Question

Prisms are used in binoculars. Collect information why prisms are used in binoculars.

Answer 1

Hint: The images formed by the binocular without the prism are inverted. Prisms are used to erect the images formed by binoculars, so that it can be suitable for humans. Also prisms decrease the optical path of light, making the binocular smaller.

Complete Step By Step Solution:
Binoculars are magnifying devices, which make an object appear larger than its actual size. Prism is a transparent, polished optical instrument which refracts light through its surfaces. Prisms are used in binoculars for two primary reasons.

The image formed by the objective lens is laterally inverted and upside down. Prism corrects the orientation of the image and produces an upright image, suitable for viewing by the human eye. Another reason for using prisms in a binocular is reducing the length of the binocular. Prism bends the light rays and reduces the length of the path of light, making the binocular smaller.

Two types of prisms are used in binoculars which are Porro prism and Roof prism. Porro prism consists of four ${45}^{\circ }$ triangular reflecting surfaces. Binoculars use two porro prisms at right angles. Light enters in the prism through a large rectangular surface and then suffers total internal reflection twice from the triangular faces. The light exits from the rectangular surface, opposite to the direction on incident light rays. Roof prism is a reflective prism in which two faces meet at ${90}^{\circ }$. The name roof prism also comes from resemblance of this intersection with the roof of the house. Roof prism split the light entering it in two parts. One part gets inverted and the other part reflects through the two surfaces. Porro prism is considered better than the roof prism because it provides more contrast.

Note: Roof prism might appear to be simpler than the porro prism, but actually it has a more complex light path. Also the angle between the reflecting surfaces has to be accurate; otherwise the quality of image is degraded. So, roof prisms are required to be made with great precision, which also increases their cost.