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Physics for Secondary Schools
I 40 cm
FOR ONLINE READING ONLY
C I F 15 cm
O
h i
V
Figure 4.26
5. Using the chosen scale, the nature,
size and position of the image can
be determined. Thus,
(a) The measured height of the
image, h = 2.4 cm, which
i
corresponds to the actual height
of 12 cm (1 cm represents 5 cm).
(b) The measured image distance,
v = 4.8 cm, which corresponds
to the actual image distance of
24 cm from the mirror.
(c) True rays form the point of
intersection, so the image is real.
(d) The image is inverted.
(e) Magnification m    12 cm  0.6, extended backwards pass through the
20 cm
the image is diminished.
Task 4.5
Use a ray diagram to observe what
happens when the object is located
very far beyond the centre of curvature
of a concave mirror. Discuss with your
classmates about your observations.
146
Physics Form 2 Final.indd 146
Images formed by convex mirrors
In convex mirrors, the the centre of
curvature, C, and the focal point, F, are
located behind the mirror, that is, on the
side of the mirror opposite the reflecting
surface. Thus, a convex mirror has a
negative focal length.
A convex mirror is sometimes referred
to as a diverging mirror because incident
light rays diverge upon reflection from the
mirror surface. As a result, the reflected
light rays never intersect on the object
side (front side) of the mirror; instead,
they appear to converge behind the mirror
when extended backwards. For this reason,
convex mirrors produce virtual images
that are located somewhere behind the
mirror. To determine the image location,
only a pair of incident and reflected rays
needs to be drawn. These rays are the
same as those used in concave mirror ray
diagrams, except that in this case, the rays
appear to diverge from a point behind the
convex mirror. The divergence point can
be determined using the laws of reflection
for convex mirrors (Figure 4.27), stated as:
1. Any incident ray travelling parallel to
the principal axis will be reflected in
such a manner that the reflected ray
focal point of the mirror.
2. Any incident ray travelling toward a
convex mirror such that its extension
passes through the focal point will be
reflected parallel to the principal axis.
3. An incident ray, not parallel to the
principal axis, that strikes the pole
of the convex mirror is reflected such
Student’s Book Form Two
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