The figure shows a ray of light that originates in an aquarium. It travels through water, is incident on the glass side, and emerges into the air. Ignore any partial reflections. Note: The figure is not drawn to scale.
-Which one of the following figures shows the smallest angle of incidence in the water for which no light emerges into air? Note: Only one figure is physically possible.
A)a
B)b
C)c
D)d

Question

The figure shows a ray of light that originates in an aquarium. It travels through water, is incident on the glass side, and emerges into the air. Ignore any partial reflections. Note: The figure is not drawn to scale. 
-Which one of the following figures shows the smallest angle of incidence in the water for which no light emerges into air? Note: Only one figure is physically possible.  
A)a 
B)b 
C)c 
D)d

Quizplus · Accepted Answer

In order for no light to emerge into air, the angle of incidence on the glass must be greater than the critical angle, which is given by sin(critical angle) = n2/n1, where n1, n2 are the refractive indices of water and glass respectively. From the figure, it can be seen that the critical angle is smallest for the smallest angle of incidence in water, which is shown in figure D. Therefore, option D is the correct choice.

The Figure Shows a Ray of Light That Originates in an Aquarium