A combination of two convex lenses is used in a simple binoculars with two small telescopes attached together.  Each side is meant for an eye.

The quality of the images seen by the person is not very good, there are color aberrations and the image is inverted when lenses are used.  If image is to be erected, one more lens is to be used and that makes the binoculars long.  Also there is a loss of light energy when lenses are used.

Prisms are used to retain the total energy of light, as total internal reflection takes place.  Prisms do not cause color aberration. The quality of the image is better and the image is erect.

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