S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
Common terminology of reflection of light on a
plane mirror
LESSON 5.1 –
Understanding reflection of light
Introduction :
Light is a form energy that enables us to see.
Light is emitted or given by hot objects, like a candle
flame, or the glowing filament of light bulb,or the sun.
An object which produces its own light is said to be
luminous.
Most of the objects we can see do not produce their
own light – they are non-luminous. What we can see
is the light is reflected off them.
Light seems to travel in straight lines. A picture of
light as rays traveling in straight lines from a luminous
source or bounching off objects, or being absorbed or
transmitted by them, is very useful one. We can think
of light ray as a very thin beam of light. A real beam
contains many rays. But by drawing just a few of
them we can get a clear idea of what happening to
the whole beam.
When rays of light strike any surface the rays are
reflacted , unless the surface is black, when they are
absorbed.
The reflection depend on how smooth the surfaces
are.Good mirrors reflect well over 90% of the light
that reaches them, with only a small amount being
absorbed.
The surface of most objects are very irregular. Light
hitting this sort of surface is scattered in all directions.
This is called diffuse reflection.
On the other hand,the surface of a polished sheet of
glass or metal can be made extremely smooth. The
reflection from this is the regular.
Normal
:
A line at right angles to the
mirror’s surface.
Incident ray
:
A ray of light that is directed
onto the mirror’s surface.
Reflected ray
:
A ray that is reflected by the
mirror’s surface.
Angle of
incidence
:
The angle between the incident
ray and the normal
Angle of
reflected
reflection
:
The angle between the
AO
ON
OB
i
r
:
:
:
:
:
ray and the normal
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Laws of reflection
The first law
:
The second law :
and
The angle of incidence is equal
to the angle of reflection.
The incident ray, the normal
the reflected ray all lie in the
same plane.
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S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
Charateristics of an Image Formed by a Plane
Mirror by drawing Ray Diagram
Example 2
The diagram shows two plane mirrors set an angle
100o to each other. A ray strikes one of the mirrors as
shown at angle of incidence of 45o . Redraw and
complete the diagram showing the path of the ray
and calculate the angle of reflection at which it
leaves the second mirrors.
(1) Draw two normals at the surface of the
mirror.
(2) Draw two incident rays and each of them
is directed to the normals.
(3) Measure the angles of incidence
respectively by using protractor.
(4) By using the Laws of reflection (The angle
of incidence is equal to the angle of
reflection draw two reflected rays in front
the mirror.
(5) Extrapolate the reflected rays behind the
with dotted line. The point of intersection is
the location of the image .
Solution
The charateristics of an image formed by a plane
mirror are :
(1) The size of the image= the size of the
object
(2) The distance of image = the distance of
object
(3) Virtual image (appears to be behind the
mirror)
(4) Laterally inverted (left and right are
interchanged)
(5) Upright
Example 1
A boy is at a distance 5 m from a plane mirror. The
mirror is moved away from the boy by 2 m. What is
the distance between the boy and his image.
Solution
2
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
Common terminology of reflection of light
on a curved mirror
Concave mirror
Radius of curvature, r
:
Distance between
the pole,P and the
centre of
curvature,C.
Focal length , f
:
The distance
between the
principal focus ,F
and the pole of the
curved mirror,P.
Relationship between f and r
r = 2f
Relationship curvature of mirror and f
The more curved the mirror , the shorter its focal
length,f.
Convex mirror
Pole of mirror , P
:
The centre point on
the curved mirror
Centre of curvature , C
centre
:
The geometric
of a hollow sphere of
which the concave
or convex mirror is a
part.
Principal axis ,PFC
Principal focus , F
:
:
A line which passes
through the centre of
curvature ,C and the
pole of a curved
mirror,P
Charateristics of an Image Formed by a curved
Mirror by drawing Ray Diagram
A point through
which all rays
traveling parallel to
the principal axis
converage to or
appear to diverge
from after reflection
by the mirror.
We must draw any two of three rays to determine the
position of the image.There are three
Ray 1
For concave mirror :
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S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
A ray parallel to the principal axis is
reflected to pass through F.
Ray 3
For concave mirror :
For convex mirror :
A ray through C is reflected back along its
own path.
A ray parallel to the principal axis is
reflected as if it came from F. F
For convex mirror :
A ray towards C is reflected back along its
own path.
Ray 2
For concave mirror :
A ray through F is reflected parallel
to the principal axis.
Image formed by a Concave Mirror
A
For convex mirror :
A ray towards F is reflected parallel to the
principal axis.
4
u < f ( Object between F and P )
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
Characteristics of image :
Application :
B
D
u = f ( Object,O is at F )
u = 2f or u = r ( Object ,O is at C)
Characteristics of image :
1.real
2.inverted
3. same size
Characteristics of image :
Application :
Application :
Light reflector in a projector
C
E
f < u < 2f or f < u < r ( Object O is between
F and C.
u > 2f or u > r ( Object, O is beyond C )
Characteristics of image :
Characteristics of image :
Application :
...................................................................................
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S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
F u = ∞ ( Object ,O very far from the lens)
Characteristics of image :
Application :
B
f < u < 2f or f < u < r ( Object O is between
F and C)
Characteristics of image :
Application :
Conclusion :
The distance
object,u
u < f
u =f
f <u < 2f
u = 2f
u > 2f
u=∞
Characteristics of image
Characteristics of image :
Application :
The object
distance,u
u < f
f <u < 2f
Image formed by a Convex Mirror
A
u < f ( Object between F and P )
Conclusion
6
Characteristics of image
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
Uses of the reflection of light
Plane mirror
(1) A mirror periscope can be built by mounting
two plane mirrors in a cardboard tube.
(2) A rear view mirror and a side mirror of a car
in order to enable the driver see objects at
the back and side of the car.
(3) Plane mirror acts as an anti-parallax error
mirror in electrical instruments such as the
voltmeter and the ammeter.
What are the angles of incidence and
reflection?
Concave mirror
(1) A shaving mirror or a make-up mirror used
a wide-aperture concave mirror with a
large radius of curvature.
(2) A torchlight or a headlight of motor
vehicles used concave parabolic mirror to
produce a beam of parallel light rays.
A
B
C
D
2
Convex mirror
(1) Convex mirror are mounted at a sharp
corner of a road so that the drivers are able
to see oncoming cars on the blind side of
the corner.
(2) Convex mirror hung on the corner of
ceilings in supermarkets serve to provide a
wide field of view activities happening in the
shopping area.
Angle of
reflection
40o
50o
40o
50o
The diagram shows a mirror is titled at an angle
of 40o to the bench. A ray of light is directed so
that it hits the mirror at an angle of 10o to the
surface of the mirror.
What is the angle of reflection?
A
C
E
3
TUTORIAL 5.1
1
Angle of
incidence
40o
40o
50o
50o
The diagram shows a single ray of light being
directed at a plane mirror.
7
10o
40o
80o
B
D
20o
500
The diagram shows an image is formed by a
mirror.
At which point the object is placed?
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
4
Which lamp’s image can be seen?
The diagram shows a ray of light from a small
bulb strikes a plane mirror.
7
Where is the image of the bulb formed and its
characteristic?
A
B
C
5
The diagram shows a wall clock .
At P and virtual
At Q and real
At R and virtual
What is the time shown by the image of the wall
clock in a plane mirror as seen by a student.
The diagram shows a girl stands at the side of a
boy in front of a mirror. They are both the same
distance from the mirror.
Where does the girl see the boy’s image?
A
C
8
01:40
07:10
B
D
04:50
10:20
Diagram shows a student looking at a plane
mirror.
The image observed by the student is
6
The diagram shows four lamps in front of a
plane mirror. The card prevents the observer at
M from seeing the lamp directly.
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S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
9
Diagram shows an object O at a distance in
front of a plane mirror CD. The plane mirror is
moved by a distance 4 cm at LM.
What is the distance of image of the chart from
the patient?
What is the distance between object and the
new image of the object?
A
C
E
10
B
D
16 cm
32 cm
12
A boy stands in front of a plane mirror a
distance 5 m . When the boy moves toward the
mirror by 2 m , what is the distance between
the boy and his new image?
A
C
E
11
12 cm
24 cm
64 cm
A
C
E
2m
6m
10 m
B
D
3m
7m
14 m
B
D
4m
10 m
The diagram shows a half-metre rule has a hole
at 15 cm mark and is hung vertically. A plane
mirror is placed in front of the ruler.
4m
8m
The diagram shows a patient sitting in a testing
room in front of a chart.
What is the region of the image of the ruler
seeing by an observer , when the observer look
through the hole?
A
B
C
D
E
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9
between 0 cm mark and 30 cm mark
between 0 cm mark and 45 cm mark
between 5 cm mark and 35 cm mark
between 10 cm mark and 25 cm mark
between 0 cm mark and 50 cm mark
The diagram shows a point object,O is
positioned in front of a plane mirror .
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
A
B
C
D
17
In order to view the image of O in the mirror,
you must be between the points
A
C
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U dan V
W dan Y
B
D
A concave mirror has a focal length 20 cm.
What happen to the size of image when an
object is placed at a distance of 40 cm in front
of the mirror?
A
C
U dan Y
Y dan Z
18
The figure shows a light ray being incident to
the surface of plane mirror MN. The mirror is
then rotated anticlock wise through an angle
10o.
A
C
15
300
100
B
D
20
magnified
real , inverted and magnified
virtual , inverted and magnified
real , upright and diminished
virtual , upright and diminished
A concave mirror has a focal length 15 cm. The
image formed by the mirror is virtual, upright
and magnified. What is the object distance?
A
C
E
What is the new angle of reflection of the light
ray?
diminished
B
same size of object
An object is placed at a distance 18 cm from a
concave mirror of focal length 10 cm. What is
the characteristics of the image formed?
A
B
C
D
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diminished, real and inverted
magnified , real, and upright
diminished ,virtual and upright
magnified , virtual and inverted
8 cm
20 cm
40 cm
B
D
15 cm
30 cm
The figure shows a candle placed in front of a
concave mirror of focal length, f.
150
50
The diagram shows five light rays A, B,
C ,D and E from an object,O and is reflected by
a concave mirror to form an image I. Which of
the light ray is not correct?
The image formed is
A
B
C
D
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The characteristics of an image , formed by a
convex mirror for all positions of the object are
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real, upright and magnified
real, inverted and diminished
virtual, inverted and magnified
virtual, upright and diminished
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
21
A
B
C
D
22
(c)
Which of the following states the right reason for
replacing a plane mirror are used as rear- view
mirrors in motor vehicles with a convex mirror ?
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To shine the object
To widen the field of view
To produce a brighter image
To produce a sharper image
Write down the word " REFLECTION "
when the word is seen by a person in a
plane mirror.
Figure (a) shows the positions of the eye of a
car driver , the wing mirror of the car and the
front of a lorry.
(a) Figure shows the positons of an object O
and the image I of the objects is formed by
a plane mirror M.
Figure (a)
(a) Draw rays to indicate the part of the front
of the lorry which can be seen in the
mirror by the car driver.
(b) Figure (b) shows a plane mirror and a
curved mirror.The mirrors have the same
size. The wing mirror of a car is often
curved .
Draw rays from observer to the plane
mirror and the curved mirror to explain
how the curved mirror give more
advantage than the plane is used as a
wing mirror of a car.
Figure(a)
Complete figure (a) showing the path of
the ray,including the in Incident ray,
reflected ray and normal.
(b)
Figure(b) shows two plane mirrors JK and
LM which have been placed at a right
angles to each other.
Figure (b)
Complete figure (b) to show the ray diagram
path until it emerges from the both the mirrors.
24
11
An object is placed at a distance ,u from a
concave mirror, M. The table shows the
S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
characteristics of image , when the distance u
is varied.
Object
distance ,
u/cm
10
…………………………………………..
(iii) What is the characteristics of the
image when the concave mirror M is
displaced by a convex mirror of focal
length same as the mirror M.
Image
Charateristics
…………………………………………..
20
30
…………………………………………..
Real,inverted,same size as
the object
25
40
Figures(a) and (b) show the parallel rays of light
directed toward the curved mirrors, J and K. CP
is the radius of curvature, and F is the focal
point of the mirrors.
(a) Name the light phenomenon that is
involved to form the image by the mirror.
....................................................................
(b) Using the information in the table,
determine the focal length of the mirror M.
(c)
....................................................................
Give the reason for your answer in (b).
............................................................
(a) What is meant by focal length.
(b) Using Figure(a) and (b) , compare the
curvanture of the mirrors and their focal
lengths.
(c) Figure(c) shows a cross section of a
parabolic reflector used in a car
headlamp.
The headlamp has two bulbs , X and Y.
Bulb X is located at the focal point of the
reflector while bulb Y is above bulb X.
............................................................
(d) Complete the table
(e) An object is placed at a distance 50 cm
from the concave mirror M.
(i) Draw a ray diagram in the space
below to show the formation of the
image.
Figure(c)
Explain what happens to the light rays
from the bulb when only:
(i) bulb X is switched on
(ii) bulb Y is switched on
(ii)
You may use diagrams to illustrate your
answers.
What is the characteristics of the
image?
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S M K Pahi
Bidang sains dan matematik
Lesson 5.1 - Understanding light reflection
(d)
You are given two pieces of plane
mirror measuring 5 cm x 6 cm to make a
periscope.
(i) Using these mirrors and other
materials, describe how you would
make the periscope.
(ii) Explain why the image observed
through the periscope using plane
mirrors are not very clear.
(iii) Suggest modification that can be
made to the periscope to produce
clearer and bigger images.
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