Principle of the periscope. A - Periscope using two plane mirrors. B - Periscope using two right–angled prisms. 1 - 2 - Plane mirrors. 3 - 4 - Right–angled prisms. 5 - 6 - Observer eye. 7 - 8 - Periscope tube. H - Periscope optical height.
Principle of the lens periscope. A - Periscope using a single lens (L2) to correct image. B - Periscope which uses two lenses (L2-L3) to correct image. 1-3 - Periscope window. 2-4 - Field stop or reticle. P - Right–angled prisms (or plane mirrors). L1 - Objective lens. L2 - for A - L2 - L3 - for B - Image erecting lens. L3 - L4 -for A - L4-L5- for B - Eyepiece. L0 - Field lens. y - Distant object. H - Periscope optical height.