Case Study:

Submarines operate deep underwater and must remain hidden to avoid detection. However, the crew inside the submarine still needs to observe ships, aircraft, and surroundings above the sea surface. For this purpose, submarines use a device called a periscope.

A periscope works based on the reflection of light using two plane mirrors placed parallel to each other at an angle of 45°. The upper part of the periscope remains above the water surface while the lower part is inside the submarine. When light from an object above the water enters the periscope, it strikes the first mirror at 45°. The mirror reflects the light downward to the second mirror placed inside the tube. The second mirror reflects the light horizontally toward the observer's eye inside the submarine.

Suppose a submarine is 20 m below the water surface. The periscope tube extends upward so that its top is 2 m above the water surface. When light from a ship reaches the periscope, it travels through the system of mirrors, allowing the submarine operator to see the ship without surfacing.

This technology is widely used in naval submarines, armored vehicles, and even simple observation devices used by soldiers. The working principle of periscopes demonstrates how basic laws of reflection can be applied in real-world engineering systems.

Understanding this concept helps students connect classroom physics with practical devices used in defense, navigation, and surveillance.

If you want to understand the basic concepts first, read these topics

Questions

Section A - MCQs

1. In a submarine periscope, the mirrors are usually placed at which angle to reflect light correctly toward the observer?
A. 30°
B. 45°
C. 60°
D. 90°

2. The main principle used in the working of a periscope is:
A. Refraction of light
B. Dispersion of light
C. Reflection of light
D. Diffraction of light

3. If the first mirror in a periscope reflects light downward, the second mirror will reflect the light:
A. Upward
B. Horizontally toward the observer
C. Back toward the object
D. Vertically upward

4. Why do submarines use periscopes instead of directly coming to the water surface?
A. To increase the speed of the submarine
B. To observe surroundings while remaining hidden
C. To reduce water pressure
D. To increase light inside the submarine

Section B - Short Answer Questions

1. Explain why plane mirrors are used in periscopes instead of curved mirrors.

2. A periscope contains two mirrors placed at 45°. Explain how the light path changes when it strikes each mirror.

3. If the mirrors in a periscope were not placed at the correct angle, what problem would the observer face? Explain.

Section C – Long Answer Question

1. A naval engineer is designing a new submarine periscope. He decides to change the angle of one mirror from 45° to 60°.

a) Predict how this change would affect the path of light inside the periscope.
b) Would the observer still see the object clearly? Explain using the laws of reflection.
c) Suggest why 45° is considered the ideal angle in standard periscopes.

Answer Key

MCQ Answers

1. 1. B - 45°
2. 2. C - Reflection of light
3. 3. B - Horizontally toward the observer
4. 4. B - To observe surroundings while remaining hidden

Section B - Short Answers

1. Plane mirrors are used because they produce clear, upright images without distortion. Curved mirrors may change the size or shape of the image, which would make observation inaccurate.

2. When light from an object strikes the first mirror at 45°, it is reflected downward according to the law of reflection. The second mirror also placed at 45° reflects the light horizontally toward the observer's eye.

3. If mirrors are not placed at correct angles, light will not reflect properly toward the observer. As a result, the image may be distorted, partially visible, or may not reach the observer at all.

Section C - Long Answer

a) If the mirror angle changes to 60°, the reflected ray will change direction because the angle of reflection equals the angle of incidence. The light will not travel along the intended path inside the periscope.

b) No, the observer will likely not see the object clearly because the reflected light may miss the second mirror or not reach the observer's eye.

c) An angle of 45° ensures that the light changes direction by 90°, which allows the periscope to redirect light vertically and then horizontally toward the observer. This arrangement makes the device efficient and practical.