50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

What is Light ?

INTRODUCTION :
Light is the form of energy which makes object visible. It means that we can see an object only when light from
an object enters our eyes. The light may have been emitted by the object or may have been reflected by it.
You may have also seen beams of light from the torch, head light of vehicles.

SOURCES OF LIGHT :
Those bodies which emit light in all directions are called sources of light. The sources can be point one or extended
one. The sources of light are of two types :
(i) Luminous sources : Those objects which by itself emit light are called luminous sources.
Eg. : Sun and stars (natural luminous sources), electric lamps, candles and lanterns (artificial luminous sources).
(ii) Non-luminous sources : Those objects which do not emit light but become visible only when light
from luminous objects falls on them. They are called non-luminous sources.
Eg. : Moon, planets (natural non- luminous sources), wood, table (artificial non-luminous sources) etc.

50 Amazing Lines of Light Physics Science

MEDIUM OF LIGHT :
Substance through which light propagates or tends to propagate is called a medium of light.
According to the medium of light objects are divided into three parts :
(i) Transparent object :
Bodies that allow light to pass through them i.e. transmit light through them, are called transparent bodies.
Eg. : Glass, water, air etc.
(ii) Translucent object : Bodies that can transmit only a part of light through them are called translucent objects.
Eg. : Froasted or ground glass, greased paper, paraffin wax etc.
(iii) Opaque object : Bodies that do not allow light to pass through them at all are said to be opaque objects.
Eg. : Chair, desk etc.

50 Amazing Lines of Light Physics Science

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50 Amazing Lines of Light Physics Science

PROPERTIES OF LIGHT :
(i) Light is invisible form of energy .
(ii) Light does not require material medium for its propagation i.e. light can travel through vacuum

Some definition related to the light :
(A) Ray of Light :
The path along which light energy travels in a given direction is called ray of light. A ray of light is
represented as a straight line. The arrow head on it gives the direction of light.

(B) Beam of Light :
A collection of rays of light is called beam of light. However, if the number of rays is too small then such a
collection of rays is called Pencil of light.

(C) Parallel Beam :
When the rays of light travel parallel to each other, then the collection of such rays is called parallel beam
of light. For example, sun rays entering into a room through a ventilator constitute a parallel beam .

BEHAVIOUR OF LIGHT AT THE INTERFACE OF TWO MEDIA :
When light travelling in one medium falls on the surface of a second medium, the following three effects may
occur :
(i) A part of the incident light is turned back into the first medium. This phenomenon is called reflection
of light.
(ii) A part of the incident light is transmitted into the second medium along a changed direction. This
phenomenon is called refraction of light.
(iii) The remaining third part of light energy is absorbed by the second medium. This phenomenon is
called absorption of light.

50 Amazing Lines of Light Physics Science

REFLECTION OF LIGHT :
When a ray of light falls upon a smooth surface like a mirror, the ray of light is reflected in another direction
in the same medium. This phenomenon is called reflection of light.
General definitions about reflection :
(i) Mirror :
A smooth polished surface from which regular reflection can take place is called mirror.

(ii) Incident ray :
A ray of light which travels towards the mirror is called incident ray. 

(iii) Point of incidence :
The point on the mirror, where an incident ray strikes is called point of incidence. 

(iv) Reflected ray :
A ray of light which bounces off the surface of a mirror, is called reflected ray. 

(v) Normal :
The perpendicular drawn at the point of incidence, to the surface of mirror is called normal. 

(vi) Angle of incidence :
The angle made by the incident ray with the normal is called angle of incidence.

(vii) Angle of reflection :
The angle made by the reflected ray with the normal is called angle of reflection.

(viii) Glance angle of incidence :
The angle which the incident ray makes with the mirror is called glance angle of incidence.

(ix) Glance angle of reflection :
The angle which the reflected ray makes with the mirror is called glance angle of reflection.

50 Amazing Lines of Light Physics Science

REGULAR AND DIFFUSED REFLECTION :
When a beam of light falls on a smooth, highly polished surface, almost entire light gets reflected in the
same medium in a definite direction. This type of reflection is called regular reflection. We can see our
image formed by a mirror due to the phenomenon of regular reflection.

When a beam of light falls on an opaque, rough and uneven surface, the light gets reflected in different
directions. This type of reflection is called irregular or diffused reflection. We can read a book or a
newspaper due to diffused reflection.

LAWS OF REFLECTION :
The reflection of light from a surface obeys certain laws called laws of reflection. They are :
(i) The angle of incidence is equal to the angle of reflection. 

(ii) The incident ray, the reflected ray and the normal at the point of incident, all lie in the same plane.
The laws of reflection are applied for all type of reflections.

50 Amazing Lines of Light Physics Science

MIRROR :
It is a highly polished surface, which is quite smooth and capable of reflecting a good fraction of light from its
surface.
(a) Object :
Anything which gives out light rays (either its own or reflected) is called an object.
(b) Image :
The reproduction of object formed by mirror or lens is called an image.

50 Amazing Lines of Light Physics Science

REAL AND VIRTUAL IMAGES :
The image which can be obtained on a screen is called a real image. It is formed when light rays, after
reflection, actually intersect each other. It is always inverted.
The image which cannot be obtained on a screen is called a virtual image. It is formed when light rays, after
refection, intersect when extended in backward direction. Its is always erect.

PLANE MIRROR :
Image Formation by Plane Mirror :
Consider a point source of light placed at a point O at a distance u in front of the plane mirror. Light rays
leave the source and are reflected from the mirror. After reflection, the rays diverge but they appear to come
from a point I located behind the mirror. Point I is called the image of the object O. Point I is at a distance v
behind the mirror.

CHARACTERISTICS OF IMAGE :
(i) Image is virtual, erect and of the same size as object.
(ii) It is as far behind the mirror as the object is in front of it
(iii) Laterally inverted : When left appears right and right appears left.

NOTE :
(i) Minimum size of the mirror required to see full image of a person is at least half of his own height.
(ii) If object moves with a speed V towards plane mirror then image moves with a speed 2V towards
object.
(iii) If mirror moves with a speed V towards stationary object then image moves with a speed 2V towards
object.

Uses of Plane Mirror :
In looking mirrors, hair saloons, reflecting periscopes & kaleidoscopes.

50 Amazing Lines of Light Physics Science

SPHERICAL MIRRORS :
Mirrors, whose reflecting surfaces are spherical or curved, are called spherical mirrors. These are of two types:
(i) Concave mirror : If the reflecting surface of the spherical mirror is curved inwards, it is called a concave
mirror. The image formed by a concave mirror can easily be taken on the screen. You must have played with
concave mirror to obtain image of sun on the ground or wall by positioning it to in a specific way. It means a
concave mirror can form real or virtual image. In concave mirror when a parallel beam of light after reflection
from a concave mirror converges at a point in front of the mirror. Due to this it is known as converging
mirror.

(ii) Convex mirror : If the reflecting surface of the spherical mirror is curved outwards, it is called a convex
mirror. It means a convex mirror always forms virtual image. In convex mirror when a parallel beam of light
after reflection from a convex surface diverges and the rays do not meet. Due to this it is known as diverging
mirror.

50 Amazing Lines of Light Physics Science

SOME POINTS :

(i) Pole : The central point of a mirror is called its pole.
(ii) Centre of curvature : The centre of the sphere of which the mirror is a part, is called centre of
curvature.
(iii) Radius of curvature : The radius of the sphere of which the mirror is a part, is called radius of curvature.
(iv) Principal axis : The straight line joining the pole and the centre of curvature is called the principal axis.
(v) Focal length : The distance between the pole and the focus is called the focal length. The focal length is half
of the radius of curvature.
(vi) Aperture : The size of the mirror is called its aperture.
(vii) Focus point :
(a) In concave mirror when a parallel beam of light after reflection from a concave mirror converges at a point
in front of the mirror. This point (F) is the focus of a concave mirror and it is real.

(b) when a parallel beam of light after reflection from a convex surface diverges and the rays do not meet.
However on producing backward, the rays appear to meet at a point behind the mirror. This point is focus
of the convex mirror and it is virtual.

USES OF CONCAVE MIRROR :
(i) Concave mirror is used by doctors for examining eyes, ears, nose and throat.
(ii) They are also used by dentist to see an enlarged image of the teeth.
(iii) The reflector of torches, headlights of cars and scooters are concave in shape.

50 Amazing Lines of Light Physics Science

USES OF CONVEX MIRROR :
(i) Convex mirrors are used as rear view mirrors to see the vehicle coming from behind. These mirror covers a
wider view and form upright small size (diminished) image.
(ii) They are used as reflector in street lamps so as to diverge light over a large area.

REFRACTION OF LIGHT :
When light travels in the same homogeneous medium, it travels along a straight path. However, when it
passes from one transparent medium to another, the direction of its path changes at the interface of the two
media. This is called refraction of light.
The phenomenon of the change in the path of the light as it passes from one transparent medium to another
is called refraction of light. The path along which the light travels in the first medium is called incident ray and
that in the second medium is called refracted ray. The angles which the incident ray and the refracted ray
make with the normal at the surface of separation are called angle of incidence.

It is observed that :
(i) When a ray of light passes from an optically rarer medium to a denser medium, it bends towards the normal.
(ii) When a ray of light passes from an optically denser to a rarer medium, it bends away from the normal.
(iii) A ray of light travelling along the normal passes undeflected.

50 Amazing Lines of Light Physics Science

Cause of Refraction :
Refraction occurs because the speed of light is different in different media. Light travels with a greater
velocity in a rarer medium like air and with lower velocity in a denser medium like glass or water.
The lower the velocity of light in the medium than that in air, the greater the bending because the greater
would be the need to take a shorter path.

REFRACTION THROUGH SPHERICAL LENSES :
A lens is a piece of transparent refracting material bounded by two spherical surfaces or one spherical and
other plane surface. In case of a lens, the light rays do not bounce off its surface, instead, the light rays pass
through it but in doing so they change their path i.e., they bend from their original path.
A lens is the most important optical component used in microscopes, telescopes, cameras, projectors etc.
Basically lenses are of two types :
(i) Convex lens or converging lens
(ii) Concave lens or diverging lens

50 Amazing Lines of Light Physics Science

(a) Convex Lens :
A lens which is thick at the centre and thin at the edges is called a convex lens. The most common form of a
convex lens has both the surfaces bulging out at the middle. When a convex lens is held close to an object, it
always forms an enlarged, virtual and erect image. In a convex lens, when rays parallel to principal axis falls
on lens after refraction they meet or appears to meet at a point. So due to this it is known is converging lens.

(b) Concave Lens :
A lens which is thin at the middle and thick at the edges is called a concave lens. The most common form of
a concave lens has both the surfaces depressed inward at the middle. When a concave lens is held close to an
object, it always forms a diminished, virtual and erect image. In a concave lens, when rays parallel to principal
axis falls on lens after refraction they move away from the lens. So due to this it is known is diverging lens.

50 Amazing Lines of Light Physics Science

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(c) Definitions in connection with Spherical Lens :
(i) Centre of curvature : The centre of curvature of the surface of a lens is the centre of the sphere of which
it forms a part, because a lens has two surfaces, so it has two centres of curvature. 
C1 and C2 are the centres of curvature.

(ii) Radius of curvature : The radius of curvature of the surface of a lens is the radius of the sphere of which
the surface forms a part. R1 and R2 represents radius of curvature.

(iii) Principal axis : It is the line passing through the two centres of curvature (C1 and C2) of the lens.

(iv) Optical centre : If a ray of light is incident on a lens such that after refraction through the lens the
emergent ray is parallel to the incident ray, then the point at which the refracted ray intersects, the principal
axis is called the optical centre of the lens. O is the optical centre of the lens.
If the radii of curvature of the two surfaces are equal, then the optical centre coincides with the geometric
centre of the lens.

(v) Focus point and focal length : When rays parallel to principal axis falls on lens after refraction they meet
or appears to meet at a point on principal axis, this point is known as principal focus. The distance between
optical centre and focus point is known as focal length.

50 Amazing Lines of Light Physics Science

SUNLIGHT WHITE OR COLOURED :
There are many phenomenons in which we can see sunlight is white or coloured. The followings are given
below :
(a) Rainbow :
White light is composed of seven colours, e.g., violet, indigo, blue, green, yellow, orange and red. Phenomenon of appearance of rainbow in the sky is a proof of seven colours. A rainbow appears usually after rain in
the morning or evening when the sun is low in the sky. It is formed when white light from the sun passes
through tiny prism-like water droplets and splits into different colours. -This phenomenon of splitting of white
light into several different component of colors is called dispersion of light. A rainbow is arch-shaped.

There are seven colours in the rainbow which gradually change from one to another. The set of colours formed on
splitting of white light is called the spectrum of white light. They are seen in the order VIBGYOR, i.e., violet,
indigo, blue, green, yellow, orange and red.
Since white light splits into seven colours, we should be able to get white light by mixing the seven colours of a
rainbow.

(b) Glass prism :
If we take a glass prism, and put a narrow beam of sunlight through a small hole in the window of a dark
room, so, we can easily see a colourful band on the paper or screen or wall.

This shows that light consists of seven colours. If we take another prism and place it in upside down position
with respect to first prism, we again get a beam of white light.

(c) Newton’s Disc :
The famous English physicist Sir Isaac Newton for the first time explained this phenomenon of mixing. To
prove his point, he made a disc on which the seven colours of the rainbow were painted in equal sections.

When’ this disc was rotated, all the colours disappeared and white colour appeared. This is
because when the disc spins fast, all the colours pass through a spot rapidly one followed by the other. This
gives the effect of putting all the colours together in one place. The colours mix and appear white. Such a disc
is popularly known as Newton’s disc.

On the basis of these experiments it is confirmed that white light is made of seven colours VIBGYOR.

50 Amazing Lines of Light Physics Science

EXTENDED LEARNING – ACTIVITIES AND PROJECTS
1. To observe the path of light :
• Take a chart paper and roll it into a pipe. Look at a lighted candle through it. Take
care not to touch the chart paper with the flame.
• Now bend the pipe from the middle and repeat the activity. Why are you not able
to see the lighted candle through the bent pipe?
Light travels in a straight line and does not bend with the pipe.

2. To prove that light travels in a straight line :
• Take three cardboards of similar size and make a hole in the centre of each. Make sure that the size and
position of the hole is the same in each cardboard .
• Now put the three cardboards in vertical position with the help of a brick of plasticine or clay in such a way
that the holes in the three cardboards are in a straight line.
• Place a lighted candle in front of the cardboards. Look through the hole of the last cardboard.
Are you able to see the flame of the lighted candle? What happens when you displace one of the cardboards?
The flame can be seen clearly through the holes in the first case. On diplacing one of the cardboards, no flame
can be seen as light cannot bend.
Light travels in a straight line. No flame can be seen when the holes of the three cardboards are not in a
straight line because light cannot bend.

3. To prove that smooth and shiny surfaces reflect light better :
• Place objects like a mirror, a shiny steel plate, a plastic plate or a sheet of paper near a screen which can
be a wall or a paper sheet.
• Shine a beam of light through a torch on these objects in such a way that reflected light falls on the screen
as shown in figure.
You will observe that in case of the plate mirror, a bright patch of light is seen on the screen. Similarly, in case
of the steel plate, a patch of light is visible on the screen. But when you use a paper sheet, no patch of light
can be seen on the screen.
Shiny and polished objects like steel plate, mirror, polished wood piece, etc. reflect more light and form a
brighter patch on the screen. Rough and dull surfaces, on the other hand, reflect less light and do not make
a patch on the screen.

50 Amazing Lines of Light Physics Science

4. To observe the reflection of light through a plane mirror :
• Take a torch and cover its glass portion with a chart paper having three slits.
• Put a plane mirror vertically on another chart paper fixed on a wooden board.
• Now switch ‘on’ the torch and direct the beam of light on the mirror through the slits at an angle.
• Put the torch in such a way that its reflected light is seen on the chart paper. You can do this activity in a
dark room also to have better results.

What do you observe? Is there any change in the direction of light rays which fall on the surface of ‘ the
mirror? Move the torch to either sides and see what happens. Do you find any change in the direction of
reflected light? Look into the mirror along the direction of the reflected light. Do you see the slits in the
mirror?
The beam of light gets reflected from the surface of plane mirror.

5. To observe the nature of an image formed by a plane mirror :
• Take a looking glass and place an object such as a candle or a matchbox or even a flower vase in front of
it.
• Note your observations.
Do you find that a similar object is placed behind the mirror ?
Now change the position of the object and observe the image carefully. Put a vertical screen or chart
paper behind the mirror and try to get the image of the object on it. Are you able to do so ?
Now place the same screen in front of the mirror and see whether you are able to get the image of the object
on it. Are you able to get the image on the screen in either case ? But why ?

• The image formed by the plane mirror cannot be taken on the screen. Thus, it is a virtual image.
• The image formed by plane mirror is erect.
• The image formed by plane mirror is of the same size as that of the object.

6. To observe that an image formed by a plane mirror interchanges sides :
• Stand in front of a looking glass (plane mirror) and observe your image. Also observe the distance
between you and your image from the mirror.
• Now raise your left hand and note which hand does your image raise.
• Touch your right ear with your left hand and observe your image. Do you find that in the image, the right
hand touches the left ear ?
• Write a name for example AMIT on a piece of paper and hold it in front of a plane mirror. Are you able
to read it ?

Now interchange the sides and see what happens ?
Image formed by a plane mirror interchanges sides, i.e., on a plane mirror, left appears right and right
appears left. This interchange of sides between the object and its image is called lateral inversion. Also the
image is as far behind the plane mirror as the object is in front of it.

7. To show that convex lens converges the rays of light whereas concave lens does not :
• Place a sheet of white paper in bright sunlight and adjust the position of the convex lens in such a way
that you are able to see a bright spot on the paper.
• Hold the lens in the same position for a few minutes. What do you observe?
You will find that the paper starts burning from that spot.

Now replace the convex lens by the concave lens and repeat the activity. Are you able to get a bright spot
on the paper this time also? Why not?
• Convex lens converges the rays of light from the sun at the focus of the lens and you see a sharp image
of the sun on the paper.
• Concave lens does not form the image of the sun on the paper as it diverges the rays of light.

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science

50 Amazing Lines of Light Physics Science