How do periscopes use the laws of reflection?

In one line

The rule

The law of reflection states: "The angle of incidence is equal to the angle of reflection."

The formula is: angle of incidence (i) = angle of reflection (r)

• Angle of incidence (i): The angle between the incoming ray and the normal (an imaginary line at 90° to the mirror).
• Angle of reflection (r): The angle between the reflected ray and the normal.

Let's understand it simply

Imagine you are at a crowded cricket match. You want to see the field, but tall people are blocking your view. A periscope helps you look over their heads. It does this by using two mirrors inside a tube. When light from the field hits the top mirror, it bounces down to the second mirror, and then into your eyes. The mirrors are set at just the right angle so the light follows this path. This is possible because of the law of reflection: light always bounces off a mirror at the same angle it hits it.

How it works

1. Light from the object (like a cricket ball on the field) travels in straight lines.
2. The top mirror (set at 45°) catches this light and reflects it downwards.
3. The light travels down the tube to the second mirror, also at 45°.
4. The second mirror reflects the light horizontally, straight into your eye.
5. Each reflection follows the law: angle in = angle out, so the image stays clear and upright.

The story behind it

The periscope was first used in the 19th century, especially in submarines and trenches during wars. It allowed soldiers and sailors to see without exposing themselves to danger. The basic idea comes from the study of how light behaves with mirrors, which scientists like Alhazen (Ibn al-Haytham) and later Isaac Newton helped explain. The periscope became famous in World War I, when it saved many lives in the trenches.

Where you see it in real life

• Submarines: Sailors use periscopes to see above water while staying hidden below.
• Trenches in war: Soldiers use periscopes to look over the top without being seen.
• Tanks: Drivers use periscopes to see outside safely.
• Crowded events: Simple periscopes help kids see over crowds at parades or fairs.
• Science labs: Students use periscopes to learn about light and reflection.
• Inspection tools: Mechanics use small periscopes to look inside engines or behind walls.

Why it matters

• Safety: Periscopes let people see without exposing themselves to danger.
• Technology: The same principle is used in cameras, binoculars, and even some medical tools.
• Understanding light: Learning about periscopes helps you understand how light and mirrors work, which is useful in many fields.
• Exams and future studies: The laws of reflection are a foundation for optics, which comes up in higher classes and many entrance exams.

Worked example

Let's see how the angles work in a periscope:

Given: Both mirrors are at 45° to the tube.
Step 1: Light hits the first mirror at 45° (angle of incidence).
Step 2: By the law of reflection, it bounces off at 45° (angle of reflection).
Step 3: The light travels down and hits the second mirror at 45°.
Step 4: Again, it bounces off at 45°, now moving straight into your eye.
Step 5: The image you see is in the same direction as the original object.

Common mistakes

• Thinking the mirrors can be at any angle: For a clear image, both mirrors must be at 45°.
• Forgetting the law of reflection: Some think the angle changes, but it is always angle in = angle out.
• Believing the image is upside down: In a simple periscope, the image stays upright.
• Assuming periscopes use lenses: Basic periscopes use only mirrors, not lenses.

Remember it

"Periscope = two mirrors at 45°, bouncing light into your eyes using the law: angle in = angle out."