A periscope uses two mirrors to let a viewer see over an obstacle.

The longer version

Two flat mirrors mounted at forty-five degrees inside a tube turn a line of sight twice. Light from the scene strikes the upper mirror, travels down the tube, strikes the lower one, and leaves horizontally toward the eye, so the viewer sees around a corner or over a wall.

Each mirror flips the image once, and two flips restore the original orientation, which is why a simple periscope shows things upright rather than inverted. Prisms often replace the mirrors because they reflect efficiently and are easier to keep aligned. Submarines use optical versions alongside electronic ones when running near the surface.

Where this entry sits

These facts explore how light behaves and how human eyes turn it into the colors we see. Along the way you meet rainbows, prisms, mirrors, shadows, lasers, and optical tricks that fool our vision every day. The full list sits on the Color and Light page, where every entry is listed in order.