Page 159 - Physics_Form_2

Nature and reflection of light

Therefore, the image is formed at a Example 4.7
distance of 18.28 cm from the mirror An electric room heater uses a concave
with a size of 8.81 cm. The negative mirror to reflect infrared (IR) radiation
sign of the image distance indicates from a hot coil. Note that IR follows
that the image is formed behind the the same law of reflection as visible
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mirror. light. Given that the mirror has a radius
of curvature of 50 cm and produces an
image of the coil 3 m away from the
Example 4.6 mirror, where is the coil located?
A 4.0 cm light bulb is placed 35.5 cm Solution
from a convex mirror with a focal Using the new Cartesian convention,
length of 12.2 cm. Determine the image  R
distance and the image size. f   25 cm, v = 300 cm
2

Solution 1  1  1
By the new Cartesian sign conversion; v f u
u  35.5 cm (Measured against the fv

direction of incident light), u  v  f
f =+ 12.2 cm  300 cm 
( 25 cm)
u   27 cm
1 1 1  300 cm ( 25 cm)
From, = +
f u v Therefore, the coil is 27.27 cm in front
of the mirror.
1  1  1
v f u Example 4.8

fu
v  An object 3 cm high is placed at a
u  f point 30 cm from the pole of a concave

 35.5 cm 12.2 cm mirror whose focal length is 12 cm.
v   9.08 cm Using the mirror formula, find the

 35.5 cm 12.2 cm
position, the height and the nature of
 v  9.08 cm the image formed.
m    0.26
u  35.5 cm Solution
12 cm, f = 30 cm, u = h = 3 cm
h  i mh  o 0.26  4 cm 1.04 cm o
1 = 1 + 1
Therefore, the image size is 1.04 cm, f u v
and the image distance is 9.08 cm 1 1 1
behind the mirror. v  f  u

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Physics Form 2 Final.indd 153 25/10/2025 10:27