Page 158 - Physics_Form_2

Physics for Secondary Schools

signs of values such as image distance, image height, and focal length. Two
sign conventions, namely the new Cartesian convention and the real is positive
convention, are commonly used. The two sign conventions are summarised in
Table 4.4.

Table 4.4: Sign conventions
FOR ONLINE READING ONLY
New Cartesian convention Real is positive
1. All distances are measured from the 1. All distances are measured from the
mirror as the origin mirror as the origin
2. Distances measured in the opposite 2. Distances of real objects and images
direction of the incident light are are positive
negative
3. Distances measured along the 3. Distances of virtual objects and
direction of the incident light are images are negative
positive

Generally, the sign conventions for the
given quantities in the mirror equation and Solution
magnification equations are as follows; Given h = 4.0 cm , u = 8.3 cm,
0
(a) f is positive if the mirror is concave. f = 15.2 cm
(b) f is negative if the mirror is convex.
Using formula
(c) v is positive if the image is real and
located on the same side of the mirror 1 = 1 + 1
as the object. f u v

(d) v is negative if the image is virtual Upon rearranging the formula
and located behind the mirror.
fu
(e) h is positive if the image is upright v  u  f
i
and, therefore, virtual.

(f) h is negative if the image is inverted v  15.2 cm  8.3 cm  18.28 cm
i
and, therefore, real. 8.3 cm 15.2 cm

Example 4.5 From m  h i   v , hence, h   vh 0
h u i u
A 4.0 cm light bulb is placed 8.3 cm 0
from a concave mirror with a focal 
( 18.28 cm) 4 cm
length of 15.2 cm. Determine the image h  i 8.3 cm  8.81 cm
distance and the image size.

152
Student’s Book Form Two

Physics Form 2 Final.indd 152 25/10/2025 10:27