Light

'On a windless day, the smooth surface of a lake reflects light and produce stunning images of the surrounding landscape and sky. The light falling on the flat, smooth, surface of lake undergoes regular reflection.'

1.0What Makes Things Visible

Look around your darkened room at night. You cannot see an object in the dark. This means that eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. Thus, light is a form of energy which excites our sense of sight.

Sources of light: During the day, the primary source of light is the Sun and the secondary source is the brightness of the sky. Other common sources are flames, electric bulbs, tube lights (fluorescent tubes), compact fluorescent lamps (CFLs) and light emitting diodes (LEDs).

Luminous objects: The objects which emit their own light are called 'luminous objects'. Examples : Sun and other stars, lamp, bulb, tubelight, candle flame, etc.

Non-luminous objects: The objects which do not emit their own light but only reflect or scatter the light which falls on them are called 'non-luminous' objects (or illuminated objects). Examples : Table, chair, animals, plants, planets, satellites, moon, etc.

Absorption, transmission and reflection: Objects can absorb light, reflect light, and transmit light - allow light to pass through them. The type of matter in an object determines the amount of light it absorbs, reflects, and transmits. An opaque material only absorbs and reflects light, no light passes through it. You cannot see through opaque materials. Materials that allow small portion of light to pass through them are described as translucent materials. You cannot see clearly through translucent materials. Transparent materials, transmit almost all the light striking them, so you can see objects clearly through them. Only a small amount of light is absorbed and reflected by transparent materials.

Q. How could we see non-luminous objects and luminous objects?

• Explanation: When the light falls on a non-luminous object, it reflects a part of the light towards us. When this light reaches our eyes, we could see such an object. A luminous object emits its own light. When this light reaches our eyes, we could see a luminous objects. Thus, for us to see an object, it must reflect or emit some light that reaches our eyes.

2.0Properties Of light

• Light consists of electromagnetic waves (non-mechanical waves) which do not require any material medium for their propagation. That is, light can travel through vacuum.
• The speed of light depends on the nature of medium. In vacuum or free space, they travel fastest and their speed is $3\times 10^8\mathrm{m}/\mathrm{s}$. In air, the speed is almost same (marginally less), whereas, in glass or water, it reduces considerably.
• The wavelength range of visible light is about $4\times 10^{-7}\mathrm{m}$ to $7\times 10^{-7}\mathrm{m}$.
• Light travels in a straight line in vacuum or in a uniform transparent medium. This characteristic is termed as rectilinear propagation of light.

• The formation of image, eclipse and shadow take place because of rectilinear propagation of light.

3.0Reflection Of light

Reflection of light is the process in which light rays, meeting the boundary between two media, 'bounces back', to stay in the first medium.

To The process of sending back of light rays which fall on the surface of an object is called reflection of light. You probably have noticed that when incoming rays of light strike a smooth reflecting surface, such as a polished mirror, at an angle close to the surface, the reflected rays are also close to the surface. When the incoming rays are quite high above the reflecting surface, the reflected rays are also high above the surface. An example of this similarity between incoming and reflected rays is shown in figure. There are laws of reflection which can explain this similarity.

• On reflection of light from a surface, the speed, wavelength and frequency of light does not change. This is because the light stay in the same medium.
• On reflection, the amplitude and intensity of reflected ray is slightly less than that of incident ray as some part of energy is absorbed at the surface.

4.0Some Basic Terms Related To Reflection Of Light

• Ray of light : A ray of light is the direction in which light travels.
• Beam of light : A bundle of light rays is called beam of light or light beam.
• Incident ray : The ray of light which falls on the mirror surface is called incident ray.
• Reflected ray: The ray of light which is sent back by the mirror is called 'reflected ray'.
• Point of incidence : The point at which the incident ray falls on the mirror is called 'point of incidence'.
• Normal : A line perpendicular to the surface of mirror passing through the point of incidence is called 'normal'.
• Angle of incidence : The angle made by incident ray with the normal at the point of incidence is called 'angle of incidence'.
• Angle of reflection : The angle made by reflected ray with the normal at the point of incidence is called 'angle of reflection'.

5.0Laws Of Reflection

First law: The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Second law: The angle of incidence is always equal to the angle of reflection.

• The angle between the surface and incident ray or reflected ray is called the glancing angle.
• If angle between incident ray and surface is given then angle of incidence $\angle \mathrm{i}=90^{\circ }$ - angle between incident ray and surface.
• A ray of light is an idealization. In reality, it is a narrow beam of light which is made up of several rays. For simplicity, we use the term 'ray' for a narrow beam of light.
• The laws of reflection hold good for all kinds of waves. Also, these laws are applicable to plane as well as curved surfaces.

Q. In the given figure, using law of reflection, identify which eye(s) can see the image of the object in the mirror? Which eye(s) cannot see the image?

• Explanation According to the law of reflection, the angle of incidence is equal to the angle of reflection. We will make ray diagrams for each of the eyes using the law of reflection. If the reflected ray enters the eye, that eye can see the image, otherwise not.
  

Using laws of reflection to identify the eye that can see the image or the eye which cannot.

From the figure, we can conclude that eyes 1, 3 and 4 can see the image of the given object while eye 2 cannot see the image of the given object.

Although a light source gives off light in all directions, your eye sees only the particular diverging cone of rays that is coming towards you. If you go to the other side of the object, a different cone of rays will enter your eye.

6.0Reflection From Plane Mirrors

A mirror is a highly polished surface used to reflect the light falling on it. Mirrors are usually made by depositing a thin layer of silver metal on one side of a plane glass sheet.

Some basic terms

Object : Anything which gives out light rays either of its own or due to reflection is called an 'object'. Point object : An object whose dimensions are negligibly small is called 'point object'. Extended object : An object whose dimensions are quite large is called 'extended object'. $\begin{array}{c}\text{ Point Objects }\\ \text{ Extended Objects }\uparrow \end{array}\}\begin{array}{c}\text{ Symbols used in }\\ \text{ ray diagrams }\end{array}$ Image : An image of an object is formed when light rays coming from the object meet or appear to meet at a point after reflection from a mirror or refraction from a lens. Real image : A real image is one formed when the light rays actually meet at a point and which can be obtained on a screen.

Virtual image : A virtual image is one formed when the rays do not actually meet at a point but they appear to meet at a point. Such images can not be obtained on the screen.

Image formed by a plane mirror

The properties of an image formed by a plane mirrors are, (1) The image is virtual and erect. (2) The distance of image from mirror is equal to distance of object from mirror. (3) The size of image is exactly equal to the size of object. (4) The image is laterally inverted.

• The most common example for a real image on the cinema screen.
• The colour of an image is identical to the colour of the object forming the image. The fact that the colour is same is the evidence that the frequency of light doesn't change upon reflection.
• If you come closer or far away from the plane mirror, the size of the image does not change.

Formation of image of a point object

(1) Let us take a point object 0 placed in front of a plane mirror M (see figure). To find the image I formed in the mirror, we take two light rays $OA$ and $OB$ incident at points $A$ and $B$ on the mirror. The ray $OA$ is incident perpendicular to the mirror, then it retraces the original path after reflection i.e., the reflected ray is $AO$. The reflected ray for incident ray $OB$ is $BX$. (2) The reflected rays AO and BX are diverging that is, they are moving away from each other. When we produce the

Formation of image of a point object reflected rays AO and BX backwards, they meet at a point I behind the mirror. This point $I$ is the image of the object 0 formed by the plane mirror.

Formation of image of an extended object

An extended object is considered as a collection of large numbers of point objects. (1) An extended object $AB$ is placed in front of a plane mirror $M$ (see figure). For point $A$, we draw an incident ray $AC$ falling perpendicularly on the mirror at $C$. This ray will be reflected back along the same path (ray CA). An another ray AD , falls on the mirror at D and reflected along DX. On producing the reflected rays CA and DX backwards, they meet at point $A^{\prime }$. Point $A^{\prime }$ is virtual image of top point $A$ of the extended object $AB$. (2) A similar work is done to find the virtual image ( $B^{\prime }$ ) of the point $B$ of the extended object $AB$. We have located the images of topmost point $A$ and the lowest point $B$ of the extended object $AB$. Images of points which are lying between the point $A$ and $B$ of the extended object $AB$, must lie between the points $A^{\prime }$ and $B^{\prime }$. Thus, joining the points $A^{\prime }$ and $B^{\prime }$ will give the image $A^{\prime }{B}^{\prime }$ of the extended object $AB$.

• If the object is shifted by the distance $d$ towards mirror, the image will also shift by the same distance $d$ towards the mirror. If an object moves with a speed $v$ towards (or away) from the mirror, the image to him will appear to move with a speed $2v$ towards (or away) from him.

7.0Lateral Inversion

When an asymmetric object is placed in front of a plane mirror, then the right side of the object appears to be the left side of image and the left side of the object appears to be the right side of its image. This change of sides of an object seen in the image is called 'left - right inversion' or 'lateral inversion'. The image is inverted side ways, thus, also called 'side ways inversion' (see figure).

The word 'AMBULANCE' is written from right to left. Since in the mirror, the image is laterally inverted so the driver of the vehicle ahead can read "AMBULANCE" written on it.

Q. In the top view of figure, the image of the stone seen by observer 1 is at $C$. Where does observer 2 see the image : at $A$, at $B$, at $C$, at $E$, or not at all ?

• Explanation: The position of image does not depend on the position of the observer i.e., the observer 2 will also see the image at C only. In fact, if there any number of observers looking into the mirror, everyone will see the image of the object at C only. which shows one ray each that are made using law of reflection for observer 1 and observer 2 that enters into their eyes.
  
  Using law of reflection to locate the position of image

Regular reflection (specular reflection)

If a parallel beam of light is incident on a smooth plane mirror, the reflected beam is also a parallel beam. This reflection is called 'regular reflection'.

Reason : The light rays strike the smooth surface at same angles of incidence. This is because all the normals are parallel to each other on the smooth surface. Thus, the light rays get reflected at same angles of reflections. That is why, the rays of reflected beam remains parallel to each other.

Irregular reflection (diffused reflection)

If a parallel beam of light is incident on a rough surface or mirror, the rays of light becomes non-parallel to each other. Such a reflection is called 'diffused reflection'.

Reason : The light rays strike the rough surface at many different angles of incidence. This is because all the normals are not parallel to each other on the rough surface. Thus, the light rays get reflected at many different angles. That is why, the rays of reflected beam become non-parallel to each other. The laws of reflection are followed in both regular and diffused reflection.

8.0Reflected Light Can Be Reflected Again

When you visit a hair dresser, he makes you sit in front of a mirror. After your hair cut is completed, he places a mirror at your back to show you how the hair has been cut (see figure). You could see the hair at the back of your head because the light reflected by the smaller mirror placed at your back is again reflected by the bigger front mirror and finally the light reaches your eyes. This is called multiple reflection of light.

Periscope: The periscope makes use of multiple reflection of light. A periscope is an instrument for observation from a hidden position. In its simplest form it consists of a tube with mirrors at each end set parallel to each other at a $45^{\circ }$ angle (see figure). This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches or bunkers during World War I.

Military personnel also use periscopes in some gun turrets and in armored vehicles like tanks. Periscopes allow a submarine, when submerged at a shallow depth, to search visually for nearby targets and threats on the surface of the water and in the air.

9.0Multiple Images

We can obtain many images of an object using two or more mirrors by multiple reflection of light.

Kaleidoscope: This child's toy is a visual delight of changing colours as the toy is rotated. The effects are produced by multi-coloured glass pieces that tumble around when the toy is turned (see figure). Here, two (or three) mirrors are positioned $60^{\circ }$ to each other and five images of the object are produced for this orientation.

10.0Dispersion Of White Light

Sunlight is referred to as white light. It consists of seven colours. When white light travels through a triangular cross-section piece of glass called a prism, a rainbow like pattern appears on the other side. This effect is called dispersion. Dispersion is a method of demonstrating that white light is composed of many different colours (wavelengths) of light.

The phenomenon of splitting up of white light into its constituent colours is called 'dispersion of light'.

• The series of colours which are obtained when white light passes through a prism is called 'visible spectrum'. Rainbow is a natural phenomenon which shows the visible spectrum i.e., showing dispersion.
• The acronym VIBGYOR (Violet-Indigo-Blue-Green-Yellow-Orange-Red) is used for remembering the spectrum of light.

11.0The Human Eye

Components and Structure Of Human Eye

(1) The eye has a nearly spherical shape, the eye ball has a diameter of about one inch (nearly 2.3 cm ). The front portion is more sharply curved and it is covered by a thin transparent protective membrane called 'cornea'. It is this portion which is visible from the outside. (2) Behind cornea, there is a liquid called 'aqueous humour' and behind that, there is a crystalline lens. Between the aqueous humour and the lens, there is a muscular diaphragm called 'iris', which has a small hole in it called 'pupil'. (3) The eye lens is composed of a fibrous, jelly like material which is hard in the middle and gradually becomes soft towards the edges. The curvature of the lens is altered by the ciliary muscles to which it is attached. (4) The space between lens and the retina is filled with another liquid called 'vitreous humour'. (5) The light entering the eye forms an image on the retina which is a delicate membrane having enormous number of light sensitive cells. It contains about 125 million receptors called 'rods' and 'cones' which receive the light rays and about one million optic nerve fibres which transmit the information to the brain. (6) The region on the retina where the optic nerve enters the eye ball is called the 'blind spot'. It is insensitive to the light, that is, any image formed at this spot is not visible. (7) The macula lutea, also called 'yellow spot' is a central part of the retina responsible for sensing fine details and for looking straight ahead. It has high concentration of nerve endings and it is slightly raised. Its function is to form a very clear image, by sensing a large number of electrical signals to the brain. (8) Sclera or sclerotic is the outermost covering of eye and it is made of white fibrous tissue. 'Choroid' is a grey membrane attached to sclera.

Outer coating of the eye called sclera is white. It is tough so that it can protect the interior of the eye from accidents. Cornea and aqueous humour provide most of the bending of the light rays entering the eye. The crystalline lens merely provides the fine adjustment required to make the images of objects placed at different distances on the retina.

Role Of Iris

Iris controls the size of the pupil and therefore, helps in regulating the amount of light entering the eye through a variable aperture (the pupil). In low intensity of light, iris expands the pupil to allow more light to enter into the eye. When the light is very bright, iris contracts the pupil and the pupil becomes very small, thus, only a small amount of light enter into the eye.

• Pupil appears black because any light falling on it goes into the eye and there is no chance of light coming back to the outside.
• At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.
• The iris is the part of that eye which gives it its distinctive colour. When we say that a person has green or brown eyes, we refer actually to the colour of the iris .
• Rods are responsible for vision at dim light and cones are responsible for vision at bright light.

Working Of Eye

(1) When the light enters the eye from air, most of the bending of light occurs at cornea. Some additional bending is done by the lens so as to form an inverted, real image of the object on retina. (2) When the eye is focused on a distant object, the ciliary muscle relaxes allowing ligaments to increase tension on the lens and cause it to flatten i.e., the lens becomes thin or less curved. Finally, the image is formed on the retina and we can see the object clearly. (3) When the eye is focused on a closer objects, the ciliary muscle contracts, allowing the lens, by virtue of its elasticity, to become more curved i.e., the lens becomes thick. Again, the image is formed on the retina and we see the object clearly. (4) The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are i.e., the brain converts the inverted image formed on the retina to again erect (or upright).

Persistence Of Vision

The impression of an image does not vanish immediately from the retina. It persists there for about $1/16$ th of a second. So, if still images of a moving object are flashed on the eye at a rate faster than 16 images per second, then the eye perceives this object as moving.

• The continuance of the sensation of the eye for a fraction of second even after the removal of the object is called 'persistence of vision'.

Principle of cinematography (or motion-picture projection): If still pictures taken by a movie camera are projected on a screen at a rate of about 24 images per second (faster than 16 per second), then the image of one picture persists on the retina till the next picture falls on the screen. Due to this, the slightly different images of the successive pictures present on the film merge smoothly into one another and gives us the feeling of moving images (or movie). This is the principle used in cinematography.

Accommodation: By contracting or relaxing the ciliary muscles connected to the lens, its shape can be changed such that we can see the nearby as well as the distant objects clearly. This process is called accommodation.

• The ability or property of the eye to change the shape of lens so as to see the object clearly is called 'accommodation'.
• The muscles cannot be strained beyond a limit and thus, an object placed too close to the eye cannot be seen clearly.

Near point: The nearest point for which the image can be formed clearly on the retina, is called the 'near point of the eye'.

Least distance of distinct vision: The minimum distance at which objects can be seen clearly without strain is called 'least distance of distinct vision (or clear vision)'. In other words, the distance of the near point from the eye is called the 'least distance of distinct vision'. This distance varies with age, increases with it because of decreasing effectiveness of the ciliary muscles and the loss of flexibility of the lens. The symbol used for least distance of distinct vision is D. Standard value of D for a young adult with normal vision is 25 cm . For a child of 10 years $D$ is nearly 7 to 8 cm , for a old man of 60 years D may be 200 cm .

Far point: The farthest point up to which the eye can see objects clearly is called the 'far point' of the eye. For normal eye, far point is at infinity. Some persons can see near objects clearly but cannot see distant objects so clearly. Such a defect of eye is called myopia. Some persons cannot see near objects clearly but they can see distant objects quite well. Such a defect of eye is called hypermetropia. Some persons can see near objects clearly but cannot see distant objects so clearly. Such a defect of eye is called myopia. Some persons cannot see near objects clearly but they can see distant objects quite well. Such a defect of eye is called hypermetropia.

12.0Care Of Eyes

• Eyes are wonderful instrument gifted by nature thus, it is necessary that you take proper care of your eyes.
• If you feel any problem you must go to an eye specialist. Have a regular checkup. If advised, use suitable spectacles.
• Wash your eyes twice everyday, preferably with boiled and cooled water.
• Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina. For that reason, never look at the sun or a powerful light directly.
• Never rub your eyes strongly. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement i.e., if irritation persists rush to a doctor.
• While playing, be careful as you can hurt your eyes.
• Always read at the normal distance for vision. Do not read by bringing your book too close to your eyes or keeping it too far.
• If food is deficient in some nutrients, it may affect eye. For example, lack of vitamin A in foodstuff is responsible for many eye troubles. Most common amongst them is night blindness. Thus, we should include such food which contains vitamin A. Food items which are rich in vitamin A are : raw carrots, broccoli and green vegetables (such as spinach), cod liver oil, eggs, milk, curd, cheese, butter and fruits such as papaya and mango.

Q. Why do we have two eyes for vision and not just one?

• Explanation: Two eyes provide a wider field of view to us. A single eye has a horizontal region of view of nearly $150^{\circ }$, but with two eyes it is about $180^{\circ }$. Also, the ability to detect faint objects increases with two eyes instead of a single eye. With a single eye and the world looks flat i.e., two-dimensional. The three dimensional effect can be experienced with two eyes only.
• Nature has provided eyes with eyelids to protect from any object entering the eye. Eyelids also shut out light when not required.
• Because our eyes are separated by a few centimeters, each eye looks a slightly separate image. Our brain combines the two images into one and tells us how close or far away the various objects are present.

13.0Eyes Of Animals

Animals have eyes shaped in different ways. (1) Eyes of a crab are quite small, but they enable the crab to look all around. So, the crab can sense even if the enemy approaches from behind. (2) Butterfly has large eyes that seem to be made up of thousands of little eyes (see figure). It can see not only in the front and the sides but the back as well.

14.0Visually Challenged People

People, including children, can be visually handicapped. They may have very little vision to see things or they or the may not see at all since birth. There are cases where people lose their eyesight because of a disease or an accident. Such people try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply. Resources for visually challenged people The resources for visually challenged people can be of two types : (1) Non-optical aids (2) Optical aids.

Non-optical aids : It include visual aids, tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids.

Visual aids can be used for those people who have little vision. Visual aids can magnify words, can provide suitable intensity of light and material at proper distances.

Tactual aids can be used for partly blind or completely blind people. It includes Braille writer slate and stylus (pointed writing tool), help these people in taking notes, reading and writing.

Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids include talking calculators, available for performing computational tasks. Closed circuit television is an electronic aid that enlarges printed material with suitable contrast and illumination. Such television can be used for partially blind people.

Optical aids : Optical aids are used for any person who is suffering from minor eye defect or for a partly blind person. It include corrective glasses, bifocal lenses, contact lenses, tinted (coloured) lenses, magnifiers and telescopic aids.

The lens combinations are used to rectify normal visual limitations like reading books, etc. The telescopic aids are available to view chalkboard and class demonstrations.

Cataract

The crystalline lens of some people in old age becomes hazy or even opaque due to development of membrane over it. This defect is called 'cataract' which leads to decrease or loss of vision of the eye.

Cataract can be removed by performing surgery to restore clear vision. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer.

• The defect of eye due to which a person cannot distinguish between colours is called 'colour blindness'. Usually, colour blind persons cannot distinguish between red, blue and green colours. Driving license for vehicles is not issued to colour blind persons.
• Nowadays, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.

15.0Braille System (Braille script)

The Braille script is a phonetic alphabet used by the blind for reading and writing. Each Braille character or "cell" is made up of six dots, arranged in a matrix of dots. A dot may be raised or flat, giving a total of possible combinations for 6 dots. Blind people use touch to read the raised dots. This kind of reading is known as 'tactile reading' or reading by touch. Braille cells are read from left to right, regardless of the language they are used to encode.

Braille is used to transcribe text in natural languages by encoding individual letters, abbreviations, and shorthand contractions per language. Braille is also used to transcribe

Louis Braille, a blind Frenchman, developed the Braille system for visually challenged persons 1821. mathematical and musical notation, and there are several standards for each.

Braille is produced manually or by computer. Manual transcription methods include a slate and stylus, and a Braille typewriter (the most famous of which is the Perkins Brailler).

The most popular resource for visually challenged persons is known as Braille. The present system was adopted in 1932. Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each.

Visually challenged people learn the Braille system by beginning with letters, then special characters and letter combinations. Methods depend upon recognition by touching. Each character has to be memorised.

16.0Mind Map

17.0Some Basic Terms

• Excites:- Give rise to.
• Luminous :- Emitting usually steady, suffused, or glowing light.
• Absorb :- To take in and hold something (a liquid, heat, etc.).
• Transmit :- To send or pass something from one person or place to another.
• Image : - A visual representation of something: such as, a likeness of an object produced on a photographic material.
• Inversion : - A situation in which something is changed so that it is the opposite of what it was before, or in which something is turned upside down
• Periscope : - A periscope is defined as an instrument used for observing over, around or through an obstacle or object which is prevented by direct line of sight.
• Kaleidoscope :- A tube containing loose bits of colored glass or plastic and two mirrors at one end that shows many different patterns as it is turned.
• Vision : - The act or power of sensing with the eyes; sight.
• Cataract : - A cataract is a clouding of the lens of the eye, which is typically clear.