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     Telescopes and binoculars are used to view distant objects.  They consist of a long tube with a big convex lens (objective lens) and a small convex lens called eye piece.  The ratio of focal length of objective to that of eye piece gives us the magnification.  The image appears inverted when we view it. Length of telescope is the sum of focal lengths of two lenses.

    To make the image erect (not inverted) for our eye, we need to add another inverting lens in the middle of the two lenses. This further increases the length of the telescope by 4 times the length of the inverting lens.

  For obtaining a good view of distant objects, the size of telescope has to be very big in diameter and in length too. They are heavy and cannot be conveniently handled.

    Prismatic telescopes solve this problem by use of two triangular prisms to invert the image between the eye piece lens and the objective lens.  The first prism receives parallel rays from distant objects and reflects (total internal reflection) rays on to another prism which totally internally reflects rays back towards the eye piece. These rays are now viewed through eye piece with a large magnification.

   In SLR cameras and binoculars, prisms are used along with a reflector mirror (concave).  This is when you see that the binocular is not cylindrical in shape.
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