High School
by SSL Technologies
Physics Ex-47
THE LAW OF REFLECTION
When light strikes a surface and is reflected, it changes direction.
The direction it takes depends upon the angle it strikes the surface.
As illustrated below, a ray of light going towards a surface is called
an incident ray while a ray of light which is reflected away from a
source is called a reflected ray.
The law of reflection says that the angle of the incident ray with the
normal equals the angle of the reflected ray (also with the normal).
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Physics Ex-47
By definition, the angle which the incident ray makes with the normal
is called the angle of incidence while the angle which the reflected
ray makes with the normal is called the angle of reflection.
Note that the incident ray,
the reflected ray,
and the normal are
all on the same plane.
Remember : The angle of incidence (i) is the angle formed by the incident
ray and the normal (not the reflecting surface).
The angle of reflection (r) is the angle between the reflected
ray and the normal.
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Physics Ex-47
The law of reflection, therefore, simply states that when a ray of
light is reflected from a surface, it is reflected in such a direction
that the incident angle equals the reflected angle.
Using the law of reflection, we can determine the location of
images formed by plane mirrors.
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Physics Ex-47
An object is placed in front
of a plane mirror.
Such as
right here!
Normal
Find the location and
characteristics of the image.
We can now draw the image.
There are two steps in finding the image of a plane mirror.
Where
the
two
reflected
raysits
meet
In
Objects
locating
consist
an image,
of anwe
infinite
take number
one
point
ofon
points.
the
object
and find
Normal
Since point
we are
not given
the
location
of
theof
object,
is
the
imageobject.
point
the object point.
corresponding
(location)
on
the
image
And each point we
hascan
an infinite
number
of
rays
radiating
outwards.
select any arbitrary location.
Object
As
shown
above,
we usually
take
the
(extreme)
top point and find
Back
of mirror
Images
also
consist
of an infinite
number
of points.
its corresponding
point on the image object.
Step-1
Step-2
Each point on the object has a corresponding point on the image.
Draw
incident
ray
slightly
upwards towards
Draw
anan
incident
ray
slightly
downwards
towardsthe
themirror.
Then draw
its then
reflected
accordance
mirror,
drawray
its in
reflected
ray. with the law
of reflection.
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Physics Ex-47
Virtual images are formed
by extended rays.
Real images are formed
by reflected rays.
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Physics Ex-47
Physics Ex-47
Given a plane mirror and the
position of an observer’s eye,
determine the field of vision.
Mirror
angle of incidence = angle of reflection
Normal
Normal
Step-1
Step-2 angle of incidence = angle of reflection
Eye
There
are
two
steps
ineye
finding
the
field
of
Draw
Draw
aa
ray
ray
from
from
the
the
eye
toto
the
one
other
endend
ofvision.
the
of the
mirror.
mirror.
This
Thisisisthe
thereflected
reflectedray.
ray. Then,
Then,inin accordance
accordancewith
with
the
thelaw
lawofofreflection,
reflection,draw
drawthe
theincident
incident ray.
ray.
Field of vision
(Any point within this area will be reflected
to the observer’s eye as an image point.)
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Physics Ex-47
P'
Image point
Image
distance
Mirror
Given a plane mirror, a point
object (P) and the position of
an observer’s eye, determine
whether or not the observer
can see object point P.
Next, since the image distance equals the object distance,
=point,
Image
distance
Object
To find the
location
ofthe
theimage
image
point,
wealong
start the
by drawing
a line.
line
Perpendicular
to
mirror
weObject
mark offdistance
P‘,
extended
To determine
whether
or
notline
the observer
can
see
the given
distance
perpendicular
to the
mirror
and not
extend
it beyond
the object,
mirror.
Look! The
does
intersect
the
mirror.
we first locate the image of the object.
Conclusion:
observerANSWER
cannot see the image.
Object
point
Next, we draw a line from
thewe
image
to theline
observer
(eye).
Finally,
drawpoint
a straight
from the
P
The
observer
cannot
see
the image.
observer’s
eye
to
the
image
point.
If the line intersects the mirror, the observer can see the image.
Eye
If the line does not intersect
theline
mirror,
then the
observer
If this
intersects
the
mirror, cannot
see the image.
the observer can see the image.
If this line does not intersect the mirror,
the observer cannot see the image.
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P'
Physics Ex-47
Image point
Given a plane mirror, a point
object (P) and the position of
an observer’s eye, determine
whether or not the observer
can see object P.
Image
distance
Mirror
ANSWER
observer
the image.
To determine whether or not the The
observer
can seecannot
the givensee
object,
We now draw
a straight line from the
Tolocate
find the
we first
thelocation
image of the image
object.point, we start by drawing a line
observer’s
eye=toImage
image
point.
perpendicular toObject
the mirror
and extend
itthe
beyond
the
mirror.
distance
distance
Next,
we
draw
a
line
from
the
image
point
to
the
observer’s
eye.
Look!
The
line
does
not
intersect
the
mirror.
Object
If this line intersects the mirror,
distance
Perpendicular
tocan
mirror
If the line intersects
thethe
mirror,
the observer
observer
can
see
the
image.
the
see
image.
Thus,
observer
cannot
seethe
the
image.
If the line does not intersect Ifthe
mirror,
then not
the intersect
observerthe
cannot
this
line does
mirror,
see the image.
the observer cannot see the image.
Eye
P
Object point
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Question-1
Physics Ex-47
State the Law of Reflection.
The angle of incidence equals the angle of reflection.
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Physics Ex-47
Question-2
The diagram below represents an object in front of a plane mirror.
Object
 Note : Diagram not drawn to scale.
a) Draw the image.
b) How high is the image?
4 cm
c) How far is the image from the object?
20 cm
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Question-3
Physics Ex-47
Explain why there is only a lateral (left-right) reversal when
we look at ourselves in a plane mirror.
Because when we turn towards a mirror, we do so by turning about
the Y-axis (left-right) and not about the X-axis (top-bottom).
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Question-4
Physics Ex-47
The diagram below illustrates the image of an object produced
by a plane mirror. Label the incident ray, the reflected ray and
the extended ray.
Reflected ray
Incident ray
Extended ray
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Physics Ex-47
Question-5
Explain how real images are formed and how virtual images
are formed.
Real images are formed by the intersection of reflected rays,
virtual images are formed by the intersection of extended rays.
NOTE
As long as there is at least one extended ray,
the image is said to be virtual.
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Question-6
Physics Ex-47
Stefania is 1.5 m tall. Prove that the shortest mirror necessary
for Stefania to see her full height is 75 cm (half her height).
Divide Stefania’s height into two parts, from her eyes to the top of
her head and from her eyes to the bottom of her feet.
Half Stefania’s height
Thus, in order for a person to see their full height in a plane mirror,
the minimum mirror required is a mirror half their height.
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Question-7
Physics Ex-47
A beam of light is reflected from a plane mirror such that the
angle between the incident ray and reflected ray is 50o.
Draw the beam and calculate the angle of incidence? .
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Question-8
Physics Ex-47
Two mirrors, M1 and M2, are at 90o to each other. As illustrated,
a beam of light strikes mirror M1 with an angle of incidence of 60o
and is reflected by mirror M2. Complete the diagram and find the
angle of incidence of the ray reflected by mirror M2.
Angle of incidence of mirror M2
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Question-9
Physics Ex-47
Two plane mirrors, M1 and M2, are at 60o to each other as illustrated
in the diagram below. A beam of light strikes mirror M1 with an angle
of incidence of 40o. Complete the diagram and determine the angle
of reflection of the beam reflected from mirror M2 ?
Angle of reflection of mirror M2
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Question-10
Physics Ex-47
Two mirrors are parallel to each other as illustrated in the diagram
below. A beam of light strikes the beginning of one mirror at an
angle of incidence of 35o. Complete the diagram and find the
number of times the beam is reflected before it emerges from
the two mirrors? .
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Physics Ex-247
Question-11
What is the angle of reflection for the ray diagram illustrated below?
Angle of reflection
Angle of incidence
A) 1
B) 2
C) 3
D) 4
REMINDER
All angles are measured
from the “normal”.
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Physics Ex-47
Question-12
Stefania wants to install a rear view mirror on her bike. She has a
choice of choosing a plane or a convex mirror as illustrated below.
Which mirror should Stefania choose in order to obtain
the maximum filed of view?
Observer
Answer:
Stefania should choose the
convex mirror in order to
obtain the maximum field
of view.
Observer
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Physics Ex-47
Question-13
The following set up consists of a light source, an obstacle,
six different locations (labeled 1 to 6) and a target.
Note thatthe
only
at location
5 can
To determine
correct
location,
At which one of the locations
can you place
a plane
mirror
we
draw
unobstructed
lines
both
We now
divide
angle between
the
two
draw
twothe
lines
each
point.
so that the light source we
is reflected
to
the from
target?
toline
theto
source
anda
to
the target.
lines
at
location
5 with
bisector.
One
the
light
source
and This
If another
required,
using
anormal
protractor,
we
can
bisector
will
be
the
the
mirror.
line
the
target.
If
both
Thus,
weto
must
placeto
the
mirror
measure
the at
angle of incidence
or
lines
aredraw
unobstructed,
5. is the
Finally,
we
thelocation
mirror that
perpendicular
to
the
angle
of
reflection.
point where
we place
the mirror.
the normal
(bisector).
No
No
No
No
No
Yes
Mirror
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Physics Ex-47
Question-14
The set up below consists of an object and its shadow cast on
a screen by a point light source.
This problem illustrates the fact that light
travels in a straight line, technically known
as rectilinear propagation .
Determine the position of the light source.
A) Point-A
B) Point-B
C) Point-C
Draw a line from the top of the shadow
to the top of the object and then extend
the line to the light source.
D) Point-D
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