1.1. A ray of light passes from air into a glass prism as shown in Figure 1.

Figure 1

As the light ray passes through the prism, it emerges back into the air.

Calculate the critical angle from the glass to the air.

1.2. Figure 2 now shows the prism rotated and one side is coated with a film of transparent gel. A ray of light strikes the prism, at an angle of incidence of 38°, and continues through the glass to strike the glass–gel boundary at the critical angle.

Figure 2

Calculate the refractive index of the gel.

1.3. A ray of light now strikes the prism at an angle of incidence which means that it now refracts straight through the gel at the glass–gel boundary. 

Without calculation, explain how the critical angle for the glass–gel boundary differs from the critical angle for the gel–air boundary.