PHYSICS 534
R
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W
S
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EXERCISE-47
A
Isidor Rabi was awarded
the Nobel prize for
physics in 1944 for his
discovery of magnetic
resonance.
Plane Mirrors
RABI
 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).
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: The incident ray, the reflected ray, and the normal,
all lie on the same plane.
Incident
ray
Reflected
ray
 Remember: The angle of incidence is the angle formed by the incident ray and the normal (not the reflecting surface).
The angle of reflection is the angle between the reflected ray and the normal.
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.
To locate the image formed by a plane mirror, follow these steps:
 Step-1: Draw three rays coming from the object;
one ray above the horizontal (Ray-1),
one ray along the horizontal (Ray-2),
and one ray below the horizontal (Ray-3).
 Step-2: Using the law of reflection, draw the
reflected rays from the mirror.
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R
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y1
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y-2
R
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 Step-3: Extend the three reflected rays until they intersect.
I
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c
N
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al
Since actual objects consist of points, once we can locate a few "image points", it is a simple matter to locate the image
of an object. Indeed, all we need to do is locate a few "strategic" points and we can locate the image of an object.E
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PHYSICS 534  OPTICS  AnsPhysics Ex-47.DOC
© 1999 S. Lancione
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1.
State the Law of Reflection.
__________________________________________________________________________
T
h
e
a
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q
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.
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10cm
4cm
10cm
4cm
2.
The diagram on the right represents
an object in front of a plane mirror.
10 cm
 Note: Diagram not drawn to scale.
Reflected rays
Extended rays
a) Draw the image.
b) How high is the image?
4 cm
________________________
c) How far away is the
image from the object?
4 cm
10 cm
20 cm
__________________________
3.
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).
_________________________________________________________________________
4. 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.
a) Reflected ray
Mirror
c) Extended ray
b) Incident ray
Object
Image
5. Explain how a real image is formed and how a virtual image is formed.
___________________________________________________________________________
Real images are formed by the intersection of reflected rays, virtual images
___________________________________________________________________________
are formed by the intersection of extended rays.
___________________________________________________________________________
PHYSICS 534  OPTICS  AnsPhysics Ex-47.DOC  Plane Mirrors
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Virtual
image
6. Tanya is 1.5 m tall. Prove that the shortest mirror necessary for Tanya to see her full height
is 75 cm (half her height).
___________________________________________________________________________
1/2 of top required
Divide Tanya’s
___________________________________________________________________________
Half Tanya’s height
height into two
parts, from her
___________________________________________________________________________
eyes to the top
1/2 of bottom required
of her head, and
___________________________________________________________________________
from her eyes to
her feet.
___________________________________________________________________________
___________________________________________________________________________
Answer: In order for a person to see their full image in a plane mirror, the
minimum mirror required is a mirror half their height.
7. 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?
dr
ay
ide
Inc
___________________________________________________________________________
ay
nt r
Ref
lect
e
___________________________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
25 o 25o
___________________________________________________________________________
___________________________________________________________________________
8. 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.
[30o]
M
M1
60
o
1
______________________________________________
30
o
Parallel
60o 60o
o
60
______________________________________________
o
30
M2
o
30
____________________________________________________________________________
____________________________________________________________________________
___________________________________________________________________________
M2
PHYSICS 534  OPTICS  AnsPhysics Ex-47.DOC  Plane Mirrors
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9. Two plane mirrors, M1 and M2, are at 60o to
each other as illustrated in the diagram on the
right. A beam of light strikes mirror M1 with
an angle of incidence of 40o. Complete the
diagram and the angle of incidence of the beam
reflected from mirror M2? [20o]
40o
o
50 40
o
20o
M1
60o
M2
______________________________________________
70o
70o
_______________________________________________________________________________
_______________________________________________________________________________
Using geometry, since the angle of incidence for mirror M1 is 40 o, and since the
angle between the two mirrors is given as 60 o, then using the triangle, we find
_______________________________________________________________________________
the other two angles. As shown in the diagram, the other angles are 50 o and 70 o.
_______________________________________________________________________________
Thus, we can find the angle of incidence to be 20 o.
10.
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? [4]
35 o 35 o
1
2
4
35 o 35 o
35 o 35 o
35 o 35 o
3
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________

PHYSICS 534  OPTICS  AnsPhysics Ex-47.DOC  Plane Mirrors
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