Reflection of Light
Simple Explanation
Reflection happens when light bounces off a surface, obeying the law of reflection: the angle of incidence equals the angle of reflection (both measured from an imaginary line, the normal, drawn perpendicular to the surface). Regular (specular) reflection happens off a smooth surface, where all reflected rays stay parallel — this is what makes a sharp, clear image possible. Diffuse reflection happens off a rough surface, scattering light in many directions.
Why Do We Need It?
This single law explains why a mirror gives a sharp, clear reflection while a piece of paper (also reflecting light) does not — it's entirely down to how smooth the reflecting surface is.
Worked Example
Apply the law of reflection
A ray of light strikes a flat mirror, making an angle of 35° with the normal. Find the angle of reflection, and the angle between the incident and reflected rays.
Why Does This Work?
The law of reflection is a direct consequence of how light interacts with a surface — it always reflects symmetrically about the normal, which is exactly why the two angles, measured on either side of that same normal line, must be equal.
Real-Life Example
Why wet roads are harder to see at night
A dry road diffusely scatters headlight beams in many directions, but a wet road can create a bright, distracting glare.
A thin layer of water makes the road surface much smoother, shifting the reflection from diffuse toward regular (specular) — concentrating the reflected headlight beam directly back toward the driver's eyes as glare.
Practice
A ray of light hits a mirror with an angle of incidence of 50°. Find the angle of reflection.
EasyCommon mistake
Measuring the angle of incidence/reflection from the mirror surface itself, instead of from the NORMAL (the perpendicular line to the surface) — the law of reflection is always defined relative to the normal, not the surface.
Quick Review
- Law of reflection: angle of incidence = angle of reflection (both from the normal).
- Regular reflection: smooth surface, parallel rays stay parallel — sharp images.
- Diffuse reflection: rough surface, rays scatter in many directions — no clear image.