Caption: "Combination of porro prisms used in binoculars."

1. A porro prism is a type of prism used in optical devices to change the orientation of an image by total internal reflections.

2. Total internal reflection occurs when angle of incidence of light ray at an interface between optical media is too large to allow refraction: i.e., no refracted light ray is transmitted.

3. In the image, there are 4 total internal reflections.

   Each pair of total internal reflections (which are caused by one prism) causes a plane reflection through a plane normal to the incident surface and aligned with the back vertex.

   Recall a plane reflection is really a back-to-front inversion across a plane.

   The 2 plane reflections effect an overall point inversion (i.e., 180° rotation) of the optical image. This point inversion cancels the point inversion the objectives of binoculars.

4. By the by, the high probability of total internal reflections for light rays entering diamond is what makes diamonds sparkly---and NOT just like cut glass.