This document provides an overview of light and vision. It defines luminous and non-luminous objects, and explains that light reflects off of non-luminous objects allowing us to see them. Reflection of light is demonstrated using a ray box experiment. The document also describes the anatomy of the human eye, including the iris, pupil, retina, and lens. It explains how the eye focuses on near and far objects by changing the thickness of the lens. Common vision defects like myopia and hyperopia are also outlined.

1. LIGHT
CLASS 8th
PREPARED BY:
BHAVYA VASHISHT

2. Introduction
 Light is form of energy
 Light is needed to see things around us
 We see various things around us with eyes but eyes alone can’t see any
object.
 We can’t see objects in dark
 We need source of light to make objects visible
 It is light which makes things visible to us.
 Light enables us to see things from which it comes or from which it is
reflected

3. LUMINOUS & NON LUMINOUS OBJECTS
 Luminous objects: Objects which emit their own light
 They are source of light
 Very small in number
 Example: Stars, Lighted electric bulb, fire etc.
 Non- Luminous objects: Objects which don’t emit their own light
 Can’t make their own light
 Most objects around us are non luminous.
 Can be seen only when light coming from luminous objects fall on them
 Non luminous objects are also called Illuminated Objects
 Example: Planets, satellites, book, chair, etc.

4. REFLECTION OF LIGHT
 Process of sending back light rays which fall on surface of an object is called
reflection of light.
 Studied by using plane mirror
 Plane mirror reflects almost all light falling on it
 Plane mirror changes direction of light falling on it
RAY BOX EXPERIMENT
O
(Goes in) (Goes out)
Point of Incidence
Plane
A B
M M’

5.  We use Ray Box to produce thin beam of light
 It has narrow slit in front of box
 When light bulb is switched on, very thin beam of light comes out of narrow slit
 This narrow beam of light used to study reflection of light from plane mirror
 Thin beam of light produced by ray box is visible on white sheet of paper, so its path
can be traced by using pencil.
 Though thin beam of light is made up of several rays of light but for simplicity &
convenience, a thin beam of light is considered to be a ray of light

6. TERMINOLOGY
 Incident Ray: *Ray of light which falls on the mirror
surface
*Tells us direction in which light from source falls
on mirror
*Always go towards the mirror
 Point of Incidence : *The point at which incident ray strikes
mirror
*It tells us where exactly light falls on mirror
surface
 Reflected Ray : *Ray of light which is sent back by mirror
*Tells us direction in which light goes after
reflection from mirror
* Always go away from the mirror
 There can be only 1 reflected ray for given incident ray falling on plane mirror.
 Same ray of light is called incident ray before it strikes the mirror & becomes
reflected ray after it rebounds from mirror

7. TERMINOLOGY
 Normal: *Line drawn at right angles to mirror at point of incidence
*It is just an imaginary line
*Lies exactly in between incident and reflected ray
Angle of Incidence: *Angle between incident ray and normal
*Represented by “i”
Angle of Reflection: *Angle between reflected ray and normal
*Represented by “r”

8. LAWS OF reflection of Light
FIRST LAW OF REFLECTION
The incident ray, reflected Ray & normal all lie on same plane
SECOND LAW OF REFLECTION
The angle of incidence is always equal to angle of reflection
∠i = ∠r

9. M
M’
N
EXAMPLE 1

10. EXAMPLE 2
EXAMPLE 3

13. FORMATION OF IMAGE IN PLANE MIRROR
 Consider the figure given above. Here object AB of height ‘h’ is placed at a distance ‘u’ from
the mirror.
 When the rays of light from this object fall on the mirror, the image of the object is formed
inside the mirror.
 Consider the incident rays that fall on the mirror: AP, AO and BO. These incident rays are
reflected back from the mirror as PA, OC and OB.
 The rays PA and OC are diverging rays hence we can extend them behind the mirror to find
the point of intersection that is A’.
 Similarly, the other reflected rays are extended backwards and virtual image A’B’ of height ‘h’
of the object AB is formed on the mirror.

15. CHARACTERISTICS OF IMAGE FORMED BY
PLANE MIRROR
 Is the same size as that of the object
 Left-right inverted (Lateral Inversion)
 Erect and virtual
 Formed behind the mirror at the same distance as that of the object
placed in front of the mirror
 The object and image are equidistant from the mirror.

21. Angle between 2 plane mirrors No. of images formed
180° 1
120° 2
90° 3
60° 5
45° 7
0° Infinite

22. 180 ° 120 ° 90 °
60 ° 45 ° 0 °

25. HUMAN EYE
 The front part of the eye is covered with a transparent spherical membrane
known as the cornea. Light enters the eye by cornea.
 Space present just behind the cornea contains a fluid known as aqueous
humour.
 Just behind the cornea is a muscular diaphragm, which is dark coloured and
is known as the iris which has a small circular opening in the middle called
the pupil. The black colour of the pupil is due to no light being reflected from
it.
 The iris is responsible for controlling the amount of light entering the eye by
adjusting the size of the pupil.
 The lens of our eyes is a convex lens made of a transparent jelly-like
proteinaceous material. The eye lens is hard in the middle and becomes soft
towards the outer edges.
 The ciliary muscles hold the eye lens in its position. The ciliary muscles are
responsible for changing the curvature and focal length of the eye lens.
 The inner back surface of the eyeball is the retina. It is a semi-transparent
membrane that is light sensitive and behaves as the screen of a camera. The
light-sensitive receptors present in the retina are rods and cone cells.
 The space between the retina and eye lens is filled with vitreous humour.

28. Functions of Iris and Pupil
 Iris automatically adjusts the size of pupil according to intensity of light received by
the eye from the surroundings.
 If amount of light is high, iris contracts the pupil & hence reduces amount of light
entering the eye.
 If the amount of light around us is small, iris expands the pupil so that more light can
enter the eye
 Adjustment of size of pupil takes some time

29. RODS, CONES, BLIND SPOT
 Rods are the rod shaped cells present in the retina of an eye which are
sensitive to dim light
 Rods don’t provide information about the color of objects
 Cones are the cone shaped cells present in retina of an eye which are
sensitive to bright light.
 Cones also cause sensation of color of objects in our eyes.
 Blind spot is small area of retina insensitive to light where optic nerve
leaves the eye.
 When the image is formed at blind spot in the eye, it can’t be seen by
the eye.

30. PERSISTENCE OF VISION
 The image of an object seen by our eyes persists on retina for about
1/16th of a second even after the object has disappeared from our view.
 The ability of an eye to continue to see the image of an object for a
very short duration even after the object has disappeared from view is
called persistence of vision.
 If still pictures of a moving object are flashed on our eyes at a rate
faster than 16 pictures per second, then eyes perceive this object as
moving.

31. RANGE OF VISION
 Far point: Farthest point from the eye at which an object can be seen clearly. It is at
infinity
 Near point: Nearest point up to which the eye can see an object clearly without any
strain. It is about 25 cm
 Normal eye can see distant objects as well as nearby objects by focusing images of
distant objects as well as nearby objects on its retina by changing thickness
(converging power) of its lens
 Thin eye lens has less converging power whereas thick eye lens has greater
converging power on the rays of light coming from an object
 When eye is looking at distant object, ciliary muscles gets relaxed due to which eye
lens is thin (less convex). Thin eye lens has smaller converging power which is
sufficient to converge parallel rays of light coming from distant object to form its
image on retina.
 When eye is looking at nearby object, ciliary muscles gets stretched due to which eye
lens is thick (more convex). Thick eye lens has greater converging power which is
required to converge diverging rays of light coming from nearby object to form its
image on retina

32. DEFECTS OF THE EYE
 Defects in the eye happen due to many reasons. Due to growing age, the vision also
decreases, and when the focal length alters, the vision also alters.
 Cataract is the common defect seen in the eye. Cataracts cause partial or sometimes
complete loss of vision, when not treated properly. When the crystalline lens at old
age becomes milky and cloudy it is known as a cataract. When a person undergoes
cataract surgery, the vision can be restored.
 When the eye loses its ability to adjust its focal length, problems appear like a person
cannot see the image properly (blurring of vision), unable to view nearby objects or
far away objects. If not taken timely care, eyes might completely lose the power of
accommodation.
 Myopia is commonly known as near-sightedness. In this condition, the person can
see the objects nearby, but cannot see distant objects clearly. Faraway objects appear
blurry and a person will not be comfortable in seeing them.
 Myopia condition takes place when the shape of eyes leads the light rays to bend in a
wrong way, focusing images in front of the retina rather than focusing on the retina.
 Correction : When a concave lens of suitable power is used, it assists in focusing
the image onto the retina.

34.  Hypermetropia or Far-Sightedness
 Hypermetropia is commonly known as far-sightedness. In this condition, the person
can see objects at distance but cannot see nearby objects clearly. Usually, the person
with this disorder squints to see nearby objects. Hypermetropia is caused when the
light rays from a closeby object are focussed at a point behind the retina.
 Correction : Using spectacles with a converging lens (Convex) imparts additional
focussing power and thus helps in forming the image on the retina.
DEFECTS OF THE EYE

38. THANK YOU