Which one among the following is used to make periscope ?

1. Fundamental Laws of Reflection (basic)

To master geometrical optics, we must begin with the Fundamental Laws of Reflection. When light strikes a polished surface, such as a mirror, it doesn't bounce off randomly; it follows two precise rules that govern its behavior. To understand these, we first imagine a line perpendicular (at 90°) to the surface at the exact point where the light hits. This imaginary line is called the Normal.

• First Law: The angle of incidence (the angle between the incoming ray and the normal) is always equal to the angle of reflection (the angle between the reflected ray and the normal). Mathematically, we say ∠i = ∠r Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135.
• Second Law: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane. Imagine these three lines drawn on a single flat sheet of paper; they will all stay flush against that paper without any ray sticking out at an odd angle Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135.

A common point of confusion for students is whether these laws only apply to flat mirrors. In fact, these laws are universal. They apply to all types of reflecting surfaces, including curved or spherical mirrors like the concave and convex mirrors found in car side-mirrors or makeup mirrors Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.160. At any specific point on a curved surface, the light behaves exactly as if it hit a tiny, flat portion of that surface.

Consider the special case of Normal Incidence. If a ray of light hits a mirror head-on, traveling exactly along the path of the normal, the angle of incidence is 0°. Consequently, according to the laws of reflection, the angle of reflection is also 0°, and the light simply bounces straight back along its original path Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.158.

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.135; Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.160; Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.158

2. Characteristics of Images in Plane Mirrors (basic)

To understand how a plane mirror works, we must first look at the Laws of Reflection. Every time light hits a smooth, flat surface, the angle at which it arrives (angle of incidence) is exactly equal to the angle at which it bounces off (angle of reflection). Furthermore, the incoming ray, the reflected ray, and the 'normal' (an imaginary perpendicular line at the point of impact) all sit on the same flat plane Science Class X, Light – Reflection and Refraction, p.135. These laws are the 'rulebook' that ensures the image you see is a faithful, though mathematically modified, representation of yourself.

The images formed by these mirrors have four distinct characteristics that every UPSC aspirant should memorize. First, the image is virtual and erect. 'Virtual' means the light rays don't actually meet behind the mirror; your brain just perceives them as if they do, so you cannot project this image onto a screen. Second, the size of the image is always identical to the size of the object Science Class VIII, Light: Mirrors and Lenses, p.156. If you are six feet tall, your image is six feet tall—unlike concave or convex mirrors which can magnify or diminish your appearance.

Third, there is a perfect symmetry in position: the image appears to be at the same distance behind the mirror as the object is in front of it Science Class VII, Light: Shadows and Reflections, p.162. Finally, we encounter lateral inversion. This is the fascinating phenomenon where the left side of the object appears as the right side of the image, and vice versa. This is why the word 'AMBULANCE' is written backwards on vehicles—so that drivers ahead can read it correctly in their rearview plane mirrors Science Class VII, Light: Shadows and Reflections, p.167.

Feature	Description in Plane Mirror
Nature	Virtual and Erect (Upright)
Size	Same as the object
Distance	Object-to-Mirror = Mirror-to-Image
Orientation	Laterally Inverted (Left-Right swap)

Sources: Science Class X, Light – Reflection and Refraction, p.135; Science Class VIII, Light: Mirrors and Lenses, p.156; Science Class VII, Light: Shadows and Reflections, p.162; Science Class VII, Light: Shadows and Reflections, p.167

3. Spherical Mirrors: Concave vs. Convex (intermediate)

To understand spherical mirrors, imagine a hollow glass sphere. If you cut a slice from this sphere and silver one side, you create a mirror with a curved reflecting surface. If the reflection happens on the inner curved surface (like the inside of a spoon), it is a Concave Mirror. If the reflection happens on the outer bulging surface, it is a Convex Mirror. Both types strictly follow the Laws of Reflection, but their geometry changes how they converge or diverge light rays Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.158.

Concave mirrors are known as converging mirrors. When parallel rays of light hit a concave mirror, they reflect and meet at a single point called the Principal Focus (F). These mirrors are highly versatile: depending on the distance of the object from the mirror, they can produce images that are real or virtual, and magnified or diminished Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.137. This ability to magnify makes them ideal for shaving mirrors or for use by dentists.

Convex mirrors, on the other hand, are diverging mirrors. They scatter parallel light rays outwards. No matter where you place an object in front of a convex mirror, the image formed is always virtual, erect, and diminished Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.160. Because the images are smaller than the objects, these mirrors offer a much wider field of view, which is why they are used as rear-view mirrors in cars and security mirrors in shops.

Feature	Concave Mirror	Convex Mirror
Reflecting Surface	Curved Inwards	Curved Outwards
Nature of Mirror	Converging	Diverging
Image Characteristics	Real or Virtual; can be magnified	Always Virtual, Erect, and Diminished
Common Use	Searchlights, Solar furnaces	Rear-view mirrors, ATM security

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.137; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.158; Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.160

4. Refraction and the Function of Lenses (intermediate)

Hello! Now that we understand how light reflects, let’s explore what happens when it actually passes through a material. This phenomenon is called refraction. Simply put, refraction is the bending of light as it travels from one transparent medium (like air) into another (like glass or water). This happens because the speed of light is different in different media; it slows down in denser materials, causing its path to pivot Science, Class X, Light – Reflection and Refraction, p.159.

To master this, we look at lenses. A lens is a piece of transparent material bound by at least one spherical surface Science, Class X, Light – Reflection and Refraction, p.150. Unlike mirrors that bounce light back, lenses use refraction to redirect light. We categorize them into two primary types based on their shape and how they behave when light hits them:

Feature	Convex Lens (Converging)	Concave Lens (Diverging)
Shape	Thicker at the middle, thinner at the edges.	Thicker at the edges, thinner at the middle.
Effect on Light	Converges parallel rays to a single point (Focus).	Spreads out (diverges) parallel light rays.
Image Type	Can form both real (inverted) and virtual (erect) images depending on object distance.	Always forms a virtual, erect, and diminished image Science, Class VIII, Light: Mirrors and Lenses, p.163.

In technical terms, the ability of a lens to converge or diverge light is called its Power, which is mathematically the reciprocal of its focal length (P = 1/f). We also use the Lens Formula (1/v - 1/u = 1/f) to calculate exactly where an image will appear based on where the object is placed Science, Class X, Light – Reflection and Refraction, p.159. Understanding these helps us design everything from the spectacles on your nose to the massive telescopes exploring the stars.

Sources: Science, Class X (NCERT 2025), Light – Reflection and Refraction, p.150, 159; Science, Class VIII (NCERT 2025), Light: Mirrors and Lenses, p.163

5. Total Internal Reflection and Optical Prisms (exam-level)

When light travels from an optically denser medium (like glass or water) to a rarer medium (like air), it generally bends away from the normal. However, if we keep increasing the angle of incidence, we reach a specific point called the Critical Angle. At this angle, the refracted ray grazes the boundary surface. If the incident angle increases even slightly beyond this critical point, the light is not refracted at all; instead, it is entirely reflected back into the denser medium. This fascinating phenomenon is known as Total Internal Reflection (TIR). Unlike ordinary mirrors, which absorb a small portion of light, TIR is nearly 100% efficient, making it invaluable for precision optics Science, Class X, Light – Reflection and Refraction, p.158.

Optical prisms leverage TIR to manipulate light paths with extreme clarity. A common example is the right-angled isosceles prism (45°-90°-45°). For standard glass, the critical angle is approximately 42°. When light enters one of the shorter faces of such a prism perpendicularly, it strikes the hypotenuse at an angle of 45°. Since 45° is greater than the critical angle of 42°, the light undergoes total internal reflection. This allows the prism to turn the light beam by 90° or even 180° without the need for silvered mirror surfaces which can tarnish over time Science, Class X, The Human Eye and the Colourful World, p.165.

While basic instruments like a simple school-project periscope use plane mirrors fixed at 45° angles to reflect light Science, Class VII, Light: Shadows and Reflections, p.164, professional-grade instruments like binoculars and submarine periscopes prefer totally reflecting prisms. The reason is twofold: prisms are more durable and they avoid the "ghost images" often caused by multiple reflections within the thick glass of a standard mirror. Whether using a mirror or a prism, the fundamental laws of reflection (where the angle of incidence equals the angle of reflection) remain the governing principle of the device's geometry Science, Class VIII, Light: Mirrors and Lenses, p.160.

Feature	Plane Mirror Reflection	Total Internal Reflection (Prism)
Efficiency	Lower (some light is absorbed by the silvering)	Virtually 100% (highly efficient)
Image Quality	Can produce "ghosting" due to glass thickness	Crisp, single image
Durability	Silver coating can peel or tarnish	Prism surfaces do not tarnish easily

Sources: Science, Class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.158; Science, Class X (NCERT 2025 ed.), The Human Eye and the Colourful World, p.165; Science, Class VII (NCERT 2025 ed.), Light: Shadows and Reflections, p.164; Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.160

6. Working Principle of a Periscope (intermediate)

A periscope is a fascinating optical instrument that essentially allows you to "see around corners" or over obstacles. It operates primarily on the laws of reflection using a pair of mirrors. In its most basic form, a periscope consists of a tube (often Z-shaped) with two plane mirrors fixed at each bend. These mirrors are positioned parallel to one another and are set at a 45-degree angle to the path of the incoming light Science-Class VII, Chapter 11, p.164. This specific orientation is the key to its functionality.

The working principle is a two-step reflection process. When light from a distant object enters the top of the periscope, it strikes the first mirror at an angle of 45°. According to the law of reflection, it is reflected downwards at another 45° (totaling a 90° turn) through the tube. At the bottom, it hits the second mirror, which is also at a 45° angle, reflecting the light another 90° into the observer’s eye. This double reflection ensures that the light exits parallel to its original path, allowing you to see an upright, virtual image of the object even if you are standing behind a wall or submerged in a submarine Science-Class VII, Chapter 11, p.164.

It is important to note why we use plane mirrors rather than spherical ones. Plane mirrors reflect parallel rays of light as parallel rays, preserving the shape and size of the object. If we were to use concave or convex mirrors, the light rays would either converge or diverge, resulting in a distorted or blurred image that would be useless for accurate observation Science, Class VIII, Chapter 10, p.160. While advanced periscopes in modern submarines use prisms for better light transmission and lenses for magnification, the underlying geometry of turning light via reflection remains the same.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 11: Light: Shadows and Reflections, p.164; Science-Class VIII . NCERT(Revised ed 2025), Chapter 10: Light: Mirrors and Lenses, p.160

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of light, this question serves as a perfect application of the laws of reflection. A periscope is designed to allow an observer to see objects that are not in their direct line of sight—essentially "looking over" obstacles. This is achieved by redirecting light beams using mirrors. In a standard periscope, light hits a top mirror and is reflected 90 degrees down a tube, where it hits a second mirror and is reflected another 90 degrees into the observer's eye. To ensure that the image remains true to size, upright, and undistorted, we rely on the specific properties of the Plane mirror.

When approaching this question, your reasoning should focus on the geometry of reflection. For the light to travel vertically down the tube and then horizontally to the eye, the mirrors must be placed parallel to each other at a 45-degree angle to the path of light. Since a Plane mirror reflects light at the exact same angle it receives it without changing the focal properties, it is the ideal component for this device. As noted in Science-Class VII . NCERT(Revised ed 2025), while advanced military periscopes may incorporate prisms for better clarity, the fundamental building block is the plane reflective surface.

UPSC often includes distractors like Concave lens or Concave mirror to test your understanding of image formation. These are common traps; a concave surface would converge light rays, leading to a magnified, diminished, or inverted image depending on the distance. Such distortion would make a periscope useless for accurate observation. Therefore, by eliminating options that would warp the visual information, we arrive confidently at the correct answer: (C) Plane mirror.