LIGHT

Light is a form of energy that produces a sensation of sight in our eyes.

Every objects we see in our surrounding is due to light.
At night there is no light so we can’t see anything.

LENS
A piece of transparent substance, usually glass, having two opposite
surfaces either both curved or one curved and one plane, used in an
optical device.
Lenses are used in various optical instruments like microscope,
telescopes, binoculars, cameras,etc.
TYPES OF LENS

CONVEX LENS
A lens that is thicker at the centre and thinner at the edge is known as
the convex lens.
TYPES OF CONVEX LENS
 Biconvex
 Concavo-convex
 Plano-convex
WORKING OF A CONVEX LENS
When a parallel beam of light is incident on a convex lens then it
converges the ray of light after refraction through the lens.
So a convex lens is also called as the converging lens.

CONCAVE LENS
A lens that is thinner at its centre and thicker at the edge is known as
concave lens.
TYPES OF CONCAVE LENS
 Biconvex
 Concavo-convex
 Plano-convex

WORKING OF A CONCAVE LENS

When a parallel beam of light is incident on a concave lens then it
diverges the ray of light after refraction through the lens.
So a concave lens is also called as the diverging lens.

SOME TERMS RELATED TO LENS

1. CENTRE OF CURVATURE
 The lens either concave or convex is assumed to be the part of
sphere bounded to each other with centres as C1 and C2 as shown
in figure below.

The centres C1 and C2 is called as the centre of curvature of lens.

2. RADIUS OF CURVATURE
The radius of sphere whose surface forms the part of lens is called
as the radius of curvature of lens.
Since a lens is bounded by 2 spherical surfaces so it has 2 radii r1
and r2 as shown in figure above.

3. PRINCIPLE AXIS
The straight line passing through 2 centres of curvature is called
as the principle axis of lens.
In figure above XY is the principle axis.

4. OPTICAL CENTRE
The centre point of a lens is called optical centre. The ray of light
passing through optical centre goes straight and does not deviate.
 5. PRINCIPLE FOCUS
When a parallel beam of light is incident on a lens parallel to the
principle axis then it either meet at a point on a principle axis after
refraction or appears to meet.
That point is called as the principle focus.
If the light rays are coming from left hand side they will converge
at right hand side of the lens and vice versa.
That is why, a lens has two foci. They are at equal distance from
the optical centre.
 6. FOCAL LENGTH
The distance between the optical centre and principle focus of the
lens is called as the focal length.
It is denoted by (f).

RULES OF REFRACTION IN A LENS

A ray of light which is originally parallel to the principal axis passes
through the focus after refraction through the lens.
A ray of light passing through the optical centre of the convex lens does
not bent after refraction but goes straight. Also, a ray of light going
along the path of principal axis of a convex lens also goes straight and
does not deviate.

When a ray of light passes through the focus of the convex lens then it
becomes parallel to the principal axis after refraction through the lens.

OBJECT AND IMAGE DISTANCE

From above figure,
F – principle focus
2F – centre of curvature
O – optical centre
AB – height of object
A’B’ – height of image
OA - object distance(u)
OA’ - image distance(v)
OF - focal length(f)
The distance between the object and the optical centre is called as the
object distance.
The distance between the image and the optical centre is called as the
image distance.

CONSTRUCTION OF A RAY DIAGRAM FORMED BY A

CONVEX LENS
The size, location, and the nature of the image formed by the convex
lens depends upon the position of the object.
i. When the object is at infinity
The parallel rays coming from the object meet at the focus
after the refraction and the image is formed at the focus.
NATURE OF IMAGE—real,inverted and highly diminished.

ii. When the object is placed beyond 2F.

When the object is placed beyond 2F then one ray of light becomes
parallel to principal axis and passes through focus after refraction
through the lens and another light ray passes through optical centre
and goes straight after refraction. The image is formed between F and
2F on the other side of convex lens.
NATURE OF IMAGE – real, inverted and diminished.
iii. When the object is placed at 2F.
When the object is placed at 2F then one ray of light becomes
parallel to the principal axis and passes through focus of the
lens after refraction. Another ray of light passes though optical
centre and goes straight after refraction. The image is formed
at 2F on the other side of the lens.
NATURE OF IMAGE – real, inverted and same size to that of the
object.

iv. When the object is placed between F and 2F.

when the object is placed between F and 2F then a ray of light
parallel to the principal axis of the lens passes through focus (F)
after refraction through the lens. Another ray of light passes
through optical centre of the lens and goes straight. The image
is formed beyond 2F.
NATURE OF IMAGE – real,inverted and magnified.
v. When the object is placed at F.
When the object is placed at the focus of the convex lens (at F)
then one ray of light becomes parallel to the principal axis of
the lens and thus, passes through another focus after
refraction through the lens. Another ray of light passes through
the optical centre of the lens and goes straight. The image is
formed at the infinity.
NATURE OF IMAGE – real,inverted and highly magnified.

vi. When the object is placed between F and O.

The object placed between optical centre(O) and focus
(F), then the first ray of light starting from the top of the
object is parallel to the principal axis which passes through
another focus after refraction through the lens. Another
ray of light from the object passes through the optical
centre of the lens and goes straight after refraction
through the lens. Thus, both the light rays diverge after
refraction through the lens and does not meet. Therefore,
both the refracted rays are produced backwards so that
they meet at a point to form an image beyond 2F on the
same side to that of the object.
NATURE OF IMAGE – virtual,erect and magnified.
RULES FOR OBTAIING AN IMAGE BY A CONACVE
LENS

A ray of light parallel to the principal axis of the concave lens

appears to be coming from focus after refraction through the
lens.

A ray of light passing through the optical centre of the concave

lens goes straight after refraction through the lens.

A ray of light going towards the focus on another side of the

concave lens becomes parallel to the principal axis after refraction
through the lens.
CONSTRUCTION OF A RAY DIAGRAM FORMED BY A
CONCAVE LENS
The size, location, and the nature of the image formed by the concave
lens depends upon the position of the object.
When an object is placed anywhere between optical centre and
infinity, the image formed is between optical centre and focus.
NATURE OF IMAGE – virtaual, erect and diminished.
When the object is at infinity then the image is formed at the
focus.
NATURE OF IMAGE -- virtual, erect and highly dimdinished.

DIFFERENCES BETWEEN REAL AND VIRTUAL

IMAGE
REAL IMAGE VIRTUAL IMAGE
i. It forms when the light i. It forms when the light
rays meet at a rays appear to meet at a
particular point after definite point, after
refraction. refraction.
ii. It is always inverted. ii. It is always erect.
 iii. It can be obtained on iii. It cannot be obtained on
the screen. the screen.
iv. Converging lens forms iv. Diverging lens forms
real image. virtual image.
v. It is usually formed on v. It is formed on the same
the other side of the side of the lens where
lens or behind the lens. object lies

MAGNIFICATION
The ratio of height of image to the height of object is called as the
magnification.
It can also be defined as the distance of image from lens to the
distance of object from the lens.
Mathematically,
height of image (I ) image distance (v)
Magnification(M) = height of object (O) = object distance (u)

INTERPREATATION OF MAGNIFICTION

Magnification(m) Remarks
m=1 Height of image is equal to the
height of object
m>1 Height of image is greater than
the height of object
m<1 Height of image is less than the
height of object
m is negative The image is virtual and erect
m is positive The image is real and inverted
 I v
TO PROVE O = u

 In the figure above a convex lens is taken.

 An object of height AB is placed perpendicular on the
principle axis beyond 2F.
 A ray of light from B parallel to the principle axis converges
at focus F and another ray passing through the optical centre
O remains undeviated.
 The ray finally meet at point B’ and forms a real and inverted
image A’B’ between F and 2F.

In △ABO and △A’B’O, we have

i. ∠BAO = ∠B’A’O [ both are 90o]
ii. ∠BAO = B’A’O [ vertically opposite angles]
iii. ∠ABO = A’B’O [remaining angle]
Therefore △ABO and △A’B’O are similar triangles.
Thus we can write their sides are proportional.
A' B' OA '
AB
= OA
height of image (I ) image distance (v)
i.e. = height of object (O) = object distance (u)

= proved.
I v
O u
 RELATION BETWEEN OBJECT DISTANCE,
IMAGE DISTANCE AND FOCAL LENGTH

1 1 1
f
= u
+ v

SIGN CONVENTIONS
Real distance is taken as positive(+)
Virtual distance is taken as negative(-)
Focal length of convex lens is positive.
Focal length of concave lens is negative.

POWER OF LENS
The capacity of lens to converge or diverge light rays falling into it
is called as the power of lens.
Mathematically,
1
Power of lens = focal length ( f ) ∈metre
SI unit of power = dioptre(D)

One diopter power : A lens is said to have power 1 dioptre if its

focal length is 1 metre.

OPTICAL INSTRUMENTS
Those instuments which make the use of mirror, prism and lens is
called as the optical instuments.
TYPES
REAL IMAGE FORMING OPTICAL INSTRUMENTS
Eg. Camera, eye, cinema projector, slide projector,etc.
VIRTUAL IMAGE FORMING OPTICAL INSTRUMENTS
Eg. Microscope, telescope, binoculars,etc.

EYE
Since eye has a convex lens so it is considered as an optical
instrument.
Eyes are the natural real image forming optical instruments which
are concerned with the sense of vision.

LAYERS OF EYE
A. SCLERA
It is the outermost layer.
It is opaque and white in colour.
Its front portion is transparent called as cornea.
 It gives a proper shape to the eyeball and protects the inner
parts.

B. CHOROID
It’s the middle layer of eye.
It is dark black in colour.
It protects and nourishes the retina.
Parts of choroid

1. Iris and pupil : Iris controls the amount of light entering

the eye and pupil allows the light to pass into the eye.

2. Eye-lens : behind the iris there is a transparent convex

lens made up of protein. The lens converges the ray of
light coming into the eye to the retina and forms
real,inverted and diminished image.

3. Ciliary muscles and suspensory ligamensts : they

together holds the lens to its position.

4. Aqueous humor and vitreous humor : the space

between the lens and the cornea is filled with the fluid
called as the aqueous humor. It protects from external
shock and keeps the lens moist.
The space between the lens and the retina is filled with the
fluid called as the vitreous humor.it maintains the shape of
eyeball and protects the retina.

C. RETINA
It is the innermost layer of eye.
It is the light sensitive layer.
It prevents the internal reflection of light.
It the place where image of an object is formed.
The optic nerves in the retina thus transfer the information
about the image formed to the brain.

Spots in retina
 YELLOW SPOT
it is present at the centre of the retina.
It is sensitive to light so image of the object is formed
at this portion.
It consists of rod and cone cells.

 BLIND SPOT
It is the place where the optic nerves enter the eye.
It is called blind spot because no image is formed here
since it is not sensitive to light.

WORKING OF AN EYE
 Light enters the eye through the cornea and passes
the aqueous humor before reaching the lens.
 The pupil adjusts in response to the light : its
adjustment is helped by the iris. In dim or low light
condition iris expands pupil to allow more light to go in
but in good light condition it contracts the pupil to
allow less light.
 The lens focuses the light onto the retina.
 Real,inverted and diminished image is formed on
the retina.
 The optic nerve transmits visual information to the
brain.

RANGE OF VISION

Range of vision of a normal human eye is the range of

distance for which human eye can see an object clearly.
It ranges from infinity to 25 cm i.e. the range between
far and near point.
The nearest point, up to which an object cab be seen
clearly by human eyes, is called the near point of the
eyes. It is at 25 cm.
It is also called as the least distance of distinct vision.
The farthest point upto which objects can be seen
clearly is called as the far point. It is at infinity.
 POWER OF ACOMODATION OF HUMAN
EYE
The ability of an eye to change the focal length of the
lens so as to always obtain the image on the retina is
called as the power of accommodation.

PERSISTENCE OF VISION
Even after the object is removed, the impression of an
object seen by the eye remains on the retina for 0.1
second is called as the persistence of the vision.

DEFECTS OF VISION
The eye which is not able to form the image of an
object lying between near and far point is said to be
defected eye.
The defect is mainly due to inability of an eye to change
the focal length of the lens.
The common defects of vision are:
 Long-sightedness(Hypermetropia)
a. It is the defect of vision in which the person cannot
see the nearby objects clearly but can see far
objects clearly.
b. Here the far point of the eye is at infinity but the
near point is at greater than 25cm.
c. So in this defect the rays of light are focused
behind the retina as shown in figure below.
d. That’s why old peoples cant read books holding in
their hand.
 CAUSES OF DEFECT
Thinning of the lens and inability of ciliary muscles
to shorten the lens resulting in the increase in the focal length of
the lens.
Shortening of eyeball resulting in the decreament of the distance
between the eye lens and the retina which ultimately increases the
focal length.

REMEDY FOR THIS DEFECT

A convex lens of suitable power is used to correct this defect.

 Short-sightedness(Myopia)
 It is the defect of vision in which the person cannot see
the far objects clearly but can see near objects clearly.
 Here the far point of the eye is some metres only not at
infinity but the near point is at 25cm.
  So in this defect the rays of light are focused in front of
retina as shown in figure below.

CAUSES OF DEFECT
Thickening of the lens and inability of ciliary
muscles to stretch the lens resulting in the decrease in the focal
length of the lens.
elongation of eyeball resulting in the increment of the distance
between the eye lens and the retina which ultimately decreases
the focal length.

REMEDY FOR THIS DEFECT

A concave lens of suitable power is used to correct this defect.

MICROSCOPE
A microscope is an optical instrument which is used to see the
magnified image of the small objects placed close to it.
TYPES
a. Simple microscope
It has one convex lens.
It is also called as the magnifying glass or a hand lens.
When an object is placed in between F and O then a
virtual,erect and magnified image is formed.

Uses
It is used in laboratory to study different parts of flowers and
small animals like bacteria.
It is used in watch shop to repair watches and also in jwellery
shop to see small parts of any object.

b. Compound microscope
it consists of 2 lens(eye piece and object lens)
both the lenses are convex lens.
The lens close to the object is called as the objective lens and
the lens close to the eye is called as the eyepiece.
Both the lenses have the lager focal length.
WORKING

The object AB is placed at a distance slightly more than focal

length of the objective so that its real, inverted and
magnified image A'B' is obtained beyond the centre of
curvature (c) of objective lens. The image A'B' becomes an
object for the eyepiece. The position of image A'B' is
adjusted such that it lies within the focal length of eyepiece.
The piece forms a virtual, erect and magnified image A"B" of
the object. Thus, the final image formed by a compound
microscope is virtual, inverted and magnified behind the
object.
USES
It is used in pathology lab to examine stool,urine and blood of the
patients.
It is used in the various research works in the lab.