Final answer:
Refraction is the optical phenomenon applied in a periscope for navigation underwater by allowing observation from below the surface. It involves the redirection of light waves at the interface of different mediums.
Explanation:
Refraction is the optical phenomenon applied in a periscope that allows for submerged navigation by enabling the observation of the surface of the sea while underwater. Refraction occurs when light waves change direction as they pass from one medium to another with different optical densities, causing the apparent displacement of objects.
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