A probe beam of light is directed at a flat polished silver surface. Which statement correctly details this reflection?

• A: The angle of reflection is the angle between the reflected ray and the reflective surface, and is equal to the angle of incidence.
• B: The incident ray, the reflected ray, and the normal line at the point of incidence all lie in different planes.
• C: The angle of incidence and angle of reflection are equal, with both measured between their respective rays and the perpendicular normal line.
• D: The reflected ray always travels back along the path of the incident ray.

A narrow monochromatic laser beam is directed from air into a synthetic diamond gemstone. The angle of incidence $i$ is $38^{\circ }$ and the measured angle of refraction $r$ is $15^{\circ }$.

1. State the term used for the imaginary line drawn perpendicular to the gemstone's surface at the point of entry.
2. Write down the equation that relates the refractive index $n$ of the gemstone to the angle of incidence $i$ and the angle of refraction $r$.
3. Calculate the refractive index of this synthetic diamond gemstone. Give your answer to 2 decimal places.

An underwater diver shines a torch upwards towards the smooth surface of a calm lake. Under certain conditions, no light escapes the water into the air.

1. Identify the wave phenomenon that describes this complete reflection of light back into the water.
2. State and explain the two physical conditions necessary for this phenomenon to occur at the water-air interface.