Question

1. Diagram alongside shows a glass prism of refractive index 1.5 and an object PQ is placed in front of face AC. Copy the diagram and complete it showing how the image is produced by the prism.
2. Name an instrument in which such prism is used.

Answer 1

(a) The critical angle of the glass is first found from μ = 1.5:

 

\[ \sin C = \frac{1}{\mu} = \frac{1}{1.5} = 0.667 \ \Rightarrow \ C \approx 42^\circ \]

1. Rays from P and Q fall normally on face AC, so they enter the prism without bending.
2. Inside, each ray strikes faces AB and CB at 45°. This is more than the critical angle (42°), so each ray is totally internally reflected twice.
3. The rays then leave through face AC normally, turned through 180°, and form the image P′Q′.
4. The image P′Q′ is inverted, P′ and Q′ have swapped places compared with P and Q.

(b) A prism used like this is called an erecting prism. It is used in binoculars to make an inverted image erect.