If we enter a dark room, objects present there are not visible. However, if a bulb is switched on, everything in the room becomes visible. It shows that for vision the presence of light is essential.
Definition of light
It is an invisible energy which causes in us sensation of sight (vision). Since light is obtained from heat energy, i.e., when an object is heated to a temperature beyond [500 0C, we can say that light is a kind of energy].
It must be kept in mind that light energy makes the surrounding objects visible, but by itself it is an invisible energy.
For example, if we are seeing a coloured poster, then we are seeing only the poster and not the coloured lights reflected fronts it. It is because light is invisible. The various colours reflected from the poster excite the retina of the eye, which in turn sends a message to the brain. It is the brain which finally makes out the colours of the poster. Thus, we can conclude that light is an invisible energy which causes in us sensation of vision.
6.2. REFLECTION OF LIGHT
When a beam of light is incident on a surface, a part of it is returned back into the same medium. The part of light which is returned back into the same medium is called the reflected light.
The remaining part of light is absorbed if the surface on which the incident light strikes is opaque or it is partly transmitted and partly absorbed if the surface is transparent.
The return of light into the same medium after striking a surface is called reflection.
Reflection of light is the process which enables us to see different objects around us. Luminous bodies are directly seen, but non luminous objects are seen only because they reflect the light incident on them which on entering into our eyes make them visible.
Note: Reflection is possible in case of plane mirror. A plane mirror is a plane glass plate which is silvered at its one surface. The other surface is then reflecting surface of the plane mirror.
Types of reflections
(a) Regular Reflection
The phenomenon due to which a parallel beam of light travelling through a certain medium, on striking some smooth polished surface, bounces off from it, as parallel beam, in some other direction is called regular reflection.
Regular reflection takes place from the objects like looking glass, still water, oil, highly polished metals, etc.
Regular reflection is useful in the formation of images, e.g., we can see our face in a mirror only on account of regular reflection.
However, it causes a very strong glare in our eyes.
(b) Irregular Reflection or Diffused Reflection
The phenomenon due to which a parallel beam of light, travelling through some medium, gets reflected in various possible directions, on striking some rough surface is called irregular reflection or diffused reflection.
The reflection which takes places from ground; walls; trees; suspended particles in air; and a variety of other objects, which are not very smooth, is irregular reflection.
Irregular reflection helps in spreading light energy over a vast region and also decreases its intensity. Thus, it helps in the general illumination of places and helps us to see things around us.
6.3. GENERAL TERMS TO THE REFLECTION
(a) Mirror : A smooth polished surface from which regular reflection can take place is called mirror. MM| is the mirror as shown in figure.
(b) Incident Ray : A ray of light which travels towards the mirror is called incident ray. AB is an incident ray in the figure.
(c) Point of Incidence : The point on the mirror, where an incident ray strikes is called point of incidence. ‘B’ is the point of incidence in the figure.
(d) Reflected Ray : A ray of light which bounces off the surface of a mirror, is called reflected ray. BC is reflected ray in the figure.
(e) Normal: The perpendicular drawn at the point of incidence, to the surface of mirror is called normal. BN is the normal in the figure.
(f) Angle of Incidence : The angle made by the incident ray with the normal is called angle of incidence. ∠ABN is the angle o£ incidence in the figure.
(g) Angle of Reflection : The angle made by the reflected ray with the normal is called angle of reflection. ∠CBN is the angle of reflection in the figure.
(h) Glance Angle of Incidence : The angle which the incident ray makes with the mirror is called glance angle of incidence. ∠MBA is the glance angle of incidence in the figure.
(i) Glance Angle of Reflection : The angle which the reflected ray makes with the mirror is called glance angle of reflection. ∠M’BC is the glance angle of reflection in the figure.
6.4. LAWS OF REFLECTION
1. The incident ray, the reflected ray and the normal lie in the same plane, at the point of incidence.
2. The angle of incidence is always equal to the angle of reflection.
Formula for the angle of deviation due to reflection
In the figure angle of incidence = i; Angle of deviation = d
Consider the straight line AOC, i + r +d = 180°
i.e., the sum of angle of incidence, angle of reflection and angle of deviation is 180°.
Therefore, for an angle of incidence i, the angle of deviation is equal to
Note: The deviation produced by n reflections from two plane mirrors inclined at an angle is given by
, if n = 2 where n is even.
When the rays of light, diverging from an object point, after reflection or refraction, either actually meet at some other point, or appear to meet at some other point, then that point is called image of that object.
Types of images
1. The rays of light after reflection or refraction appear to meet at some other point.
The rays of light after reflection or refraction actually meet at some point.
2. It cannot be caught on the screen.
It can be caught on the screen.
3. It is always erect.
It is always real.
4. Image of our face in plane mirror is a virtual image.
Image formed on a cinema screen is a real image.
6.6. GEOMETRICAL CONSTRUCTION OF AN IMAGE OF AN EXTENDED OBJECT IN A PLANE
MIRROR : (TWO-RAY DIAGRAM)
Consider an extended object P situated in front of a plane mirror. For geometrical construction, we will consider two points X and Y on this object. First of all we will locate the position of the image, keeping in mind that image formed in a plane mirror is as far behind as the object is in front of it, by taking normal incidence.
In order to construct two-ray diagram, from point X| draw two rays straight towards eye along X|E and X|F, cutting the mirror at A and B respectively. Join XA and XB to form incident beam on mirror. Similarly, from point Yj draw two rays towards the eye along Y|E and Y|F cutting at C and D. Join YC and YD to form incident beam on mirror.
Lateral Inversion: The phenomenon clue to which the image of an object turns through an angle of 180″ through vertical axis rather than horizontal axis, such that right side of image appears as left or vice versa is called lateral inversion.
Characteristics of an image formed by a plane mirror
1. The image is formed behind the mirror and has the same size as the object
2. The image is inverted laterally.
3. The image is as far behind the mirror as the object is in front of it.
4. The image is virtual. It cannot be received on a screen.
5. The image is erected w.r.t object
When two or more mirrors are placed at some angles to each other, we get to see multiple images. Let us take an example in which two mirrors are placed opposite to each other. If an object is placed between them, its image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will result in the formation of multiple images. The number of images formed depends on the angle between the two mirrors. This can be calculated by using the following formula:
Number of Images =
So, if the given mirrors are at a right angle to each other, 3 images will be formed. If the given mirrors are at 30° angle, we shall get 11 images. When the mirrors are kept opposite and parallel to each other, there would be infinite number of images formed.
6.7. APPLICATION OF REFLECTION
A periscope is an instrument for observation from a concealed position.In its simplest form it consists of a tube with mirrors at each end set parallel to each other at a 45-degree angle. This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during World War I
It consists of a cardboard or wooden tube bent twice at right angles and is provided with two openings. Two plane mirrors (a) are fixed at the bends of the tube at an angle 45°. Both the mirrors face each other. The tube is completely blackened from inside to avoid and reflection from its sides. The parallel rays coming from an object strike the first plane mirror at an angle of 45°. The rays get reflected at an angle of 45°. These reflected rays strike the second mirror and further reflected through an angle of 45°.
Thus we can see the image.
1. It is used to see above the head of crowds.
2. It is used by soldiers in trench warfare.
1. The final image is not brightly illuminated as light energy is absorbed due to tow successive reflections.
2. Any deposition of moisture or dust on the mirror reduces the reflection almost to nil and hence the periscope cannot be used in places where there is a lot of dust or moisture.
(Kaleido = beautiful + edos = image + scope = viewing). It is an instrument used to view beautiful images formed by the reflection of the two or more mirrors (usually three) when placed at different angles.
Principle: Multiple reflections take place in plane mirrors. This is the principle of ‘Kaleidoscope’.
Cut out a 16 × 16 cm piece of cardboard. Use a pencil to divide the cardboard into 4 strips each 4 cm wide. Use a sharp knife to score along the pencil lines so that the cardboard can be easily folded into 4 strips. Use paste to attach aluminium foil over 2 of the strips at one end. Colour the next strip black. Fold the strips to make a tube with a triangular cross-section. Two of the inside walls of the triangular tube have aluminium foil on them and the inside of the third wall is black. Use adhesive tape to keep the folded cardboard in shape. Attach clear plastic to each end of the triangular tube. Put coloured beads or small coloured shapes on the clear plastic at one end. Make a lid out of tracing paper to cover the beads or coloured shapes. The lid must be high enough to let the beads or shapes them move about. Look through the clear plastic at the other end of the triangular tube which is now a kaleidoscope. Turn the kaleidoscope while looking through it a see the changing patterns .formed by light bouncing off the aluminium foil.
A better kaleidoscope can be made if you can use small mirrors instead of aluminium foil. The kaleidoscope was invented by Sir David Brewster (1781-1868).
1. Kaleidoscope is used as a toy for children.
2. Kaleidoscope is used by costume – designers in cloth mills and fashion designing institutes.
6.8. PLANE MIRROR– ITS USES
1. They are used as looking glass.
2. They are used by opticians to provide false dimensions.
These false dimension can be obtained when two mirrors are fixed parallel to each other.
3. They are used in the construction of reflecting periscope.
4. They are used in solar cookers for reflecting the rays of sun into the cooker.
5. They are used for signalling purposes.
6. They are used by barbers to show the customer the back of his head.
6.9. SPHERICAL MIRRORS
Raj and Laxmi were waiting for their dinner. Raj lifted a stainless steel plate and saw his image in it. Oh! This plate acts as a plane mirror. My image is erect and is of the same size. Laxmi saw her image using the back of a steel spoon. “Raj look here! I can also see my erect image though it is smaller in size. This spoon also acts as a mirror of some kind”, said Laxmi. You can also use a spoon or any curved shining surface to see your image.
Now try to do these activities on your own.
Take a stainless steel spoon. Bring the outer side of the spoon near your face and look into it.
Do you see your image in it ? Is this image different from what you see in a plane mirror? Is this image erect? Is the size of the image the same, smaller or larger? Now look at your image using the inner side of the spoon.
This time you may find that your image is erect and larger in size. If you increase the distance of the spoon from your face, you may see your image inverted . You can also compare the image of your pen or pencil instead of your face.
The curved shining surface of a spoon acts as a mirror. The most common example of a curved mirror is a spherical mirror. Let’s understand something more about these spherical mirrors.
A highly polished plane surface is called a plane mirror. A mirror in which the reflecting surface is curved is called a spherical mirror.
In spherical mirrors the polished reflecting surface is a part of a hollow sphere of glass. Depending upon the nature of the reflecting surface of the mirror, spherical mirrors are of two types.
Different types of spherical mirrors
Concave mirror: A spherical mirror whose inner hollow surface is the reflecting surface is called a concave mirror.
Convex mirror: A spherical mirror whose outer surface is the reflecting surface is called a convex mirror.
6.10. TERMS RELATED TO SPHERICAL MIRRORS
The width (distance) of the spherical mirror from which reflection can take place is called its aperture. It is denoted by MM’
The centre of a spherical mirror is called its pole. It is denoted by P.
Centre of curvature
The geometric centre of the hollow sphere of which the spherical mirror is a part is called the centre of curvature of the spherical mirror. It is denoted by C.
Radius of curvature
The radius of the hollow sphere of which the spherical mirror is a part is called the radius of curvature of the spherical mirror. In other words, the distance between the pole and centre of curvature of the spherical mirror (PC) is called its radius of curvature. It is denoted by r.
The straight line passing through the centre of curvature and the pole of a spherical mirror is called its principal axis (PX).
If a beam of light parallel to the principal axis falls on a concave mirror, all the rays after reflection meet at a point.
This point is called the focus (F) of the concave mirror.
If a beam of light parallel to the principal axis falls on a convex mirror, all the rays after reflection diverge. If the reflected rays are extended backwards, they appear to come from a point on the principal axis. This point is called the focus of the convex mirror.
Focal length: The distance between the pole (P) and focus (F) is called the focal length (f). It is denoted by f.
Types of images formed by spherical mirrors
A concave mirrror forms both real and virtual images. The size and nature of the image formed by a concave mirror depends on the position of the object. A convex mirror forms only virtual, erect and diminished images. The different types of images formed by a concave mirror as follows.
Position of the object
Position of the image
Relative size of the image
Nature of the image
At focus (F)
Real and inverted
Between C and F
Real and inverted
Real and inverted
Between C and F
Real and inverted
Real and inverted
Between F and P
On the other side of the mirror.
Virtual and erect
6.11. USES OF SPHERICAL MIRRORS
I. Uses of concave mirror
Concave mirrors are commonly used in torches, search-lights and vehicles head lights to get powerful parallel beams of light. They are used as shaving mirrors to see a lager image of the face. The dentists use concave mirrors to see large images of the teeth of patients. Large concave mirrors are used to concentrate sun light to produce heat in solar furnaces.
II. Uses of convex mirrors
Convex mirrors are commonly used as rear-view mirrors in vehicles. These mirrors are fitted on the sides of the vehicle, enabling the driver to see traffic behind him/her to facilitate safe driving. Convex mirrors are preferred because they always give an erect image. Also they have a wider field of view as they are curved outwards.
6.12. IMAGES FORMED BY LENS
What is a Lens?
A lens is a piece of glass or any transparent material bound with two surfaces, atleast one of which is a curved surface. The curved surface is spherical (part of a sphere) in nature. Based on the shape of the curve on the surface of a lens, lenses are grouped into two main types:
(i) A convex lens, having a bulge in the centre and with narrow edges.
(ii) A concave lens, having a depression in the centre and thick at the edges.
Formation of Spherical Lenses different Shapes of Spherical Lenses
A. Double convex (both the sides convex)
B. Plano convex (converging lens)
C. Concavo convex (convex meniscus)
D. Convexo concave (concave meniscus)
E. Plano concave (diverging lens)
F. Double concave (both the sides concave)
We call these lenses as glasses or specs (short for a pair of spectacles) which are used by people with poor sight to watch things through them. These are nothing but a pair of lenses made from transparent glass and fixed to a frame which is held on to the eyes. A lens, which is a magnifying glass, is used by watchmakers to see very small parts of the machine as big (large) through them.
A convex lens makes the object magnified, when viewed through it. A convex lens is thick in the middle and thin at its edge. When light rays pass through a convex lens, they bend inwards and converge at a common point to form an image of the source of light.
Rays from the sun converge to form its image as a bright spot. A convex lens converges light rays. Therefore, it is also called a converging lens. The image formed when the object is placed close to a convex lens is virtual, erect and magnified. Virtual images cannot be caught on a screen. Images that are caught on a screen are called real images. When the object is placed at a distance from a convex lens, the image formed is real, inverted and diminished.
It is a lens that possesses at least one surface that curves inwards. When light rays are incident on a concave lens, they bend outwards or diverge. The rays diverge away from each other. Thus, a concave lens is also called a diverging lens. A concave lens is thinner at its centre than at its edges, and is used to correct short sightedness. It does not focus at a single point. The image formed by a concave lens is upright, virtual and smaller than the object. For example, the images seen through a peephole are different from normal holes, because these peep holes contain concave lenses.
Applications of Lenses
Lenses are used in magnifying glasses, peep holes, cameras, bioscopes, binoculars, telescopes, microscopes and projectors. A refracting telescope uses a concave mirror and a convex lens.
A band of colours extending from violet to red is a rainbow. A rainbow is formed by the refraction and reflection of the sun’s rays through raindrops.
A band of colours extending from violet to red is a rainbow. A rainbow is formed by the refraction and reflection of the sun’s rays through raindrops. When it is raining in one part of the sky and sunny in another, a rainbow appears. The centre of the rainbows arc is always directed away from the sun. It is believed that in the past, Norsemen saw rainbows as bridges for gods to come to the earth from their home in the sky. Norsemen were the inhabitants of Norway. A rainbow lasted for about 9 hours on 14th March, 1994, at a place called Wetherby in Yorkshire, England. Although sunlight appears white, it is composed of seven colours. The colours in a rainbow are the colours of sunlight. A rotating disc has a pencil that serves as a rotator. The disc is covered with violet, indigo, blue, green, yellow, orange and red coloured papers
When the disc is rotated, it appears white instead of the individual colours. This is because a mixture of colours of the rainbow in proper proportions produces white colour. The colours of a rainbow can be represented by the acronym:
VIBGYOR: V – Violet, I – Indigo, B – Blue, G – Green, Y – Yellow, O – Orange and R – Red.
Dispersion of Light through a Prism
Take a glass prism. Allow a narrow beam of sunlight to pass through a small hole in the window of a dark room to fall on one face of the prism. The light bends when it passes through the prism. Now allow the light coming out of the other face of the prism to fall on a white sheet of paper or a white wall. Different component colours of white light bend differently, and so the constituent colours can be seen separately. Thus, the colours are said to have dispersed after passing through the prism.
6.14. WHAT IS INSIDE OUR EYES
Our eyes enable us to see the beautiful world around us. The most important part of our eyes is a convex lens inside it that is made of living cells.
The human eye is like a camera having a lens on one side and a sensitive screen called the retina on the other.
The essential parts of a human eye are shown in figure.
a) Sclerotic: It is the outermost converging of the eye ball. It is made of white tough fibrous tissues. Its function is to house and protect vital internal parts of eye.
b) Cornea: It is the front bulging part of the eye. It is made of transparent tissues. Its function is to act as a window to world. i.e., to allow the light to enter in the eye ball.
c) Choroid: It is a grey membrane attached to the sclerotic from the inner side. Its function is to darken the eye from inside and, hence, prevent any internal reflection.
d) Optic Nerve: It is a bundle of approximately 70,000 nerves originating from brain and entering eye ball from behind. Its function is to carry optical message (visual messages) to the brain.
e) Retina: The optic nerve on entering the ball, spreads like a canopy, such that each nerve and attaches itself to the choroid. The nerve endings form a hemi-spherical screen called retina. These nerve endings on the retina are sensitive to visible light. On the retina are two important areas which we will discuss separately. The function of retina is to receive the optical image of the object and then convert it to optical pulses. These pulses are then sent to the brain through optic nerve.
f) Yellow spot: It is a small area facing the eye lens. It has high concentration of nerve endings and is slightly raised as well as slightly yellow in colour. Its function is to very clear image by sending a large number of optical pulses to brain.
g) Blind spot: It is a region on the retina, where the optic nerve enters the eye ball. It has no nerve endings and hence, is insensitive to the light. It does not seem to have any function. Any image formed on this spot is not visible.
h) Crystalline lens: It is a double convex lens made of transparent tissues. It is held in position by a ring of muscles, commonly called ciliary muscles. Its function is to focus the images of different objects clearly on the retina.
i) Ciliary muscles: It is a ring of muscles which holds the crystalline lens in position. When these muscles relax, they increase the focal length of the crystalline lens and vice versa. Its function is to alter the focal length of crystalline lens so that the images of the objects, situated at different distances, are clearly focussed on the retina.
j) Iris: It is a circular diaphragm suspended in front of the crystalline lens. It has a tiny hole in the middle and is commonly called pupil. It has tiny muscles arranged radially around the pupil. These muscles can increase or decrease the diameter of the pupil. The iris is heavily pigmented. The colour of eyes depends upon colour of pigment. The function of iris is to control the amount of light entering in eye. This is done by increasing or decreasing the diameter of pupil.
k) Vitreous humour: It is a dense jelly like fluid, slightly grey in colour, filling the part of eye between crystalline lens and retina. Its function is (i) to prevent the eye ball from collapsing due to change in atmospheric pressure (ii) in focussing the rays clearly on the retina.
l) Aqueous humour: It is a watery, saline fluid, filling the part of the eye between the cornea and the crystalline lens. Its function is (i) to prevent front part of the eye ball from collapsing with the change in atmospheric pressure (ii) to keep the cornea moist.
Power of Accommodation
Have you wondered why the eye is able to focus the images of objects lying at various distances?
It is made possible because the focal length of the human lens can change i.e., increase or decrease, depending on the distance of objects. It is the ciliary muscles that can modify the curvature of the lens to change its focal length.
To see a distant object clearly, the focal length of the lens should be larger. For this, the ciliary muscles relax to decrease the curvature and thereby increase the focal length of the lens. Hence, the lens becomes thin. This enables you to see the distant object clearly.
To see the nearby objects clearly, the focal length of the lens should be shorter. For this, the ciliary muscles contract to increase the curvature and thereby decrease the focal length of the lens. Hence, the lens becomes thick. This enables you to see the nearby objects clearly.
The ability of the eye lens to adjust its focal length accordingly as the object distances is called power of accommodation.
• The minimum distance of the object by which clear distinct image can be obtained on the retina is called least distance of distinct vision. It is equal to 25 cm for a normal eye. The focal length of the eye lens cannot be decreased below this minimum limit of object distance.
• The far point of a normal eye is infinity. It is the farthest point up to which the eye can see objects clearly.
The range of vision of a normal eye is from 25 cm to infinity.
Have you ever thought why animals’ eyes are positioned on their heads?
This is because it provides them with the widest possible field of view. Our eyes are located in front of our face. One eye provides 150° wide field of view while both eyes simultaneously provide 180° wide field of view. It is the importance of the presence of two eyes as both eyes together provide the three-dimensional depth in the image.
Defects of Vision
The loss of power of accommodation of an eye results in the defects of vision.
There are three defects of vision called refractive defects. They are myopia, hypermetropia, and presbyopia. In this section, we will learn about these defects of vision in detail.
1. Myopia (short sightedness)
Myopia is a defect of vision in which a person clearly sees all the nearby objects, but is unable to see the distant objects comfortably and his eye is known as a myopic eye.
A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of its retina as shown in the figure.
Myopia is caused by
i. increase in curvature of the lens
ii. increase in length of the eyeball
Since a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision. The image is allowed to form at the retina by using a concave lens of suitable power as shown in the given figure.
2. Hypermetropia (Long sightedness)
Hypermetropia is a defect of vision in which a person can see distant objects clearly and distinctively, but is not able to see nearby objects comfortably and clearly.
So, now you can easily represent the problem with a hypermetropic eye with the help of a diagram. It is shown in the given figure.
A hypermetropic eye has its least distance of distinct vision greater than 25 cm.
Hypermetropia is caused due to
i. reduction in the curvature of the lens
ii. decrease in the length of the eyeball
Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision, as you already have seen in the animation. The ray diagram for the corrective measure for a hypermetropic eye is shown in the given figure.
6.15. CARE OF EYES
Eyes are very important organ and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:
• Do not read in too bright or too dim light.
• Do not look directly at a bright object or at the sun.
• Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
• If something gets into the eye, do not rub the eye. Wash it with cold water.
• In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.
6.16. VISUALLY CHALLENGED PERSONS CAN READ AND WRITE
Some people face with disability of vision. This disability can be partial or complete. Such persons are called visually challenged persons. For a visually challenged person; life can be very difficult. These people usually show a marked development of other senses; like the sense of hearing and sense of touch. Many aids have been devised to make their life easy. They can be divided into two categories, viz. optical and non-optical aids.
1. Optical Aids: Optical aids can help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. TV monitors, magnifying devices and telescopic devices come under this category.
2. Non-optical Aids: Non-optical aids are helpful for a person who is completely visually challenged. These aids rely on the senses of hearing and touch. Aids which rely on the sense of touch are called tactual aids. Tactile buttons on the pedestrian light and in public transport are examples of tactual aids. Tactile strips at the edge of the platforms are also meant for visually challenged persons. Even the currency notes have tactile markings so
that a visually challenged person can recognize notes of different denominations.
In some countries; specially trained guide dogs are pressed into the service of a visually challenged person.
6.17. WHAT IS A BRAILLE SYSTEM?
The most popular resource for visually challenged persons is known as Braille. Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821. 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.
Patterns of dots to represent some English alphabets and some common words are shown below.