Answer-A periscope works on the principle of

Total internal reflection TIR is the phenomenon that involves the reflection of the entire incident light off the boundary. Glass prisms can be shaped to produce total internal reflection and as such are employed in binoculars, periscopes, telescopes, and other optical instruments. For the prism used in periscope, the light rays hitthe inside surface at ananglegreaterthanthecriticalangleof42and hence they are internally reflected.