Physics

NAME : SHALLINI R.V.
CLASS : X
SUB : SCIENCE
TOPIC :LIGHT-REFLECTION
AND REFRACTION

AGENDA
LIGHT
NATURE OF LIGHT
REFLECTION OF LIGHT
IMAGE
MIRROR
USES OF SPHERICAL MIRROR CONVEX
MIRRORS
USES OF PLANE MIRRORS
REFRACTION OF LIGHT
EFFECTS OF REFRACTION OF LIGHT
LAWS OF REFRACTION OF LIGHT

LIGHT
i) Light is a form of energy which helps us to see objects.
ii) When light falls on objects, it reflects the light and when the
reflected light reaches our eyes then we see the objects.
iii) Light travels in straight line.
iv) The common phenomena of light are formation of shadows,
formation of images by mirrors and lenses, bending of light by a
medium, twinkling of stars, formation of rainbow etc.

UNITS OF LIGHT
Any object which has an ability of producing light energy at its own is called as
the source of
denoted by letter “d”.
SOURCE OF LIGHT
Any object which has an ability of producing light energy at its own is called as
the source of
light. e.g., Sun, Firefly, Battery, Cell Candle, Bulb etc.
KINDS OF SOURCES OF LIGHT
There are two main kinds of sources of light viz:
1) Natural Sources:- These include all those sources which are natural and possess
their own
light for example stars, firefly etc.
2) Artificial sources:- These include all those sources which are man-made or
artificial but
possess the ability of emitting light for example candle, lantern, moon etc.

NATURE OF LIGHT
1. Dual Nature of Light
Light has dual nature, it behaves not only as a particle (photon) but also as a
wave. This is called dual nature of light.
2. Dispersion of Light
When a beam of sunlight falls on a prism, the light is split up in seven colours.
This phenomenon is called Dispersion of Light.
3. Rainbow
The rainbow is an arc of spectral colours formed across the sky during or after
rainfall in the morning or when the sun is behind us.
4. Photons (Quantum)
Photons are tiny packets of energy. They behave as particles but actually they are
not particles.

Newton’s Corpuscular Theory of Light
This theory which was proposed by Newton is as follows:
* Light is emitted from a luminous body in the form of tiny particles called corpuscles.
* The corpuscles travel with the velocity of light.
* When corpuscles strike the retina they make it sense light.
* Medium is necessary for the propagation of light.
* Velocity of light is greater in denser medium.
Wave Theory of Light
In 1676, Huygen proposed this theory. According to this theory:
* Light propagates in space in the form of waves.
* It can travel in space as well as in a medium.
* Light does not travel in a straight line but in sine wave form.
* Velocity of light is greater in rarer medium.
* Medium is not necessary for propagation.
Quantum Theory of Light
According to this theory of Max Plank:
* Light is emitted from a source discontinuously in the form of bundles of energy called
Photons or Quantum.
* It travels in space as well as a medium.
* Speed of light is greatest in space or vacuum.

REFLECTION OF LIGHT
When light falls on a highly polished surface like a mirror most of the light is
sent back into the same medium. This process is called reflection of light.
Laws of reflection of light :-
i) The angle of incidence is equal to the angle of reflection.
ii) The incident ray, the reflected ray and the normal to the mirror at
the point of incidence all lie in the same plane.

RAY AND BEAM
RAY:-
A line drawn in the direction of propagation of light is called ray.
BEAM:-
A groups of rays of light emitted from the source of light is called a beam of light.
A beam of light is three types:-
Parallel beam
Convergent beam
Divergent beam.

PARALLEL BEAM:
A group of rays of light is parallel to each other is known as parallel beam of
light.
CONVERGENT BEAM :
A group of light rays meeting at a same point is called a convergent beam of
light.
DIVERGENT BEAM :
A group of light rays spreading out from a source of light is called divergent beam of
light.

IMAGE
When light rays meet or appear to meet after reflection
from a mirror then it is called image.
Types:
 Real image
 Virtual image

REAL IMAGE:
It is a kind of image which is formed by the actual intersection of light rays after
reflection.
VIRTUAL IMAGE:
It is a kind of image which is formed by producing reflected rays backward after
reflection.

CHARACTERISTICS OF IMAGE:
i) The image is erect.
ii) The image is same size as the object.
iii) The image is at the same distance from the mirror as the object is in
front of it.
iv) The image is virtual (cannot be obtained on a screen).
v) The image is laterally inverted.

MIRROR
A mirror is an object that reflects light in such a way that, for incident light in some range
of wavelengths, the reflected light preserves many or most of the detailed physical
characteristics of the original light.
This is different from other light-reflecting objects that do not preserve much of the
original wave signal other than colour and diffuse reflected light.
TYPES OF MIRROR:
Plane mirror
Spherical mirror

PLANE MIRROR:
Plane mirror is a pieces of glasses whose one side is polished by using silver paint which is
covered by a coating of red paint to protect the silver layer.
SPHERICAL MIRROR:
Spherical mirror is a curved mirror which is a part of a hollow sphere. Spherical mirrors are
of two types. They are concave mirror and convex mirror.
i) Concave mirror :- is a spherical mirror whose reflecting surface is curved inwards. Rays of
light parallel to the principal axis after reflection from a concave mirror meet at a point
(converge) on the principal axis.
ii) Convex mirror :- is a spherical mirror whose reflecting surface is curved inwards. Rays of
light parallel to the principal axis after reflection from a convex mirror get diverged and
appear to come from a point behind the mirror.

TERMS USED IN THE STUDY OF SPHERICAL MIRRORS :-
i) Center of curvature :- is the centre of the sphere of which the mirror
is a part (C).
ii) Radius of curvature :- is the radius of the sphere of which the mirror
is a part (CP).
iii) Pole :- is the centre of the spherical mirror (P).
iv) Principal axis :- is the straight line passing through the centre of
curvature and the pole (X-Y).
v) Principal focus :-
In a concave mirror, rays of light parallel to the principal axis after
reflection meet at a point on the principal axis called principal
focus(F).
In a convex mirror, rays of light parallel to the principal axis after
reflection get diverged and appear to come from a point on the
principal axis behind the mirror called principal focus (F).
vi) Focal length :- is the distance between the pole and principal focus
(f). In a spherical mirror the radius of curvature is twice the focal
length.
R = 2f or f = R
2

Reflection by spherical mirrors :-
In a concave mirror a ray of light parallel to the principal axis after
reflection passes through the focus.
In a convex mirror a ray of light parallel to the principal axis after
reflection appears to diverge from the focus.
C F P P F C

ii) In a concave mirror a ray of light passing through the
focus after reflection goes parallel to the principal axis.
In a convex mirror a ray of light directed towards the focus after
reflection goes parallel to the principal axis.
C F P P F C

iii) In a concave mirror a ray of light passing through the
centre of curvature after reflection is reflected back along
the same direction.
In a convex mirror a ray of light directed towards the centre
of curvature after reflection is reflected back along the same
direction.
C F P P F C

iv) In a concave or a convex mirror a ray of light directed
obliquely at the pole is reflected obliquely making equal
angles with the principal axis.
C F i P i P F C
r r

Images formed by concave mirror :-
C F P

Images formed by convex mirror :-
i) When the object is at infinity, the image is formed at F
behind the mirror, it is highly diminished, virtual and erect.
P F

ii) When the object is between infinity and pole, the image
is formed behind the mirror, it is diminished, virtual and
erect.
P F C

Uses of spherical mirrors :-
Concave mirrors :-
Concave mirrors are used in torches, search lights and head lights of
vehicles to get parallel beams of light.
They are used as shaving mirrors to see larger image of the face.
They are used by dentists to see larger images of the teeth.
Large concave mirrors are used to concentrate sunlight to produce
heat in solar furnaces.

Convex mirrors :-
Convex mirrors are used as rear-view mirrors in vehicles. Convex
mirrors give erect diminished images of objects. They also have a
wider field of view than plane mirrors.

New Cartesian sign convention for spherical mirrors :-
i) The object is always placed on the left of the mirror and light from the
object falls from the left to the right.
ii) All distances parallel to the principal axis are measured from the pole.
iii) All distances measured to the right of the pole are taken as + ve.
iv) All distances measured to the left of the pole are taken as – ve.
v) The height measured upwards perpendicular to the principal axis is
taken as + ve.
vi) The height measured downwards perpendicular to the principal axis is
taken as – ve.
Direction of incident light
Distance towards the left ( - ve
)
Distance towards the right ( + ve )
Height
downwards ( - ve )
Height
upwards ( + ve )
Concave mirror
Object
Image

Mirror formula for spherical mirrors :-
The mirror formula for spherical mirrors is the relationship between
the object distance (u), image distance (v) and focal length (f).
The mirror formula is expressed as :-
1 1 1
+ =
v u f
Magnification for spherical mirrors :-
Magnification for spherical mirrors is the ratio of the height of the
image to the height of the object.
Height of the image hi
Magnification = m =
Height of the object ho
The magnification is also related to the object distance and image
distance. It is expressed as :-
hi v
Magnification m = =
ho u

Refraction of light :-
When light travels obliquely from one transparent medium into
another it gets bent. This bending of light is called refraction of light.
When light travels from a rarer medium to a denser medium, it bends
towards the normal.
When light travels from a denser medium to a rarer medium to a
rarer medium, it bends away from the normal.
Denser medium Rarer medium
Rarer medium Denser medium
Normal Normal

Refraction of light through a rectangular glass slab :-
When a ray of light passes through a rectangular glass slab, it gets
bent twice at the air- glass interface and at the glass- air interface.
The emergent ray is parallel to the incident ray and is displaced
through a distance.
i
e
Normal
Incident ray
Emergent ray
Refracted ray
Glass
Air
Normal
r
Glass
Air
Rectangular glass slab
displacement
Angle of emergence
Angle of incidence
Angle of refraction

1. coin inside a glass filled with water looks at closer distance.
2. A fish in pond or aquarium, actual position is different than what it looks.
3. Occurrence of mirage effect in deserts in summer.
4. Pencil partially dipped in glass filled with water seems to be bent at point of
immersion.
5. We see sun before it actually come to horizon during sunrise and see it even
after sunset.
6. paper weight having bubble inside looks bigger and moving with rotation.
7. Twinkling of star- light reaching to us has gone several refraction because of
mediums.
8. looking through a transparent glass filled with water makes everything
magnified.
9. similarly if you look through transparent ruler things with be magnified.
10. sparkling of diamond - refraction of light at each side
11. passing of light through fiber.- total internal refraction inside fiber
12. Tyndall effect on stream of water.- light follow the path of water
13. passing of light through prisms. -you will see spectrum
EFFECTS OF REFRACTION OF LIGHT

Laws of refraction of light :-
i) The incident ray, the refracted ray and the normal to the
interface of two transparent media at the point of incidence, all lie in the
same plane.
II) The ratio of the sine of angle of incidence to the sine of angle
of refraction is a constant, for the light of a given colour and for the
given pair of media.( This law is also known as Snell`s law of refraction.)
sine i
= constant
sine r
Refractive index :-
The absolute refractive index of a medium is the ratio of the
speed light in air or vacuum to the speed of light in medium.
Speed of light in air or vacuum c
Refractive index = n =
Speed of light in the medium v
The relative refractive index of a medium 2 with respect to a
medium 1 is the ratio of the speed of light in medium 1 to the speed of
light in medium 2.
n
21
= Speed of light in medium 1 n 21
= v
1 / v2
Speed of light in medium 2

A spherical lens is a transparent material bounded by two surfaces
one or both of which are spherical.
Spherical lenses are of two main types. They are convex and concave
lenses.
i) Convex lens :- is thicker in the middle and thinner at the edges.
Rays of light parallel to the principal axis after refraction through a
convex lens meet at a point (converge) on the principal axis.
ii) Concave lens :- is thinner in the middle and thicker at the edges.
Rays of light parallel to the principal axis after refraction get diverged
and appear o come from a point on the principal axis on the same side
of the lens.
F F

Refraction by spherical lenses
:-i) In a convex lens a ray of light parallel to the principal
axis after refraction passes through the focus on the other
side of the lens. In a concave lens it appears to diverge
from the focus on the same side of the lens.
2F1 F1 O F2 2F2 2F1 F1 O F2 2F2

ii) In a convex lens a ray of light passing through the focus
after refraction goes parallel to the principal axis. In a
concave lens a ray of light directed towards the focus after
refraction goes parallel to the principal axis.
2F1 F1 O F2 2F2 2F1 F1 O F2 2F2

iii) In a convex lens and concave lens a ray of light passing
through the optical centre goes without any deviation.
2F1 F1 O F2 2F2 2F1 F1 O F2 2F2

Images formed by convex lens :-
i) When the object is at infinity the image is formed at the
focus F2, it is highly diminished, real and inverted.
2F1 F1 O F2 2F2

ii) When the object is beyond 2F1, the image is formed
between F2 and 2F2, it if diminished, real and inverted.
2F1 F1 O F2 2F2

iii) When the object is at 2F1, the image is formed at 2F2, it
is the same size as the object, real and inverted.
2F1 F1 O F2 2F2

iv) When the object is between 2F1 and F1, the image is
formed beyond 2F2, it is enlarged, real and inverted.
2F1 F1 O F2 2F2

v) When the object is at F1 the image is formed at infinity, it
is highly enlarged, real and inverted.
2F1 F1 O F2 2F2

vi) When the object is between F1 and O, the image is
formed on the same side of the lens, it is enlarged, virtual
and erect.
2F1 F1 O F2 2F2

i) When the object is at infinity, the image is formed at the
focus F1 on the same side of the lens, it is highly
diminished, virtual and erect.
F1 O

ii) When the object is between infinity and F1, the image is
formed between F1 and O on the same side of the lens, it is
diminished, virtual and erect.
FI O

The sign convention for spherical lenses is the same as in
spherical mirrors except that the distances are measured from the
optical centre (O).
The focal length of a convex lens is positive ( + ve ) and the focal
length of a concave lens is negative ( - ve ).
O
Direction of incident light
Distance towards the left (- ve )
Height
downwards ( - ve )
Height
upwards ( + ve )
Convex lens
Object
Image
Distance towards the right ( + ve )

The lens formula for spherical lenses is the relationship between the
object distance (u), image distance (v) and focal length (f).
The lens formula is expressed as :-
1 1 1
=
v u f
Magnification for spherical lens is the ratio of the height of the image to
the height of the object.
Height of the image hi
Magnification = m =
Height of the object ho
The magnification is also related to the object distance and image
distance. It can be expressed as :-
hi v
Magnification m = =
ho u

The power of a lens is the reciprocal of its focal length
(in metres).
I 1
P = or f =
f (m) P
The SI unit of power is dioptre (D).
1 dioptre is the power of a lens whose focal length is 1
metre.
The power of a convex lens is positive ( + ve ) and the
power of a concave lens is negative ( - ve ).

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