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Prisms

Prisms are defined as a transparent medium that is bound by two plane sides that are inclined at an angle to each other. Prisms are used to deviate light, but do not change the vergence and for this reason, they do not focus light. With prisms, light is bent towards the base. The image of an object formed by a prism is a virtual image. The image will appear displaced towards the apex of the prism.

A Prism Diopter (∆) (See Figure 28) is defined as a deviation of 1 cm at 1 meter. For angles under 45° (or 100 ∆), each degree (°) of angular deviation equals approximately 2 ∆. (Approximation Formula).

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Question: A 6 PD prism will displace a ray of light how far at 1/3m?

Answer: A 6PD prism will deviate light 6cm at 1m. Therefore at 1/3m x 6PD = 2cm.

Question: What is the power of a prism that displaces an object 10cm at a distance of 50cm?

Answer: 10/50 = x/100 = 20PD.