Page 202 - Physics_Form_2
P. 202
Physics for Secondary Schools
Some rules are to be followed when one 2. A ray of light travelling through the
constructs ray diagrams for both convex optical centre continues to travel
and concave lenses. These rules originate
from the principles of refraction of light. along the same path for both convex
When a ray of light strikes the first face and concave lenses as seen in Figure
of a lens, it travels from a less dense to a 5.36.
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denser medium, thus it bends towards the
normal. Upon exiting on the other side of
the lens, the ray of light is now travelling
from the denser medium to a less dense O
medium. Therefore, the ray bends away F 1 F 2
from the normal as it exits the lens. Three
important rules are useful in constructing Convex lens
ray diagrams. The rules are:
1. Convex lens: A ray of light travelling
parallel to the principal axis of a
convex lens (Figure 5.35 (a)) is O
converged to the principal focus.
F 1 F 2
Concave lens: A ray of light travelling
parallel to the principal axis of a
concave lens (Figure 5.35(b)) is Concave lens
diverged in such a way that it appears
to be coming from the principal focus. Figure 5.36: Refraction of light rays
incident through the optical
centre
3. Convex lens: An incident ray of
F 1 F 2 light travelling through the principal
focus of a converging lens is
refracted parallel to the principal
(a)
axis of the convex lens as shown in
Figure 5.37 (a).
Concave lens: An incident ray of
light that appears to travel through
F F
1 2 the principal focus of a diverging lens
is refracted parallel to the principal
(b) axis. Figure 5.37(b) shows the path
Figure 5.35: Refraction of incident light rays of a ray through the principal focus
that are parallel to the principal axis for concave lenses.
196
Student’s Book Form Two
Physics Form 2 Final.indd 196 25/10/2025 10:28
