Principle
A prism binocular is a double, field telescope for seeing final image erect in large field of view by using two totally reflecting prisms to get erect image of an inverted image without lateral inversion.
Construction: It consists of two objective lenses of large focal length and large aperture. To totally reflecting prisms are provided with each objective as shown in the figure. The eye pieces in the left and right sections are separated by some distance as the normal distance between the two eyes. Both eye pieces are movable forward and backward simultaneously.
Working
The rays from the objective O, travel thrice the length of the tube by means of two prisms before reaching the erecting eye peice E. The purpose of the terrestrial telescope is served in a short length and by using an objective of large focal length, sufficient magnification and a wider field of view can be obtained.
It has a benefit of binoculars vision (vision with two eyes). High magnification is achieved by increasing effective length by the pair of totally reflecting prisms in between the two lenses. Field of view is increased by using convex eye pieces instead of concave. The use of prisms, in addition to the help given in increasing the magnifying power of the instrument, also helps us in increasing the field of view, because it allows use of convex eye pieces without any lateral inversion. The inverted image formed by the object is made erect by the first prism and second prism corrects the same for lateral inversion as shown in the figure.
Merits
- It has a benefit of binocular vision (vision with two eyes).
- Field of view is large due to the use of convex eye pieces instead of concave eye prieces.
- Roundness of object seen becomes more clear as we see through two widely separated objectives.
- It is suitable for terrestrial observations because the final image seen is erect without lateral inversion.
