The diagram shows a diver looking up through calm water at the sky. The surface is horizontal. Rays from objects above the water reach the diver only within...
Question 1 Report
The diagram shows a diver looking up through calm water at the sky. The surface is horizontal. Rays from objects above the water reach the diver only within a circular region often called Snell's window. A ray from an object at the edge of this region refracts at the water-air boundary and reaches the diver's eye. Another ray inside the water meets the surface at a larger angle and is totally internally reflected. The normal is drawn at the point where the edge ray meets the surface.
(a) What is the refracted ray at the edge of Snell's window travelling along? [1]
(b) When does a ray at a water-air surface undergo total internal reflection? [2]
(c) Which ray shown has a larger angle of incidence, the edge ray or the reflected ray? [1]
(d) What change in speed occurs when light passes from air into water? [1]
(e) Sketch an arrow to show the direction of the reflected ray after it meets the water surface. [1]
Answer Details
(a) At the edge of Snell’s window, the refracted ray travels along the water surface, so it is at \(90^\circ\) to the normal. [1]
(b) Total internal reflection requires both conditions: light must travel from water to air, and its angle of incidence must be greater than the critical angle. [2]
(c) The reflected ray has the larger angle of incidence. It is the ray that reaches the surface at an angle greater than the critical angle. [1]
(d) Light slows down when it passes from air into water. [1]
(e) The reflected ray remains in the water and obeys \(\text{angle of reflection}=\text{angle of incidence}\). The arrow must point away from the surface, back into the water. [1]
Remember: at the critical angle the refracted ray is along the boundary; above it, no refracted ray emerges and total internal reflection occurs.
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