Geometrical optics

Geometrical optics
 The Sun is our natural
source of light.
 Objects such as the
sun, a light bulb, a
fire, which give off
their own light are
called luminous.
 Objects which can
only be seen when
light falls on them are
called non-luminous
 Geometrical optics is
the study of how light
rays interact with
 In order to understand
how light behaves it is
necessary to
understand some of
the properties of light.
Light travels in straight lines.
Light rays, beams.
 The fact that light
travels in straight lines
explains the formation
of shadows as well as
the fact that we are
unable to see around
 In our study of optics
we represent a ray of
light as an arrow.
 Several arrows drawn
together represent a
beam of light.
Parallel Beam
Converging Beam
Diverging Beam
Reflection of Light
 When light strikes an
object some of the
light is absorbed and
some bounces back.
 The bouncing of light
off an object is known
as reflection.
 The reflection of light
can either be regular
reflection or diffuse
Diffuse reflection!
 Most objects reflect
light in a diffuse
manner which means
that the light is
reflected in all
 Such an object can be
seen when viewed
from any direction.
Light rays
 Objects which are perfectly
smooth or polished will
reflect light in a regular
Regular reflection!
 Light which is reflected in
Light rays
this way will bounce back
from an object at one
particular angle or in one
particular direction.
 Objects which reflect light
in a regular manner are
general called mirrors.
 Mirrors can either be flat
(plane) or curved
Ray diagrams.
 We can use simple
diagrams to explain
how light is reflected,
called ray diagrams.
 Ray diagrams help us
to describe how
images are formed by
both plane and
spherical mirrors.
 The ray of light striking the mirror is called
the incident ray
 The ray of light reflected is called the
reflected ray.
 The point where the incident ray strikes the
mirror is called the point of incidence.
 A line drawn at right angles to the mirror at the
point of incidence is called the normal, ‘N’.
 The angle between the incident ray and the
normal is called the angle of incidence, ‘i’..
 The angle between the reflected ray and the
normal is called the angle of reflection, ‘r’.
The Laws of Reflection
When light is reflected
from the surface f a
plane mirror;
 The incident ray, the
normal and he
reflected ray all lie in
the same plane.
 The angle of
incidence ‘i’ is equal
to the angle of
reflection ‘r’.
Reflected ray
Incident ray
Images formed by plane mirrors.
 The image formed by
Virtual Image
a plane mirror will be
1. A virtual image
2. The same distance
behind the mirror as
the object is in front
of the mirror
3. Laterally inverted,
(reversed left to
A virtual image is one formed
by the apparent intersection
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of light rays.
Uses of plane mirrors.
 Because of ‘lateral
inversion’, the
practical uses of plane
are limited.
 Emergency vehicles
will print names right
to left so that when
viewed in a rear view
mirror they can be
read properly, i.e. left
to right.
Plane mirrors are used to
view one own reflection.
The periscope.
 The periscope is an
instrument used to
view around obstacles.
 Two plane mirrors are
placed at 45º and used
to reflect light in a
regular manner.
 Submarines, sports
Locating images produced by
plane mirrors.
 The apparent movement of one object relative
to another due to the motion of the observer is
called parallax.
 The object which is farthest away moves with
the observer.
 No parallax occurs if the two objects are at the
same point.
 The method of no parallax is used to locate
the image formed by a plane mirror.

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