Question

why are prism used in binoculars ?

Answer 1

Prism are used in binoculars in addition to the convex and/or concave lenses used in binoculars or terrestrial telescopes.

1.  Binoculars show the image of distant objects in inverted way.  Then to make them upright, one needs another lens.  That makes binoculars very long.  Instead two prisms are used to reflect light from objective to prism1, then from prism1 to prism 2,  then from prism 2  to eye piece lens.
 
   The prisms reflect using total internal reflection.  So The image is brighter than if there are only lenses.

2.  There is color aberration or a little distortion with lenses.  Prisms reflect perfectly.  So there is no aberration.

Due to prisms the binoculars, becomes bulky in the middle.

Answer 2

Answer:

1. If 2 telescopes are mounted parallel to each other so that an object can be seen by both the eyes simultaneously the arrangement is called binocular.

2. The length of each tube is reduced by using a set of totally reflecting prisms.

3. They provide intense, erect image free from lateral inversion.

f0-focal length of objective

fe- focal length eyepiece

4. Through a binocular we get 2 images of the same object from different angles at same time.

5. Their super position gives the perception of depth also with length and breadth.

6. Binocular vision gives proper 3D image.

7. By using total reflecting prisms there is no loss of intensity.