracting wave fronts are converged by means of a convex lens to produce diffraction pattern.

Fresnel Diffraction- In Fresnel diffraction,
•    Source and screen are not far away from each other.
•    Incident wave fronts are spherical.
•    Wave fronts leaving the obstacles are also spherical.
•    Convex lens is not needed to converge the spherical wave fronts.

Diffraction Grating- A diffraction grating is an optical device consists of a glass or polished metal surface over which    thousands of fine, equidistant, closely spaced parallel lines are been ruled.

Polarisation of Light-

Light waves can vibrate in many directions. Those that are vibrating in one direction -- in a single plane such as up and down -- are called polarized light. Those that are vibrating in more than one direction -- in more than one plane such as both up/down and left/right -- are called unpolarized light.

Generally, unpolarized light can be considered to be vibrating in a vertical and a horizontal plane. To polarize light, one can transmit the light through a polariod filter which will only allow light of single polarity to pass. The resulting light will be polarized light of half intensity. If two polaroid filters are used and placed so that one is rotated 90 degrees to the other, no light will be able to pass.

Some polarization will also occur during reflection, refraction, and scattering of light. When reflecting off non-metallic surfaces, the resulting light will be polarized parallel to the reflected surface. During refraction, a beam of light will be split up into two polarized beams, one polarized parallel and one perpendicular to the boundary. Scattering also causes partial polarization.

Human Eye

•    Least distance of distinct vision is 25 cm

Defects of Human Eye and The Corrections
Myopia - Nearsightedness, or myopia, is the most common refractive error of the eye, and it has become more prevalent in recent years.

If you're nearsighted, the first number ("sphere") on your eyeglasses prescription or contact lens prescription will be preceded by a minus sign (–). The higher the number, the more nearsighted you are.

Nearsightedness can be corrected with glasses, contact lenses or refractive surgery. Depending on the degree of your myopia, you may need to wear your glasses or contact lenses all the time or only when you need very clear distance vision, like when driving, seeing a chalkboard or watching a movie.

Hyperopia - Hyperopia (farsightedness), is a refractive error, which means that the eye does not bend or refract light properly to a single focus to see images clearly. In hyperopia, distant objects look somewhat clear, but close objects appear more blurred.

Eyeglasses or contact lenses are the most common methods of correcting hyperopia symptoms. They work by refocusing light rays on the retina, compensating for the shape of your eye. They can also help protect your eyes from harmful ultraviolet (UV) light rays. A special lens coating that screens out UV light is available.

Presbyopia - Presbyopia is a condition in which the lens of the eye loses its ability to focus, making it difficult to see objects up close.

Presbyopia can be corrected with glasses or contact lenses. In some cases, adding bifocals to an existing lens prescription is enough. As the ability to focus up close worsens, the bifocal prescription needs to be strengthened.

Around age 65, the eyes have usually lost most of the elasticity needed to focus up close. However, it will still be possible to read with the help of the right prescription. Even so, you may find that you need to hold reading materials farther away, and you may need larger print and more light by which to read.

People who do not need glasses for distance vision may only need half glasses or reading glasses.

People who are nearsighted may be able to take off their distance glasses to read.

Astigmatism-Astigmatism is an eye condition with blurred vision as its main symptom. The front surface of the eye (cornea) of a person with astigmatism is not curved properly - the curve is irregular - usually one half is flatter than the other - sometimes one area is steeper than it should be.

When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.

The two most common types of astigmatism are:
•    Corneal astigmatism - the cornea has an irregular shape
•    Lenticular astigmatism - the lens has an irregular shape

Corrective lenses bend the income light rays in a way that compensates for the error caused by faulty refraction so that images are properly received onto the retina. Whether the corrective lenses are in glasses or contact lenses is up to the patient - they are equally effective.

Simple Microscope

A simple microscope is a microscope which only has one lens, as opposed to the compound lenses used in more complex microscope designs. Magnifying glasses and loupes are two well-known examples of the simple microscope. This design is classically used for basic microscopes used to introduce children to science and microscopy, and they may be utilized in some industries as well. Jewelers, for example, use loupes to examine specimens for grading and quality determinations.

The Magnifying power of a simple microscope is given by M = 1+ D/f; where D = 25cm and f = focal length of lens

Compound Microscope

The compound microscope consists essentially of two or more double convex lenses fixed in the two extremities of a hollow cylinder. The lower lens (nearest to the object) is called the objective; the upper lens (nearest to the eye of the observer), the eyepiece. The cylinder is mounted upright on a screw device, which permits it to be raised or lowered until the object is in focus, i.e., until a clear image is formed. When an object is in focus, a real, inverted image is formed by the lower lens at a point inside the principal focus of the upper lens. This image serves as an "object" for the upper lens which produces another image larger still (but virtual) and visible to the eye of the observer.


A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light). The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century, using glass lenses. They found use in terrestrial applications and astronomy.

Telescopes may be classified by the wavelengths of light they detect:

•    X-ray telescopes, using shorter wavelengths than ultraviolet light
•    Ultraviolet telescopes, using shorter wavelengths than visible light
•    Optical telescopes, using visible light
•    Infrared telescopes, using longer wavelengths than visible light
•    Submillimetre telescopes, using longer wavelengths than infrared light

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