___X___ is a device which is used to see around corners and is based on the…

Concept: Three optical behaviours can redirect or alter a line of sight: reflection turns a light ray back at an angle equal to its angle of incidence when it strikes a surface such as a mirror, without changing the ray's colour; refraction bends a ray as it crosses between two transparent media of different optical density (as through a lens or a prism); and dispersion splits white light into its spectrum of colours because different wavelengths refract by slightly different amounts.

Application: The stem asks for a device that lets someone see around corners or over obstacles without stepping into view — building such a device needs a way to carry the line of sight through one or more bends while keeping the image intact and undistorted in colour, which reflection off angled surfaces such as flat mirrors, or right-angled prisms working by total internal reflection, provides: each surface turns the incoming ray through roughly 90°, so an instrument built this way — a periscope — delivers the image to the observer's eye even though the object itself is not in a direct line of sight.

• Telescope: forms a magnified image of distant objects along a straight line of sight using lenses or curved mirrors; it cannot bend that line of sight around a corner.

• Kaleidoscope: also uses mirror reflections, but arranged to multiply the view of loose coloured pieces into symmetric decorative patterns, not to relay a view past an obstruction.

• Refraction: bends a ray crossing between media of different density (as through a lens or a prism); it changes a ray's direction but does not describe how a periscope's mirrors work.

• Dispersion: the splitting of white light into its spectrum of colours (as when a prism forms a rainbow); it explains why a spectrum appears, not how a device can relay an image around a bend.

Cross-check: tracing the light path through such a device shows it changes direction only at its reflecting surfaces, with no colour separation (which dispersion would cause) and no bending of the ray at a transparent block's interfaces (which refraction would cause) — confirming reflection, not refraction or dispersion, as the operating principle.

Hence the device is the periscope, and it operates on the principle of reflection.