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Looking at this question, first, I thought for a while about redundancy. A triangular prism. I searched on Wikipedia for the terms prism that says At least two of the flat surfaces must have an angle between them. (means triangle, isn't it? and triangular prism that says a three-sided prism.

In addition, I searched Google Images with triangle prism and prism. Most of them were triangle!

Of course, the latter one talks about geometry but still, a triangular prism? Why simply not say prism and it includes its shape i.e. triangle! Fun is, I searched for round prism and rectangle prism - both have results! Google is great!

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    "At least two of the flat surfaces..." – Helix Quar Apr 10 '14 at 5:31
  • @helix true but what else shape you can come up for being a prism? – Maulik V Apr 10 '14 at 5:38
  • I can conceive of a trapezoidal prism, though I have no idea of its practical application – toandfro Apr 10 '14 at 5:41
  • @MaulikV lots – Helix Quar Apr 10 '14 at 5:42
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    A cylinder is a prism. So are these. Why say "triangular prism"? Because a prism isn't necessarily triangular. A prism's base can have any shape or number of sides. – J.R. Apr 10 '14 at 9:30
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Although there are lots of other kinds, people often use the word prism to refer to a triangular prism.

From the same Wikipedia article you linked...

The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use "prism" usually refers to this type.

From Wiktionary

A transparent block in the shape of a prism (typically with triangular ends), used to split or reflect light.

  • Yes, that's what I was a bit confused about! – Maulik V Apr 10 '14 at 5:58
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In a single lense reflex (SLR) photo camera, you often have a pentaprism to transfer the immage that is seen through the lens, and reflected from the mirror before the shutter to the ocular (eyepiece).

pentaprism

If you look at the shape, you will see it fits your definition, and yet, it is not triangular :)

  • Thank you. True..it has to be specified to mean a triangular prism. +1 :) – Maulik V Apr 10 '14 at 5:59
  • I have to admit that before I started photography, I had no idea about this either :) – oerkelens Apr 10 '14 at 6:04

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