A ray of light is reflected at a plane mirror. Which statement correctly describes this reflection?

• A: The angle of incidence is measured between the incident ray and the mirror surface.
• B: The angle of incidence is equal to the angle of reflection, with both angles measured relative to the normal.
• C: The reflected ray is always perpendicular to the incident ray.
• D: The angle of reflection is measured between the reflected ray and the mirror surface.

A ray of light passes from air into a rectangular block of transparent plastic. The angle of incidence $i$ is $50^{\circ }$ and the angle of refraction $r$ is $31^{\circ }$.

1. State the name of the line drawn perpendicular to the surface of the plastic block at the point where the light ray enters.
2. State the relationship between refractive index $n$, angle of incidence $i$, and angle of refraction $r$.
3. Calculate the refractive index of the plastic block. Give your answer to 2 decimal places.

Light is able to travel along a curved glass optical fibre without escaping through the sides.

1. What is the name of the phenomenon that allows light to travel along the fibre in this way?
2. Explain the two conditions required for this phenomenon to occur at the boundary inside the fibre.