iGCSE Physics (0625) 3.2.2 Refraction of light-Exam Style Questions- New Syllabus
▶️ Answer/Explanation
Detailed solution:
The angle of refraction is defined as the angle between the refracted ray and the normal at the point of incidence.
The normal is a dashed line perpendicular to the surface of the transparent block.
From the diagram, the angle between the refracted ray and the horizontal dashed line (the normal) is explicitly labeled.
This value is given as $25^{\circ}$.
Note that the angle $65^{\circ}$ is the angle between the refracted ray and the boundary, not the normal.
Therefore, the angle of refraction is $25^{\circ}$, matching option A.
▶️ Answer/Explanation
Detailed solution:
Total internal reflection (TIR) can only occur when light travels from an optically denser medium to a less dense medium.
The critical angle $c$ is defined as the angle of incidence in the denser medium for which the angle of refraction is $90^{\circ}$.
In the provided diagram, the ray moves from the glass prism into the air at the second surface.
The angle of incidence is the angle between the incident ray and the normal inside the denser medium.
Angle $D$ is the angle of incidence at the boundary where the ray is attempting to exit the prism.
When TIR “just occurs,” the angle of incidence $i$ must be exactly equal to the critical angle, so $D = c$.
Angles $A$ and $B$ are at the first surface where light enters the prism, so TIR cannot occur there.
▶️ Answer/Explanation
Detailed solution:
When light passes from a less dense medium (air) to a more dense medium (glass), it slows down and bends toward the normal.
This means the angle of refraction $r$ is always less than the angle of incidence $i$ ($r < i$).
Option A is incorrect because the angle of incidence is measured from the normal, not the surface.
Option B is incorrect as the angle of incidence can be any value between $0^{\circ}$ and $90^{\circ}$.
Option C is incorrect because refraction occurs normally for any incident angle when entering a denser medium.
Therefore, statement D is the only correct physical description of this interaction.