Light_Reflection and Refraction-converted.pptx

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Light_Reflection and Refraction-converted.pptx

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LIGHT


REFLECTION AND

REFRACTION

Notations
Focal
length
Ma
g
n
i
f
i
cati
o
n

Distance
of the
object from
the
mirror
or
lens
Distance
of the
image from
the
mirror
or

lens

Size
of the
object (say,

height)

Size
of the
image

Least
distance
of
distinct vision
Pole
of
mirror

Optical
centre
of a

lens

f
m
u
v

h
1
or

ho

h
2
or
hi
D

P
O

Light:

(i)
It
is
an
invisible energy, which
causes the
sensation
of
sight.

(ii)
It

is

the

form

of

energy,

which

gives

in

us

a

sensation

of

sight.

It,

itself

is

not

visible
but
helps
us
in seeing

objects.

Laws
of

Reflection:

(i)
Angle
of
incidence is equal
to the
angle
of
reflection.
(


i
=


r
)

(ii)
The
incident
ray, the reflected ray and the
normal
at the
point
of incidence,
all
lie
in
the
same

plane.


Total
internal

reflection:

This

is

a

phenomenon

when

the

surface

of

a

refracting

medium

behaves

as

a

reflecting
medium (for certain angles
of

incidence).


Laws
of
refraction:

(i)

The
ratio
of
sine
of the
angle
of
incidence
to the
sine
of the
angle
of
refraction

for

sin

r

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a
particular pair
of
media is constant, i.e.,
sin
i
=
constant
=

.
This is also called

Refractive

Index and
also denoted
by

n
.

(ii)

The

incident

ray,

the

refracted

ray

and

the

normal

at

the

point

of

incidence

all

lie

in

the

same

plane
.


Refraction

through

prism
:

When

light

passes

through

a

prism

(i)
It

always

bends

towards

the

base

of

the

prism
.

(ii)
A

prism

splits

the

light

passing

through

it

into

its

corresponding

wavelengths
.

This

process

is

called

dispersion

of

light
.

When

sunlight

passes

through

prism

it

disperses

into

seven

colours,

i
.
e
.
,

seven

wavelengths

(VIBGYOR)
.


Sign
convention
for
spherical mirrors
and

lenses:

According to
the
new
Cartesian sign

convention:

(i)

The
pole ‘
P
’ of
the
mirror is taken as
the
origin and
the
principal axis of
the

mirror

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is
taken as the x
-
axis of
the coordinate

system.

(ii)
The object
is always placed
to the
left
of
the
mirror i.e.
the
light
(incident rays)
from
the
object falls
on
the mirror from
the
left hand

side.

(iii)
All

the

distances

parallel

to

the

principal

axis

of

the

spherical

mirrors

are

measured
from
the
pole ‘
P

of
the
mirror.

(iv)
All
the distances measured to the
right
of
the
origin (along
+ve
x
-
axis)
are taken as

positive.

(v)
All
the distances
measured
to the
left
of the
origin (along

ve
x
-
axis) are taken

as
negative.

(vi)
The distances
(heights) measured upwards (i.e.
above the
x
-
axis)
and
perpendicular
to the
principal axis
of the
mirror are
taken as

positive.

(vii)
The distances (heights) measured downwards (i.e. below
the
x
-
axis)
and
perpendicular
to the
principal axis
of the
mirror are
taken as

negative.

The
following illustrates all
the
points
of the
new
Cartesian
sign convention

stated

above.




P

M

B

A

X

Principal

Axis

Distances

along

Incident
light

(+)

Y

Y





incident light

(

)

Direction

of

Incident
light
Distances

against




D

Upward
Height

(+)

X


B




D
o
w
n
w
a
r
d
Height

(

)

N


Origin
of
Cartesian plane is similar
to
pole
of
mirror
and
optical centre
of

lenses.

y

+ve

+
v
e


ve

x


ve


According

to

the

sign

convention

for

mirrors,

the

focal

length

of

a

concave

mirror

is

negative

and

that

of

a

convex

mirror

is

positive
.

The

focal

length

of

convex

lens

is

positive
.

While

that

of

a

concave

lens

is

negative
.

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SPHERICAL MIRRORS

IMAGE FORMATION BY
A
CONCAVE MIRROR
FOR
DIFFERENT POSITIONS

OF

THE

OBJECT

Position of

the
object

Position of

the
image

Size
of
the

image

Nature
of

the
image

At

infinity

At
the focus

F

Highly

diminished,
point

sized

Real
and

inverted

Beyond the

centre
of
curvature
C

Between
F
and

C

Diminished

Real
and

inverted

At

C

At

C

Same
size

Real
and

inverted

Between
C
and

F

Beyond

C

Enlarged

Real
and

inverted

At

F

At

infinity

Infinitely large
or
highly

enlarged

Real
and

inverted

Between the
pole

P
of the
mirror
and
focus F

Behind
the

mirror

Enlarged

Virtual
and

erect

IMAGE FORMATION BY
A CONVEX

MIRROR

Position of the

object

Position of the

image

Size
of
the

image

Nature
of
the

image

At

infinity

At
the focus
F
,

behind
the

mirror

Highly

diminished
point
-
sized

Virtual
and

erect

Between
infinity and

the
pole
P
of the

mirror

Between
P
and
F
,

behind
the

mirror

Diminished

Virtual
and

erect

The
corresponding
ray
diagrams
are
included in annexure

4

Mirror
Formula
:
1


1


1
is called
the
mirror

formula.

f

v

u

Magnification:
The
ratio
of the
size
of the
image
to that of the
object is
called
magnification.
For a
mirror, magnification (
m
) is given

by.

m




v

u


LENSES

Lens
:

A

piece

of

any

transparent

material

bound

by

two

curved

surfaces

is

called

a

lens
.

A

lens

which

is

thicker

in

the

middle

and

thinner

at

the

edges

is

called

a

convex

lens
.

A

convex

lens

is

also

called

converging

lens
.

A

lens

which

is

thicker

at

the

edges

and

thinner

at

the

centre

is

called

a

concave

lens
.

A

concave

lens

is

called

a

diverging

lens
.

Optical

center

of

a

lens
:

The

centre

point

of

a

lens

is

called

its

optical

center
.

A

ray

of

light

passing

through

the

optical

center

does

not

suffer

any

deviation
.

Power

of

a

lens
:

Reciprocal

of

the

focal

length

of

a

lens

measured

in

meters

is

called

the

power
.

Power

of

a

lens

is

described

in

dioptre

(
D
)

units
.

Images

formed

by

a

lens
:

A

convex

lens

forms

a

real

and

inverted

image

for

all

the

positions

of

an

object

outside

its

focus

(
F
)
.

However,

when

the

object

is

placed

between

F

and

O
,

the

image

formed

by

a

convex

lens

is

virtual

and

erect
.

A

concave

lens

always

forms

a

virtual,

erect

and

a

diminished

image,

whatever

may

be

the

distance

of

the

object

from

the

lens
.

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f

v

u

Lens
formula:
1


1


1

u

Magnification
:
m
=

v

IMAGE FORMATION BY
A
CONCAVE

LENS

Position of

the
object

Position of

the
image

Size
of

the
image

Nature
of

the
image

At

infinity

At focus
F
1
point
-

sized

Highly

diminished

Virtual
and

erect

Between

infinity

and

optical

centre

O

of

the

lenses

Between focus
F
1
and
optical

centre

Diminished

Virtual
and

erect

IMAGE FORMED BY
A
CONVEX LENS
FOR
DIFFERENT POSITIONS
OF THE
OBJECT

Position of

the
object

Position of

the
image

Size
of
the

image

Nature
of

the
image

At

infinity

At focus

F
2

Highly

diminished,
point
-
sized

Real
and

inverted

Beyond

2F
1

Between
F
2
and

2F
2

Diminished

Real
and

inverted

At

2F
1

At

2F
1

Same
size

Real
and

inverted

Between
F
1
and

2F
1

Beyond

2F
2

Enlarged

Real
and

inverted

At focus

F
1

At

infinity

Infinitely large

or
highly

enlarged

Real
and

inverted

Between
focus F
1
and optical
centre

O

On the
same
side
of
the
lens
as
the

object

Enlarged

Virtual
and

erect

The
corresponding
ray
diagrams
are
included in annexure

4


IMPORTANT FORMULAE

Mirror

formula

f

v

u

1


1


1

where,
f
= focal
length
of
mirror,
u
=
Distance
of
the object,
v
=
Distance of
the
image
from

pole.

Lens

formula

f

v

u

1


1


1

where,

f

=

focal

length

of

the

lens,

v

=

Distance

of

the

image,

u

=

Distance

of

the

object
from optical

centre.

Power
of

lens

Focal
length

(in

meters)

Focal

length

(in

cm)

1

1
0
0



P



1

1
0
0

f

(
m
)

f

(
cm
)

P





size of
the

object

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Magnification
by
a
lens
=
size of
the

image

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m


h
i

h
o

Magnification
by
a
lens
=
Distance
of
the
image from
the
optical
centre
or
m


v

Distance

of

the

object

from

the

optical

centre

u


REFRACTIVE

INDEX



Absolute
refractive
index (n)
of a
medium is
the
ratio
of
speed of
light in vacuum
or

air

(c) to the speed of
light
in
the
medium
(v)

i.e.

n


c

v


Refraction

of

light

is

the

phenomenon

of

change

in

the

path

of

light

in

going

from

one

medium
to

another.


In

going

from

a

rarer

to

a

denser

medium,

the

ray

of

light

bends

towards

normal

and

in going
from
a
denser
to a rarer medium, the ray
of
light
bends
away from

normal.


Snell’s law of

refraction:

sin
i


n
2


1
n
2

sin

r

n
1


No refraction occurs,

when

(i)
light is incident normally
on a

boundary,

(ii)
refractive
indices
of the
two
media in
contact are

equal.



Refractive index
=
n
21
=
speed
of
light
in
medium
1


refractive
index
of
medium

2

speed

of

light

in

medium

2

refractive
index
of
medium

1

31

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21

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