Plane
Mirrors
The Ray Model of Light
• Light sources radiate light in all directions.
The direction in which light travels is
represented by lines with arrows (rays) on
diagrams.
• The basic property of light is that when it
travels within the same medium, it moves
in straight lines.
• When light hits an object, depending on
the properties of the object, light may be:
Reflected
• (bounced off the surface)
• Ex. Mirror , metal plate
Absorbed
• stuck in the object and turn into other
forms of energy, most often into heat).
• If these either of these two properties are
observed.
• It is an Opaque Object
• No light passes through the object.
Transmitted
• light rays go through the object in such a
way that the pattern of the light rays does
not change, or bend
• Transparent
• Transmitted pattern of the light rays
changes (light get scattered)
The way we see
• Out of all the rays
coming out of the
object, a beam of
narrow diverging light
rays gets into the eye.
• Brain extends these
rays backward and
estimates that there is
an object at their
intersection.
The way we see
• The eye receives a set of
diverging light rays out of
each point of the object.
• The brain "draws" the
object point-by-point,
estimating each point at
the intersection of
diverging light rays.
For simplicity, on this diagram only extreme points
are analyzed
The way we see in mirrors
When the eye detects the light rays reflected by the
mirror, the brain still assumes that what we see is at
the intersection of the extensions of the light rays
entering the eye.
Observer has a sensation that
the object is behind the mirror,
but there is nothing behind the
mirror.
What we see is the Image: a
pattern created by the light rays
giving us sensation of the object
reproduction.
Terminology of Reflection
The ray of light
approaching the
mirror is the
incident ray
The angle between
the incident ray and
the normal is the
angle of incidence
A line drawn perpendicular to
the surface of the mirror at the
point of incidence where the ray
strikes the mirror, is known a
normal line
The ray of light
which leaves the
mirror is the
reflected ray
The angle between the
reflected ray and the
normal is the angle of
reflection.
Incident ray, reflected ray, and normal are in the same plane
The Law of Reflection
When a ray of light
reflects off a
surface, the angle
of incidence is
equal to the angle
of reflection.
Smooth surface vs Rough surface
Same pattern of
Pattern of reflected rays is
reflected and incident different from the incident
rays
rays pattern
Clear image
Fuzzy picture or none
Which reflection is diffuse? specular?
Specular
Diffuse
Plane Mirror
• A mirror is an object with
smooth surface producing
specular reflection of light
• A plane mirror is a mirror
with a flat reflective surface
Image of a Point
Image of a point
Due to the similarity of the
triangles on the diagram, all
the reflected rays extend
backward to one point.
No matter where the observer
is, the image is always:
•Behind the mirror
•On the perpendicular line
drawn from the object, A.
•At the same distance
from the mirror as object.
A
do
di
A
’
Image Formation in Plane Mirrors
by a Point Source of Light
Because all the reflected
light rays intersect at one
point anyway, there is no
need to construct many of
them, but only one: the
perpendicular to the
surface of the mirror.
A
1
do
A’
di
To construct the image of a point,
-sketch a line perpendicular to the mirror through A
- measure do (from A to the mirror surface)
- construct image A' on the other side such as do= di
Image Formation in Plane Mirrors
To construct the image of an object, there is no need to
construct an image of each single point of it.
Construct an image of the extreme points (e.g. top and
the bottom), then connect these edge points
Properties of Images Formed by
Plane Mirrors
S ize
A ttitude
L ocation
Type
The image has the same size as the
object (no magnification)
The image is up-right but Laterally
Inverted (Left / Right)
Image is at the same distance from
mirror as the object
(see the next slide first)
Type of the images formed by
plane mirrors
Our eye catches diverging rays reflected from mirror.
Observer has a sensation that image is at the
intersection of the light rays extended behind the
mirror. There is nothing behind the mirror.
Light only appears to us to
come from this point.This
image is imaginary, or virtual
image because there is no
real light rays going to or
coming from behind the
mirror.
Properties of Images Formed by
Plane Mirrors
S ize
A ttitude
The image has the same size as the
object (no magnification)
The image is up-right but Laterally
Inverted (Left / Right)
L ocation
Image is at the same distance from
mirror as the object
Type
Plane mirrors form virtual images