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Can a ray of light go undeviated from a prism?

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Borys Shumyatskiy eNotes educator | Certified Educator

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Yes, it can, but only in very particular situations.

The first possibility. A ray does NOT change its direction on a divide between two matters if it is perpendicular to this divide (boundary). So a ray may keep its direction after the first face of a prism. Usually we call "a prism" in this context an object with non-parallel sides, but formally a cube is also a prism.

The second possibility. Even if a ray is not perpendicular to the divide between two matters, it changes its direction only if the speeds of light in these matters are different. (Or we may say that they have different refractive indices.) If a prism is made of glass and it is in the air, then the indices are different. But if a prism is immersed into some dense transparent liquid, then the speeds may occur equal. For example, water and water ice have close refractive indices.

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