The following devices use plane mirrors in their operations, except

Analyze the question: We need the device that does NOT use plane mirrors in its operation.

Concept: A plane mirror is a flat reflecting surface. It forms virtual images of the same size as the object and changes the direction of light by reflection.

Checking each option:

• A. Periscope: It uses two plane mirrors (or reflecting prisms) set at 45° to let you see over obstacles. It uses plane mirrors.
• B. Sextant: It uses an index mirror and a horizon glass (both plane mirrors) to measure the angle between two objects, such as the sun and the horizon. It uses plane mirrors.
• C. Kaleidoscope: It has two or three plane mirrors inclined at an angle, producing multiple images and symmetrical patterns. It uses plane mirrors.
• D. Binoculars: They use objective and eyepiece lenses, along with prisms (which use total internal reflection) to erect the image and shorten the instrument. They do not use plane mirrors.

Answer: D, binoculars.

Tip: Remember it this way: "Periscope, Sextant, Kaleidoscope = Mirrors. Binoculars = Lenses and prisms." When a question says "except," find the odd one out. Here, three devices rely on reflection from flat mirrors, while binoculars rely mainly on refraction through lenses.