Totally reflecting prisms are preferred to mirrors in periscopes because I. mirror gives multiple images II…

Analyze the question: We need to decide which reasons explain why totally reflecting prisms are preferred to plane mirrors in periscopes.

Concept: A prism used in a periscope is a right-angled isosceles glass prism. Light enters one face at normal incidence and hits the hypotenuse at 45°. The critical angle of glass is about 42°, so 45° is greater than the critical angle and total internal reflection occurs.

Evaluate each statement:

• I. Mirrors give multiple images: TRUE. A glass mirror reflects partly from the front glass surface and mainly from the silvered back surface. This produces a faint secondary image (ghost image) alongside the main one, which blurs the view. A prism gives one sharp image.

• II. Silvering on mirrors is easily damaged: TRUE. The silver coating tarnishes, peels, or scratches with age and exposure to moisture. A prism has no coating, so it lasts longer.

• III. Mirrors reflect less light than totally reflecting prisms: TRUE. Total internal reflection reflects essentially 100% of the light. A silvered mirror absorbs some light, so the image from a prism is brighter.

Answer: E (I, II and III)

Exam tip: Remember the three advantages of prisms over mirrors with the idea "Sharp, Strong, Stays": a sharp single image, a strong, bright reflection (nearly 100%), and a coating that stays intact because there is none to damage. If a question asks about prisms in periscopes or binoculars, all three usually apply.