Page 144 - Physics_Form_2

Physics for Secondary Schools
FOR ONLINE READING ONLY
Figure 4.18: Plane mirrors placed at a
right angle
5. Observe and count the number of
images formed.
6. Reduce the angle by closing the
mirrors gradually onto each other,
such that the angle between them is
60 , 45 , 30 and 0 .    Note that at
0the mirrors are parallel to each
other.
7. Record your results using a table as
shown in Table 4.2.
Table 4.2: Number of images for various
angles between the mirrors
Angle between Number of
the mirrors images
90
60
45
30
0
Questions
(a) What happened to the number of
images when the angle is reduced?
(b) Account for the number of images
formed in the mirror.
138
Physics Form 2 Final.indd 138
The number of images will increase as
the angle between the mirrors becomes
smaller. When the angle is 0, the mirrors
are parallel to each other and the number
of images (n) is infinite. The number of
images formed in multiple mirrors relates
to the angle θ between two mirrors, as
follows: 360 !
n = −1,
θ
!
For θ = 90 ,
360
n  90  1, n  3
From the explanation above, we can see
that as the angle between the mirrors
decreases, the number of images
increases. As the angle between the
mirrors approaches zero, the number of
images approaches infinity. That is, there
is a very large number of faint images.
Parallel mirrors are commonly used in
salons and barbershops.
Example 4.1
An object is placed 2 cm from a plane
mirror. If the object is moved by 1 cm
toward the mirror, what will be the new
distance between the object and the
image?
Solution
Given an initial distance of the object
from the plane mirror = 2 cm and the
object moves 1 cm toward the mirror:
Then, the new distance of the object
from the mirror = 2 cm – 1 cm = 1 cm.
Since in a plane mirror, the image is
formed at the same distance behind the
Student’s Book Form Two
25/10/2025 10:27