Reflection of Light

What is Reflection of Light?

When a wave of light traveling through a given medium strikes the boundary of another medium, instead of passing through, some or all of the wave energy bounces off the boundary and returns into the initial medium. This phenomenon is known as the reflection of light.

Reflection is what allows us to see the world around us. Non-luminous objects (objects that do not produce their own light, like books, tables, or trees) must reflect light from a source (like the Sun or a light bulb) into our eyes to become visible.

The Normal Line, Incident Ray, and Reflected Ray

To mathematically describe the reflection of light at a boundary, we define three key geometric elements at the point of contact:

• Incident Ray: The incoming ray of light representing the direction in which the light energy travels toward the reflecting surface.
• Point of Incidence: The specific point on the reflecting surface where the incident ray strikes.
• Normal Line: An imaginary line drawn perpendicular (90°) to the reflecting surface at the point of incidence. All angles are measured relative to this normal line, rather than the surface of the mirror.
• Reflected Ray: The outgoing ray of light that bounces off the surface and travels back into the original medium.

The Laws of Reflection

The reflection of light at any boundary obeys two fundamental laws of physics:

Mathematically, if the angle of incidence is denoted as i and the angle of reflection is denoted as r, the law states:

This simple relationship holds true for every individual ray of light reflecting off any surface, regardless of whether the surface is a highly polished mirror or a rough asphalt road.

Regular (Specular) vs. Diffuse Reflection

How we perceive a reflection depends entirely on the smoothness of the reflecting surface:

How a Periscope Works

A periscope is an optical instrument that allows an observer to view objects that are out of their direct line of sight (for example, looking over a wall or from a submerged submarine). A basic periscope uses the principle of multiple reflections:

Two plane mirrors are mounted parallel to each other at opposite ends of a tube, both inclined at an angle of 45° relative to the path of the incoming light:

1. Light from a distant object enters the top opening horizontally and strikes the first mirror at 45° (incidence angle = 45°).
2. It reflects at 45° (reflection angle = 45°), turning by 90° to travel vertically downwards.
3. The light travels down the tube and strikes the second mirror at 45° (incidence angle = 45°).
4. It reflects at 45°, turning by another 90° to emerge horizontally into the observer's eye.

Because the two mirrors are parallel, the total rotation is 90° + 90° = 180°, meaning the exiting light ray travels in the exact same direction as the entering light ray, resulting in an upright virtual image.