OPTICS

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 OPTICS

Light is a form of energy that makes us to (ii) Angle of incidence is always equal to the
see. An object reflects the light that falls on it. angle of reflection.
This reflected light, when received by our eyes,  Silver metal is one of the best
enables us to see. We are able to see through a reflectors of light.
transparent medium as light is transmitted  Laws of reflection apply to all kinds of
through it. The speed of light in vacuum or in air reflecting surface.
is 3 × 108 m/s Mirror:-
It is the branch of science in which we study Mirror is a polished surface like glass, which
about light and its properties, nature etc. Optics reflects almost all the light that is incident on it.
is classified into two types; ray optics and wave Types of Mirror
optics There are two types of mirror
1. Plane Mirror
Ray Optics If the reflecting surface of a mirror is plane, then
It deals with light rays linear propagation of light the mirror is called a plane mirror. Image formed
such as reflection, refraction, dispersion etc. by a plane mirror has following properties
i. It is always virtual and erect.
Wave Optics ii. The size of image is equal to the size of the
It deals with wave nature of light such as object.
polarization, diffraction etc. iii. The image formed is far behind the mirror as
the object is infront of it.
Properties of Light Iv Linear magnification produced by place mirror
(i) Light travels in a straight line. is 1.
(ii) A straight line drawn in the direction of v. The minimum size of the mirror required to
propagation of light is called a ray of light. see the full image of an observer is half the
(iii) A bundle of adjacent light rays is called height of the observer. If the plane mirror is
beam of light. rotated in the plane of incidence by an angle
(iv) The speed of light in vacuum is 3 × 108 m/s 𝜃, then the reflected ray rotates by angle 2𝜃
but it is different in different media. vi. Focal length of a plane mirror is infinity (i.g.,
(v) The speed and wavelength of light power is zero).
changes when it travels from one medium vii. If the object is displaced by a distance ‘𝛼’
to another but its frequency remains towards or away from the mirror, then its
unchanged. image will be displaced by a distance ‘𝛼’
towards or away form the mirror.
The sun, stars and other astronomical bodies in
the universe are the natural source of light. There 2. Spherical Mirror
are some artificial sources of light like electric A highly polished curved surface whose
bulbs. Candles etc. reflecting surface is a cut part of a hollow glass
sphere is called a spherical mirror.
Reflection of Light i. Concave Mirror:- The spherical mirror whose
The phenomenon of rebouncing back of light reflecting surface is inwards and outer surface is
rays in same medium on striking a smooth polished is called concave mirror.
surface, is called reflection of light. It is also called converging mirror because, it is
The laws of reflection are generally used to converge the beam of light after
(i) The incident ray, the reflected ray and reflection.
the normal to the reflecting surface at
the point of incidence all lie in the same
plane.

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 OPTICS

M1
Principal axis Focus

Centre of M2 M2 M2
Curvature

R R

ii. Convex Mirror:- A spherical mirror whose

Here, F= focal length, R= radius of curvature.
outer surface is reflecting the inner surface is
polished is called convex mirror. It is also called
Image
diverging mirror because it generally diverges
If light rays coming from a point after reflection
beam of light after reflection.
meet at another point or appear to come from
another point, then the second point is called the
Important terms related to spherical image of the first point.
mirror Types of Image
i. Centre of Curvature (C):- The centre of the There two types of image
sphere of which the mirror is a part, is called i. Real Image:- If the light rays coming from a
the centre of curvature of the mirror. point actually, meet after reflection. Then the
ii. Radius of Curvature (R):- The radius of the image formed is called a real image.
sphere of which the mirror is a part is called ii. Virtual Image:- If the light rays coming from a
the radius of curvature of the mirror. point, after reflection does not meet actually,
iii. Principal Axis:- The straight line joining the but appear to come from another point, then
pole and the centre of curvature of the mirror the image formed is called a virtual image.
and extended on both sides is called the  If half of the mirror is covered, the image
principal axis of the mirror. formed is complete but is intensity reduces
iv. Pole (p):- The central point of the reflecting (because less amount of light is reflected
surface of the mirror is called the pole of the from the mirror).
mirror.  The origin of multiple images is the multiple
v. Aperture:-The diameter of the reflecting reflection of light between the front and back
surface of spherical mirror is called its surfaces of glass. At the front surface of
aperture. glass, light is partially reflected and partially
vi. Focal Plane:- The plane perpendicular to the reflected and partially refracted. The
principal axis and passing through the refracted light gets reflected at the back
principal focus of the mirror is called the focal surface and then multiple reflections follow
plane of the mirror. within the thickness of glass, which is
vii. Focal Length (f):- The distance between the responsible for multiple images.
principal focus and pole of the mirror is called
the focal length of the mirror. If the aperture Image Formation by spherical Mirror
𝑅
of mirror is small then, 𝑓 − 2 . Image formation by a concave and a convex
viii. Principal Focus(F):- The point of the mirror is shown separately.
principal axis at which light rays parallel to the
principal axis, after reflection from the mirror, Image Formation By a Concave Mirror
actually meet or appear to come from, is The table given below illustrates the ray
called the principal focus of the mirror. diagrams along with the position and nature of

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 OPTICS

image, formed b concave mirror for various Between Between the Virtual,
infinity and the principal focus erect and
positions of the object. pole (i.g., at and the pole diminished
finite distance)
Position of Ray Diagram Position of Nature of the
the object the image image

At infinity At focus or in Real,

the focal Inverted
plane. extremely
diminished in Uses of Mirrors
size Uses of mirrors can be explained in different ways
i. Uses of Plane Mirrors
Beyond the Between Real,  Plane mirrors are commonly used as looking
Centre of focus and the Inverted
curvature centre of diminished
glass because the reflection that forms the
but at finite curvature image is always erect latterly inverted but they
distance are always virtual.
At the centre At the centre Real,
 Used in making periscopes which is used in
of curvature of curvature Inverted and submarines.
equal to the  Used at blind turns of some busy roads, to see
object
the vehicles coming from other side.
Between Beyond the Real,  They are used to make kaleidoscope, a toy
focus and centre of Inverted and which produces beautiful patterns from
centre of curvature bigger than
curvature object
coloured paper, pieces of glass or small
coloured beads.
At the focus At infinity Real, ii. Uses of Concave Mirrors
Inverted
extremely
 Concave mirrors are commonly used in
diminished torches, search lights and vehicles headlights
to get powerful parallel beams of light.
Between the Behind the Erect ,virtual
pole and mirror and  Concave mirrors are used as shaving mirrors
focus magnified to see larger image of the face.
 Dentists used concave mirrors to see large
images of the teeth.
Image Formation by convex mirror  Large concave mirrors are used to
concentrate sunlight to produce heat in solar
For studying the image formed by mirror, there
furnaces.
are two positions of the object. Firstly, when the iii. Uses of Convex Mirrors
object is at infinity and the second position is  Convex mirrors are commonly used as rear
when the object is at a finite distance from the view mirrors in vehicles because they always
mirror. The table given below illustrates the ray give an erect image and have wider field of
diagrams along with the position and nature of view as they are curved outwards.
image, formed by convex mirror for the above  Big convex mirrors are used as shop security
two positions of the object. mirrors; the shop owner can keep an eye on
the customers to look for thieves and
Position of Ray diagram Position of Nature and shoplifters among them.
Object image size of
image
Refraction of Light
At Infinity At the principal Virtual, Change in path of a light ray as it passes from one
focus erect and
extremely
medium to another is called refraction. When light
diminished travels from a rarer medium to a denser one, it bends
towards the normal (i< r) and when travels from
denser medium to a rarer one, it bends away from the
normal (i>r).

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 OPTICS

depends on the nature of two media and colour of

light.
Refractive index of denser medium (when rarer
medium is air)

(i>r) (i<r) 1
𝜇=𝑠𝑖𝑛𝐶

Here, i= angle of incidence r= angle of refraction Atmospheric Refraction

 A medium in which the speed of light is more The Earth’s atmosphere is not uniform throughout, its
is known as optically rarer medium and the density goes on changing as we move up or down. It
medium in which speed of light is lesser is can be considered to be consisted of layers of
known as optically denser medium. different densities, which acts as rarer or denser
medium with respect to one other. The refraction of
Cause of Refraction light due to these layers, is called atmospheric
Speed of light is different in different media. It is lesser refractions.
in denser medium and greater in rarer medium. So,
when light enters a denser medium, its speed Some Phenomena Based on Atmospheric
reduces and it bends towards the normal and when it Refraction
enters rarer medium, its speed increases and it bends
away from the normal. Twinkling of Stars:- The twinkling of a star is due to
atmospheric refraction of starlight. As the light from
the star enters the Earth’s atmosphere, it undergoes
refraction due to varying optical densities of air at
Refractive Index various altitudes. The continuously changing
The ratio of speed of light in vacuum to the speed of atmosphere refracts the light by different amounts. In
light in any medium, is called refractive index of the this way, the starlight reaching our eyes increases
medium. and decreases continuously and the star appears to
The refractive index of a medium relative to another twinkle at night.
medium, is known as the relative refractive index of
given pair of media. The Stars Seem Higher than They Actually
Appear:- As the light from a star enters the Earth’s
Laws of Refraction (Snell’s Law) atmosphere, it undergoes atmospheric refraction and
There are two laws of refraction bends towards the normal at position each time.
i. The incident ray, the refracted ray and the normal The upper layers of atmosphere are rarer than the
at the point of incidence all three lie in the same lower layers. The apparent position of the star is
plane. slightly different from its actual position. The star
ii. The ration of sine of angle of incidence to the sine appears slightly higher (above) than its actual
of angle of refraction remains constant of a pair of position, when viewed near the horizon.
media.
Scattering of Light
𝑠𝑖𝑛𝑖
= constant (1𝜇2) The reflection of light from an comparably smaller
𝑠𝑖𝑛𝑟
sized particle in all directions, is called scattering of
= light. The colour of scattered light depends on the size
There law is also called Snell’s law. The constant 1𝜇2 of scattering particles. Very fine particles scatter
is known as relative refractive index. mainly blue light while particles of larger size scatter
light of longer wavelength (red light). If the size of the
Critical Angle scattering particles is large enough then the scattered
The angle of incidence in a denser medium for which light may even appear white. The blue present in
the angle of refraction in rarer medium becomes 90o, sunlight is scattered 10 times more than the red light.
is called critical angle (C). The value of critical angle

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 OPTICS

Why is the Colour of the Sky blue? During the day ii. The angle of incident should be greater than critical
time sky appears blue. This is because the size of the angle for the two media.
particles in the atmosphere is smaller than the
wavelengths (blue end of spectrum). Practical Applications of Total Internal Reflection
When sunlight passes though the atmosphere, the 1. Optical Fibre
fine particles scatter the blue colour more strongly The working of optical fibre is based on total
than red. The scattered blue light enters our eye. internal reflection. Its inner part is core of
Hence, the sky appears blue. It should be noted that higher reflective index surrounded by another
the sky appears black to the passengers flying at layer of glass of lower refractive index. It is
higher altitudes because scattering of light is not surrounded by a plastic jacket.
prominent at such height due to the absence of When light enters from the one end of the core
particles. and moves towards cladding, then total
Colour of the Sun at Sunrise and Sunset:- At internal reflection takes place again and
sunrise and sunset, the Sun and they sky appears again, and light propagates through it. Optical
red. Light from the Sun near the horizon passes fibres are used in decorative table lamps.
through thicker layers of air and covers larger
distance in the atmosphere before reaching our eyes. Uses of Optical Fibre
Near the horizon most of the blue light and shorter i. These are used to send an electrical
wavelengths are scattered away by the particles. signal by transforming it into a light signal
Therefore, the light that reaches our eyes is of longer and vice-versa.
wavelengths. This gives rise to the reddish ii. These are used to send laser light rays
appearance. inside the human body.
iii. Today optical fibres are frequently used in
telecommunication.
iv. These are used in decorative table lamps.
v. These are used in networking, because
each fibre an can carry many signals,
each using a different wavelength of light.

2. Mirage
Mirage is the optical illusion of water appears in
desert in a hot summer day. In a hot summer day
in desert, the layers of air near the earth surface
remains hot and their temperature decreases with
altitude and become denser. When a ray of light
coming from the top of a tree or sky, moves
However at the noon, the light from the Sun overhead towards the earth and deviates away from the
would travel relatively shorter distance. So, it appears normal gradually. When angle of incidence
white as only a little of the blue and violet colour are becomes greater than critical angle, total internal
scattered. reflection takes place. After that light rays bend
upward. When, light rays enter the eyes of an
Total Internal Reflection (TIR) observer, inverted image of tree is obtained which
When a light ray, travelling from a denser medium produces illusion of water.
towards a rarer medium is incident at the interface at
an angle of incidence greater than critical angle, then
light rays are reflected back into the denser medium
(i.g., same medium). This phenomenon is called total
internal reflection.
Necessary conditions for total internal reflection to
take place are
i. The ray incident on the interface of two media
should travel from denser medium to rarer medium.

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 OPTICS

When light is incident on an object, it reflects only a

part of it. The reflected light gives the objects with
Cool air their colours.

Colours
Colours can categorised into following three
Hot air categories

Primary colours:- primary colours are sets of colours

that can be combined to make a useful range of
colours. e.g., red, green and blue are primary colours.

Secondary colours:- The colours which are

3. Diamond obtained by the mixing of two primary colours, are
Brilliance of diamond is mainly due to total internal called secondary colours.
reflection of light inside them. The critical angle for e.g., yellow, magenta and cyan are secondary
diamond air interface is very small, therefore once colours.
light enters the diamond, it is very likely to
undergo total internal reflection. Brilliance of Red+ Green= Yellow
diamond depends on tis cutting. By cutting the Red+ Blue= Magenta
diamond suitably, multiple total internal reflections Green+ Blue= Cyan
can be made to occur.

Colour of Object

Complementary Colours:- Those primary and  A convex lens is also known as converging
secondary colours which on mixing produce white lens because it converges a parallel beam of
colour, are called complementary colours. light rays passing through it
e.g., Red + Cyan = White  A double convex lens is simply called convex
Green + Magenta = White lens.
Blue + Yellow = White
2. Concaveor Diverging lens
Refraction by Spherical Lenses A lens which is thinner at the centre and thicker at its
Lens is a transparent medium bounded by two end, is called a concave lens. Concave lenses are of
surface of which, one or both surfaces are spherical. three types (as shown)
Lenses are of two types

1. Convex or converging lens

A lens which is thicker at the centre and thinner at its
end, is called convex lens.

(a)Double concave lens (b) Pleno-conceve lens (c)Convexo-concave lens

 A concave lens is also known as diverging

lens because it diverges a parallel beam of
(a)Double convex lens (b) Plano-convex lens (C) Convex lens light rays passing through it.

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 OPTICS

A double concave lens is simply called

concave lens. Posi t i on of
Object Image Image Ray Diagram
Image formation by Lenses
Location Image Nature Size
Image formation by a convex and concave lens is
given separately. Infinity At focus on Virtual Highly
Formation of Image by a Convex Lenses same side of and Erect Diminished
The table given below illustrates the ray diagrams lens as object
along with the position and nature of image, formed
Beyond Between focus Virtual Diminished
by convex lens for various positions of the object.
Infinity and and optical and Erect
Formation if Image by Convex Lens for Different
Zero centre on the
Positions of Objects
same side of
lens as object
Image formation by Convex Lens

Object Image Image Image Ray Diagram

location locatio size nature
n

At Infinity At F2 Diminis Real and

hed Inverted

Beyond 2 Betwee Small Real and

F1 n 2F2 size Inverted
and F2

Between At 2F2 Small Real and

2F1 and size Inverted
F1

At 2 F1 Beyon Magnifi Real and

d 2F2 ed Inverted

At F1 At Highly Real and

infinity Magnifi Inverted
ed

Between On the Magnifi Virtual

lens and same side ed and Erect
F1 as the
object

Formation of Image by a Concave Lens

The table given below illustrates the diagram along
with the position and nature of image, formed by
concave lens for the above two positions of the
object.
Formation of image by Concave Lens for Different
Positions of Object
Formation of Image by Concave Lens for Different
Positions of Object

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 OPTICS

1. The nature of Light? (b) Due to light absorption

(a) Like a particle (c) Due to total internal reflection
(b) Like wave (d) Due to some other inherent property
(c) Wave and particle both
(d) Neither particle and nor wave 11. An image is formed on the retina
(a) is equal to the size of the object, & inverted
2. The speed of light is: (b) Small from the object and erect.
(a) 9x102 m / s (b) 3x1011 m / s (c) Small from the object and inverted.
(d) is equal to the object and erect.
(c) 3x108 m / s (d) 2x104 m / s
12. The diffusion of light in the atmosphere is
3. Speed of light will be minimum in which due to
medium? (a) Carbon dioxide (b) Dust particles
(a) Glass (b) Vacuum (c) Helium (d) Water vapor
(c) Water (d) Air
13. The color of the light is determined, by it’s?
4. The time it takes for sunlight to reach the (a) Amplitude (b) Wave length
surface of the Earth is approximately (c) Intensity (d) Velocity
(a) 4-2 second (b) 4-8 second
(c) 8-5 minutes (d) 3-6 hours 14. Which of the following colors has the longest
wavelength?
5. When total internal reflection occurs, light (a) Green (b) Yellow
goes out (c) Blue (d) Red
(a) In glass from diamonds
(b) from Water in glass 15. The light with the shortest wave length is.
(c) from air to Water (a) Red (b) Yellow
(d) from air to Glass (c) Blue (d) Violet
6. The principle on which the optic fiber works 16. Which of the following has the highest
(a) Total internal reflection energy
(b) Refraction (a) Blue light (b) Green light
(c) Dispersion (c) Red light (d) Yellow light
(d) Interference
17. We see the sun a few minutes before the
7. How the signal flows in fiber optics actual sunrise, why?
communication (a) Scattering of light
(a) Light wave (b) Radio wave (b) Diffraction of light
(c) Microwave (d) Electrical wave (c) Total internal reflection of light
(d) Refraction of Light
8. "Endoscope" used by doctors to test the
stomach of a patient works on which of the 18. Red light is used for danger signals, because
following principles? (a) The scattering is the lowest.
(a) Works on light (b) It is comfortable for the eyes
(b) Scattering light (c) It has the least chemical effect.
(c) Light refraction (d) It is least absorbed by air.
(d) Total internal reflection of light
19. The sky appears blue, due to
9. The reason for Mirage is (a) Due to diffraction of light
(a) Interference of light (b) Due to reflection of light
(b) Scattering of light (c) Due to refraction of light
(c) Refraction of light (d) Due to scattering of light
(d) Total internal reflection of light
20. The Sun appears red at sunset
10. What makes a diamond brilliance? (a) Due to reflection
(a) Due to its molecular structure (b) Due to scattering

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(c) Due to refraction 30. A plane mirror has a minimum height to see
(d) Due to diffraction the whole image of a person.
(a) Equal to the height of the person
21. The color of the sky is visible to the (b) Half of the height of a person
astronaut. (c) One fourth height of a person's height
(a) Blue (b) White (d) Double of the height of a person
(c) Black (d) Red
Answer key
22. What type of mirror is used for visualization
of traffic, behind the car?
(a) Concave mirror 1 (c) 2 (c) 3 (a) 4 (c) 5 (a)
(b) Cylindrical mirror 6 (a) 7 (a) 8 (d) (d) (c)
(c) Convex mirror
(d) Plane mirror (c) (b) (b) (d) (d)
23. What type of mirror used as a shaving (a) (d) (a) (d) (b)
mirror?
(a) Concave mirror (b) Plane mirror (c) (c) (a) (d) (d)
(c) Convex mirror (d) none of the above
(b) (c) (b) (a) (b)
24. When an object is placed between two
parallel plane mirrors, the number of images
formed is?
(a) Two (b) one
(c) Six (d) infinite

25. The correct order of rainbow colors is

(a) Blue, Green, violet
(b) Violet, green, blue
(c) Blue, yellow, green
(d) Blue, green, yellow

26. The three colors are the primary colors.

These are
(a) Blue, yellow and red
(b) Blue, green and red
(c) Yellow, green and red
(d) Blue, Yellow and Green

27. Which lens is used to correct long

sightedness?
(a) Concave lens
(b) Convex mirror
(c) Convex lens
(d) Concave mirror

28. Myopia is corrected by?

(a) By using convex lens
(b) By using concave lens
(c) By using plane-concave lenses
(d) By using a plain glass

29. The minimum distance of clear vision is (cm).

(a) 25 (b) 5
(c) 75 (d) 100

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