Question

What device other than a plane mirror can be used to turn a ray of light through $180^\circ$ ? Draw a diagram in support of your answer. Name an instrument in which this device is used.

Answer 1

Before we go into the details, let's get a sense of what a fully reflecting prism is. When a light ray falls in a precise way on a specific side of this unusual form of prism, it can deviate a light ray by ${180^ \circ }$ . The name "total reflecting prism" comes from the fact that any light beam striking on one of its faces causes total internal reflection inside the prism.

Complete step by step answer:
To turn a ray of light ${180^ \circ }$ , a complete reflecting prism is employed. The diagram below clarifies things much more. The prism effect is employed in binoculars.

Explanation:- Conditions are as follows to get deviation of ${180^ \circ }$,
1. The use of a Total Reflecting Prism is required (right-angled isosceles prism with a ${90^ \circ }$ angle between two equal sides, and $2$ more angles with ${45^ \circ }$ each)
2. Light should be incident normally on the side of the Total reflecting prism (right-angled isosceles prism) that forms a ${45^ \circ }$ angle with the other two sides.

Take a light ray $PQ$ that is generally incident on $AC$ in the diagram above (making a ${90^ \circ }$ angle with $AC$ ). It enters the prism with no deviation and strikes the $AB$ at a ${45^ \circ }$ angle of incidence, which is greater than the critical angle for the air-glass pair $\left( {{{42}^ \circ }} \right)$ .As a result, the light ray is subjected to total internal reflection at the surface $AB$.

As a result, it is completely internally reflected and strikes the side $BC$ inside the prism with a ${45^ \circ }$ angle of incidence once more. Obviously, thorough internal reflection takes place here as well.It's clear that the ray of light deviates twice, once by ${90^ \circ }$ after reaching the $AB$ side, and then by another ${90^ \circ }$ after hitting the $BC$ side.With a prism, we may achieve a total ${180^ \circ }$ deviation of the light ray.

Note: The angle of incident must be greater than the critical angle of the prism for total internal reflection to occur, and the value of the critical angle of glass, $42^\circ$ , should be kept in mind when solving prism and its applications issues.