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LIGHT :
REFLECTION
AND
REFRACTION
LIGHT



Light is a form of electromagnetic radiation
that causes the sensation of sight.
It is an indispensable tool without which we
cannot explore the colorful beauty of nature.
The blue sky, the rainbow, the red of the
sunrise and sunset, the twinkling of stars, the
radiance of sparkling diamonds and pearls,
and shining color of gems are just some of the
natural wonders of light and color.
NATURE OF LIGHT
 Light
is an electromagnetic wave.
 These waves do not require any medium
for their propagation.

The wavelength of visible light waves is very
small, only about 4 x 10-7m to 8 x 10-7m
 The
speed of light wave depends on the
nature of the medium through which they
pass.
 The speed of light waves in vacuum is
very high, being 3 x 108 m/s.
Light
provides us means of
communication.
The fibre-optic cables
consisting of many glass
fibres transmit hundreds of
telephone conversations
over long distances.
REFLECTION OF LIGHT
 When
light falls on the surface of an
object, it may be
 i) absorbed ii) transmitted iii) reflected
 When light falls on the surface of an
object, some of it is sent back. The
process of sending back the light rays
which fall on the surface of an object, is
called reflection of light.
IMAGES
An image is formed when the light rays coming
from an object meet ( or appear to meet) at a
point, after reflection from a mirror ( or
refraction from a lens).
 Real Image – If the light rays actually meet
after reflection or refraction is called real
image. It can be obtained on a screen.
 Virtual Image – If the light rays appear to meet
after reflection or refraction, then it is called
virtural image. It can’t be obtained on a screen.

LAWS OF REFLECTION OF
LIGHT
 FIRST
LAW – The angle of
incidence (i) is equal to the angle
of reflection (r )
 SECOND
LAW – The incident ray, the
normal to the mirror at the point of
incidence, and the reflected ray, all lie
in the same plane.
FORMATION OF IMAGE IN A
PLANE MIRROR
 The
characteristics of image formed in a
plane mirror are
 i) image is virtual
 ii) image is erect
 iii) image is of the same size of the object.
iv) image is formed as far behind the
mirror, as the object is in front of it.
 v) image is laterally inverted.
SPHERICAL MIRROR
A spherical mirror is that mirror whose
reflecting surface is the part of a hollow sphere
of glass.
 It is of two types :
 i) concave mirrors
 ii) convex mirrors

RULES FOR FORMATION OF IMAGE
BY SPHERICAL MIRRORS

When an object is placed before a spherical
mirror, an image is formed. The image is
formed at that point where at least two
reflected rays intersect (or appear to
intersect).

Now to find out the position of an image
formed by a concave mirror, only two rays
light is required. We use those rays whose
path is certain, the diagram formed in this
way is called as ray diagram.
Continued…
To draw ray diagram, the following rules are
used:
 i) A ray of light parallel to the principal axis of
the mirror, passes through the focus after
reflection from the mirror.
 ii) A ray of light passing from the center of
curvature of the mirror is reflected back along
the same path.
 iii) A ray of light passing through the focus of a
concave mirror becomes parallel to the principal
axis.
 An image of any point is formed at that point
where at least two reflected rays intersect or
appear to intersect.

IMAGE FORMATION BY CONCAVE
MIRROR
position of
the object
Position of
the image
Size of the
image
At infinity
At the focus F Highly
Diminished
Real and
Inverted
Beyond 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
Real and
Inverted
Between P
and F
Behind the
mirror
Highly
enlarged
Enlarged
Nature of the
image
Virtual and irect
IMAGE FORMATION BY
CONVEX MIRROR
Position of the Position of the Size of the Nature of
object
image
image
the image
At infinity
At the focus F, Highly
Virtual
behind the
diminished, and erect
mirror
point size
Between
infinity and
the pole of
the mirror
Between p
and F, behind
the mirror
diminished Virtual
and erect
NEW CARTESIAN SIGN
CONVENTION
Objects on the left
A
Height upwards
(+ve)
X’
M
Y
Direction of
Incident light
Direction against
Incident light (-ve)
P
Distance along
Incident light (+ve)
B’
B
Height downwards (-ve)
Height downwards (-ve)
A’
N
Height downwards (-ve)
Y’
X
MIRROR FORMULA
M
A
h
B’
B
C
u
F
h’
P
f
A’
v
N
Fig 2.3
R
FORMATION OF IMAGE BY
CONVEX MIRROR
M
A
D
E
B
A’
B’ F
P
N
C
 We
will now obtain a relation between the
object-distance (u), the image –distance (v)
and the focal length (f) of the spherical
mirror having small aperture (much less than
the radius of curvature (R). This relation is
called Mirror Formula. It remains the same in
all types of physical situations, whether the
image is real or virtual
 We
now derive the mirror formula for a
concave mirror producing a real image in
fig 2.3.
 When the object AB is of size or height (h)
is placed on the left in front of the
concave mirror MN, beyond its centre of
curvature C, The image formed is real,
inverted and diminished in size (h’)
Using the New Cartesian Sign Convention, we
have
 Object distance = PB = - u
 Image distance = PB’= -v
 Focal length
= PF = -f
 Radius of curvature = PC = -R
 Now in fig 2.3 , the right angle triangles ∆ A’B’P
and ∆ ABP, are similar, so that
 A’B’
PB’
-v
v
3,1
AB
PB
-u
u

Similarly in the right angled triangles, ABC and
A’B’C’ are similar, so that
 A’B’
CB’
 AB
CB
 As we measure all distances from the pole P,
we have
 CB’ = PC – PB’
 CB = PB – PC
 Using equation 3.2 we get
 A’B’
PC – PB’
(-R)-(- v)
-R + v
 AB
PB – PC
(-u)–(-R)
-u + R

3.3
Comparing Eqs. (3.1) and (3.3), we get
 -R+v
v
 -u+R
u
 Or, uR + vR = 2uv
 Dividing both sides by uvR, we get
 1/v + 1/u = 2/R -------------- (3.4)
 When the object AB is taken at a very large
distance ( at infinity), as shown the image is
formed at the focus F. Thus, when u =∞ , v =
f, putting the values in eq 3.4 , we get
 1/f + 1/∞ = 2 / R or f = R/2 ------ (3.5)

MAGNIFICATION

The ratio between the height of the image
produced by the spherical mirror to the height
of the object is called linear magnification.
Height of the image
Linear magnification = -------------------------Height of the object
hi
M = --------- = v/u
ho
REFRACTION OF LIGHT AND ITS
LAWS
the ratio of sine of angle of incidence to the sine of
angle of refraction is constant. Thus, angle of
incidence I and the angle or refraction r are related as
Sin i
------- = n21
sin r
n21 is a constant and is
called the refractive index of second medium with
respect to first medium. It is also known as Snell’s law
of refraction.
 ii) The incident ray, the refracted and the normal at
the point of incidence lie in the same plane.

REFRACTIVE INDEX
Ans:- For two media and for a light of a particular color, the
ratio of sine of incidence angle and sine of refraction angle is
called refractive index of second medium with respect to first.
Sin i
--------------------- = n21
sin r
 if motion of light is in reverse direction means from medium 2
to medium 1, refractive index is reversible
n12 = sin r/ sin i = 1/n21



if velocity of light is v1 in first medium and v2 in second
medium,
n21 = velocity of light in first medium(v1)/ velocity of
light in second medium (v2)
Refractive index of water is 4/3 and glass is
3/2 with respect to air. What is refractive of
glass with respect to water.
Ans;- refractive index of air , n1 = 1.00
 Thus, refractive index of water w.r.t. air, n21 =
n2 = 4/3
 Refractive index of glass w.r.t. air = n31 = n3
= 3/2
 Refractive index of glass w.r.t water = n32
 n32 = n31 x n12 = n31/ n21 = n3/n2 =
(3/2)/(4/3) = 9/8 = 1.125
RULES FOR IMAGE FORMATION IN
SPHERICAL LENSES
1)
2)
A ray from the object parallel to the principal
axis after refraction passes through the
second principal focus F2 ( in a convex lens)
or appears to diverge ( in a concave lens)
from the first principal focus F1
A ray of light passing through the first
principal focus ( in a convex lens), or
appearing to meet at it ( in a concave lens)
emerges parallel to the principal axis after
refraction.
3) A ray of light passing through the optical
centre of the lens, emerges without any
deviation after refraction.
IMAGE FORMATION BY CONVEX
LENS
position of
the object
Position of
the image
Size of the
image
Nature of the
image
At infinity
At the focus
F2
Highly
Diminished
Real and
Inverted
Beyond 2F1 Between F2
and 2F2
Diminished
Real and
Inverted
At 2F1
Same size
Real and
Inverted
Between F1 Beyond 2F2
and 2F1
Enlarged
Real and
Inverted
At F1
Highly
enlarged
Enlarged
Real and
Inverted
At F2
At infinity
Between F1 On the same
and O
side of lens
Virtual and irect
IMAGE FORMATION IN CONVEX
LENS

CONVEX LENS
M
C
A
B’
B
2F1
F1
O
F2
A
A’
N
LENS FORMULA
M
C
A
B’
B
2F1
F1
O
F2
2F2
A’
N
Here object distance = OB = -u
 Image distance
= OB’ = v
 Focal length
= OF2= f
 AB = OC
 ∆ ABO ~ ∆ A’B’O are similar
 A’B’/AB = OB’/OB = v/-u ……….eq. –(i)
 Similarly ∆ OCE2 ~ ∆A’B’F2
 A’B’/OC = F2B’/OF2, As AB = OC, then
 A’B’/AB = F2B’/OF2 = ( OB’- OF2)/OF2 = (v-f)/f

……..eq. – (ii)

From eq. (i) & (ii) we get, -v/u = v-f/f
 On cross multiplication and dividing both sides
by uvf, we get,

1/v – 1/u = 1/f
IMAGE FORMATION IN CONCAVE
LENS
2F1
F1
O
Object is at infinity and the image is at F1
When the object is between O and
Infinity
A
A'
2F1
B
F1 B’
O
Image formation in concave lens
Position of
the object
Position of
the image
Size of the
image
Nature of
the image
At infinity
At focus F1
Highly
diminished,
point sized
Virtual and
erect
Between
infinity and
optical
centre O of
the lens
Between
focus F1 and Diminished
optical centre
O
Virtual and
erect.
POWER OF A LENS
The power of a lens is a measure of the degree
of convergence or divergence of light rays
falling on it.
 The power is defined as the reciprocal of its
focal length (f) as
 P = 1/f
 The SI unit of power of a lens is dioptre
denoted by symbol D.
 If f is expressed in metres so that 1 D= 1m-1

Human Eye
Different parts of Human eye are as follows:








Retina
Pupil
Iris
Aqueous humour
Vitreous humour
Cornea
Lens
Cilliary muscle
Suspensory ligament
Choroid
 Blind spot
 Optic nerve
 fovea

SIMPLE MICROSCOPE
A
simple microscope is a convex lens of
short focal length. It is also called
magnifying lens.
 The convex lens is held near the object to
be magnified, such that it is in between
the optical centre and principal focus, but
close to principal focus.
 An erect virtual and enlarged image A1B1
is formed at the least distance of distinct
vision as shown in fig. Next page.
Magnification
Size of the image
M
= Size of the object

= Distance of the image from eye

Distance of object from eye
 M = A1B1/AB = D/f ( because eye is held close
to lens) , Thus, if a convex lens of focal length
6.25 cm is used its magnification is
 m = D/f = 25 cm/ 6.25 cm = 4

COMPOUND MICROSCOPE
 Construction
– A compound microscope
consists of two metallic tubes such that
they can easily slide in one another.
 The tubes are blackened from inside to
prevent any internal reflection.
 On the side of the smaller tube a convex
lens of very small focal length called
objective lens is fitted which faces
towards the object.
 On
the side of bigger tube, another
convex lens of larger focal length is
fitted. This lens is called eye lens.
 Working – A tiny object AB is placed
in between F0 and 2F0 of the
objective lens, when it forms a real,
inverted and magnified image A1B1
on the other side of objective lens,
i.e. within the tubes.
Now the eye lens tube is moved backward or
forward, such that the real image A1B1 falls
between the principal focus (Fe) and its optical
centre (O) of the eye lens.
 The rays starting from the real image A1B1 on
passing through the eye lens give rise to a
divergent beam of light.
 When these divergent rays are received by the
eye, they appear to come from the points A2
and B2.
 Thus A2B2 is the virtual, but highly enlarged
image of the object AB.

The image formed here is inverted with respect
to the object.
 However, this does not make any difference as
most of the biological specimens seen under the
microscope are round and oval or not very well
defined in shape.

magnification
The magnifying power of a compound
microscope is the ratio between the final
size of the virtual image to the actual size
of the object.
 M = (D x L) where D = least distance of

F0 x fe distinct vision, L= tube
length, f0 = focal length of objective,

fe = focal length of eyepiece.

ASTRONOMICAL TELESCOPE
Astronomical telescope is an optical device used
to for seeing heavenly bodies such as the stars,
the Sun, the Moon, etc, closely.
 Construction – It consists of two convex lenses,
i.e. an objective lens of very large focal length
and eye lens of very small focal length.
 The two lenses are mounted on separate tubes
which can slide in one another.

working
The rays coming from a distant heavenly
body are parallel to one another, but
generally not parallel to the principal axis.
 These rays, on passing through the
objective lens, suffer refraction and hence,
converge in the plane of the principal
focus to form a real, inverted and
diminished image AB of a distant body.

Now the eye lens tube is moved backward or
forward so that the real image AB falls within
the principal focus (fe) and the optical centre(O)
of the eye lens.
 The rays starting from the real image AB on
passing through the eye lens give rise to a
divergent beam of light.
 When this divergent beam reaches the eye to it
the rays appear to come from A1B1. Thus, A1B1
is the final virtual and enlarged image formed.

Magnification
The ratio between the angle subtended by
the final image on the eye to the angle
subtended by the object on the unaided
eye is called the magnifying power of
telescope.
 m = fo/fe , fo – focal length of objective

fe – focal length of eye lens

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