Laws of Reflection and Plane Mirrors Explained for School

Reflection is the bouncing back of light when it hits a surface. A plane mirror is a flat, smooth mirror that forms clear images. The laws of reflection tell us exactly how a light ray bounces, so we can predict where the image will appear.

These laws matter every day. They explain how we see ourselves in a mirror, how periscopes work, and why still water acts like a mirror. Understanding them is the first step in all of geometrical optics.

Key Concepts

Incident ray: The ray that strikes the surface. Reflected ray: The ray that bounces away. Normal: A line perpendicular to the surface at the point of incidence.

Angle of incidence i: Angle between incident ray and normal, measured in degrees. Angle of reflection r: Angle between reflected ray and normal, also in degrees.

Plane mirror: A flat reflecting surface, usually glass coated with silver or aluminium on the back.

• The image in a plane mirror is virtual, erect and same size
• Image distance behind mirror equals object distance in front
• Left and right are reversed, called lateral inversion

Formulas

First law: The incident ray, reflected ray and normal all lie in the same plane.

Second law: i = r. Angle of incidence equals angle of reflection. Both angles are measured from the normal, not from the surface. This holds for every smooth surface.

There is no magnification formula separately because magnification for a plane mirror is always +1. The plus sign means erect, 1 means same size.

Worked Example

A ray of light hits a plane mirror at 35 degrees to the surface. Find the angle of reflection and the angle between incident and reflected rays.

1. Angle with surface is 35 degrees, so angle with normal i = 90 minus 35 = 55 degrees.
2. By law of reflection, r = i = 55 degrees.
3. Angle between rays = i + r = 55 + 55 = 110 degrees.

So the reflected ray makes 55 degrees with the normal and the total deviation between incoming and outgoing rays is 110 degrees.

Common Misconceptions

• Angles measured from mirror surface: Students often take i from the surface. Correction: i and r are always from the normal.
• Mirror image is real: The image appears behind the mirror where no light actually goes. Correction: it is virtual and cannot be caught on a screen.
• Image is smaller if you move away: The image size stays equal to object size. It only appears smaller because it is farther from your eyes.

Real-World Applications

• Dressing mirrors and bathroom mirrors for daily use
• Periscopes in submarines and simple periscopes made with two plane mirrors
• Rear-view mirrors in vehicles use plane mirrors with warning about distance
• Solar cookers and optical instruments use plane mirrors to redirect light

Reflection from a plane mirror follows two simple laws, i equals r and everything lies in one plane. These rules predict image position, size and orientation perfectly.