A simple periscope used in submarines and observation instruments works mainly on which optical principle?

Introduction / Context:
Periscopes are devices that allow an observer to see over obstacles or out of a submerged submarine without exposing themselves. They use mirrors or prisms to redirect light from the object to the eye. Although simple periscopes can use plane mirrors, many optical periscopes and related instruments rely on total internal reflection inside glass prisms. This question asks which optical principle is mainly used in a standard periscope design taught in basic physics.

Given Data / Assumptions:

• We consider a typical prism based periscope as described in school physics.
• Light from an object enters a glass prism and is turned through right angles.
• The prism is designed so that light undergoes total internal reflection at its faces.
• The device allows the observer to see around an obstruction by redirecting the ray path.

Concept / Approach:
Total internal reflection occurs when light travels inside a denser medium towards a boundary with a rarer medium at an angle greater than the critical angle. In this case, all the light is reflected back into the denser medium with no loss, acting like a perfect mirror surface. Prisms in periscopes are cut so that light striking the internal faces at suitable angles experiences total internal reflection. This produces a brighter image than ordinary mirrors, which may absorb some light. Therefore the main working principle behind a prism periscope is total internal reflection.

Step-by-Step Solution:
Step 1: Recall that a basic periscope uses two right angled prisms placed at the top and bottom. Step 2: Light from the distant object enters the first prism and strikes the hypotenuse face at an angle greater than the critical angle. Step 3: At this face, the light undergoes total internal reflection and is turned through about 90 degrees. Step 4: The light then travels down the periscope tube and enters the second prism, where it again experiences total internal reflection and is turned toward the observer eye. Step 5: Because both reflections are total internal reflections, the image remains bright and undistorted, showing that total internal reflection is the key principle.

Verification / Alternative check:
If ordinary plane mirrors were used instead of prisms, the image would be formed by simple reflection at the mirror surface, but some light would be absorbed and the mirrors could tarnish over time. In high quality optical instruments such as binoculars and periscopes, prisms are preferred for their durability and brightness. The internal faces of these prisms are not silvered; they work purely by total internal reflection. This confirms that periscopes in standard physics illustrations are examples of total internal reflection in action.

Why Other Options Are Wrong:
Refraction only refers to bending of light at the interface, but in a periscope the main turning of the ray is due to reflection inside the prism.
Diffraction of light occurs when waves bend around obstacles or pass through narrow apertures and is not the primary effect here.
Simple combination of reflection and refraction at a single surface does not describe the ideal prism behaviour, which relies on complete internal reflection.
Interference of light concerns the superposition of waves and does not explain the functioning of a periscope.

Common Pitfalls:
Students sometimes remember that periscopes use mirrors and mistakenly think of ordinary reflection rather than total internal reflection. Others may confuse refraction through lenses with the reflection process in prisms. To keep concepts clear, associate prisms in optical instruments with total internal reflection, which gives a bright, undimmed image by reflecting almost all the incident light inside the denser medium.

Final Answer:
A standard prism based periscope works mainly on the principle of total internal reflection.