Page 178 - Physics_Form_2

Physics for Secondary Schools

light travels from a denser medium
to a lighter one (η < η ) , the
1
2
refracted light ray bends away from
the normal. It is also known that
the angle of refraction and hence
the amount by which light bends
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away from the normal depends on
the angle of incidence. Therefore,
the larger the angle of incidence,
the more the light bends away from
normal. Yet, one could ask, when
light travels from a denser medium
to a less dense medium, how far Figure 5.8: Occurrence of the total internal reflection
away from normal can light bend?

Total internal reflection only takes place when
The answer to this question is
that the bending of light can only both of the following two conditions are met:
get as far as 90° without leaving 1. Light travels from an optically denser
the medium. Therefore, there is medium towards a less dense medium.
an angle of incidence of light for 2. The angle of incidence is greater than the
which the angle of refraction is 90°. critical angle.
This angle of incidence is known
as the critical angle and is typically Total internal reflection will not take place
denoted by a letter c. When the unless the incident light is travelling within the
angle of incidence becomes greater more optically dense medium towards the less
than the critical angle, light is not optically dense medium.
refracted anymore; instead, light
is reflected back into the same
medium from which it is coming. For example,
This phenomenon is known as TIR will happen for light travelling from water
total internal reflection. Therefore, towards air, but it will not happen for light
travelling from air towards water. On the other
hand, TIR occurs only if the angle of refraction
Total internal reflection (TIR) exceeds 90°. When the angle of incidence
is a physical phenomenon in equals the critical angle, the angle of refraction
which all the light travelling in
an optically denser medium is is 90° (Figure 5.9 (a)). Also, when the angle of
reflected back upon striking the incidence is greater than the critical angle, all
interface with an optically less the light undergoes total internal reflection as
dense medium. shown in Figure 5.9 (b).

172
Student’s Book Form Two

Physics Form 2 Final.indd 172 25/10/2025 10:28