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CHAPTER – 10

LIGHT-REFLECTION
& REFRACTION
Light is a form of energy, which enable us to see the object
...
e
...

Reflection of Light
When the light is allowed to fall on highly polished surface, such as mirror, most of
the light gets reflected
...


2
...

—
i=—
r
The incident ray, reflected ray and the
normal to the reflecting surface at the
point of incidence lie in the same plane
...
)

2)

Laterally inverted (The left side of object appear on right side of image)

3)

The size of image is equal to that of object
96

The image that do not form on

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4
...


Reflection of light by spherical Mirrors
Mirrors, whose reflecting surface are curved inward or outward spherically are
called spherical mirror
...

If it is curved inward fi
Act as concave mirror
If it is curved outward fi
Act as a convex mirror
...


Principal axis : Line joining the pole and centre of curvature of the spherical
mirror
...


Pole : The geometrical central point of the reflecting spherical surface
...


3
...


4
...
It has a centre, which is known as centre of curvature, denoted by
(C)

5
...
ie
...


Focus point : The point on the principal axis, where all parallel rays meet
after reflection, denoted by (F)

7
...
e
...


Relationship between focal length and Radius of curvature
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i

C

P

—
i=—
r

r

F

—
i
—
r

99

—
i=—
r
F

C

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Note : A ray of light passes through centre of cus-valerie reflecting spherical
surface is always act as normal at the point of incidence
...
Image
formation by a concave mirror for different position of the object
1
...


Object
Beyond C

F

Size of
Image
Highly diminished
(point size)

A
object

B

B1

P

F

C image

—
i
—
r

Object
At C

A
B1 B

P
F

Position of
Image
Between F&C

Nature
Real and
Inverted

Size of
Image
Small

A1

3
...


Object
Between C&F

—
i=—
r
Position of
Nature
—
i
Image
P
Real and
—
r
Beyond C
Inverted

A
Object
B1

B

C

F

Image
A

5
...


Object
Between F&P
(Special Case)

A

C

F

P
—
i
r
B —

B1

Position of Image
Behind the mirror
Size of Image
Enlarged

Nature
Virtual
and
Erect

Image formation by Convex Mirror
1
...


Object
Anywhere between
infinity and pole
of the mirror

A
A1

P
B

B1

Position of Image
Between P & F

Size of Image
Very small

F

Nature
Virtual & erect

Uses of Concave Mirror
1
...


2
...


Used by dentist to see large images of the teeth

4
...


Uses of Convex Mirror
1
...
It also helps
the driver to view large area
...


The object is always placed to the left side of mirror
...


All distance should be measured from pole (P); parallel to principal axis
...


Take 'P' as origin
...


1
1
1
F = v + u
R
where f = 2

MAGNIFICATION
It is expressed as the ratio of the height of the image to height of the object
m=

height of image h1
=
height of object h

1

It is also related to 'u' and 'v'
–v
m= u
2
\
from 1 and 2 equation
1

m=

h
h

1
image height from principle axis
– v where h fi
1
= u
h fi
Object height from principle axis
...

A ray of light bends as it moves from one medium to another

103

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Refraction is due to change in the speed of light as it enters from one transparent
medium to another
...

normal

normal
Incident
Ray

Denser medium

Raver medium
Denser medium

Rarer medium

Refracted Ray

When ray travel from Rarer to Denser it bends When ray travel from denser to
towards normal after refraction
rarer medium it bends away
from normal
Some Commonly observed phenomenon due to Refraction
1
...


2
...


3
...


Refraction through a Rectangular Glass Slab
A

N

Incident ray

Air (Rarer Medium)

i1
K

L
O
r1
i2 N

N

Here light ray changes is
1
direction at O and O , the
point at the interface of
transparent medium
...
O on interface AB, it will bends towards the normal
...
If the incident ray is extended to C, we will observe that
emergent ray O1B is parallel to incident ray
...

Note : When a ray of light is incident normally to the interface of two media it will
go straight, without any deviation
...


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
...


The ratio of sine of angle of incidence to the sine of angle of refraction is a
constant ie
...
It is the refractive
index of the second medium with respect to first medium
...

ng
Speed of light in air
c
nga = n =
= v
a
Speed of light in glass
C fi
Speed of light in vacuum = 3·
108 m/s
speed of light in air is marginally less, compared to that in vacuum
...
33
Refractive index of glass (ng) = 1
...

CONVEX LENS
A lens may have two spherical surfaces, bulging outwards, is
called double convex lens (or simply convex lens
...

CONCAVE LENS
A lens bounded by two spherical surfaces, curved inwards is
known as double concave lens (or simply concave lens)
It is also known as diverging lens because it diverges the light
...

R
Principal
Axis

C1
or (2F1)

f
O

F1

F2
Optical
centre (O)

R
Principal
Axis

C1

C2
or (2F2)

Optical centre (O)
O

F1

F2

C2

Convex
Lens

Concave
Lens

f

C1

106

O

C2

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1
...
Each of these surfaces form a part of sphere
...


2
...


Optical Centre - The central point of lens is its optical centre (O)
...
e
...


4
...


5
...


O

F1

Principal
Axis

F2

The distance OF2 and OF1 is called as focal length
Tips for drawing Ray diagram
a)

After refraction, a ray parallel to principal axis will pass through F
...

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