How Light Reflects
When a ray of light hits a smooth (specular) surface such as a mirror, it bounces off according to the Law of Reflection.
Law of Reflection
Angle of incidence (θᵢ) = Angle of reflection (θᵣ). Both measured from the normal.
Key Terms
- Normal: an imaginary line perpendicular to the surface at the point of contact
- Angle of incidence: angle between the incoming ray and the normal
- Angle of reflection: angle between the reflected ray and the normal
Specular vs. Diffuse Reflection
| Type | Surface | Result |
|---|---|---|
| Specular | Smooth (mirror, calm water) | Clear image |
| Diffuse | Rough (paper, wall) | Scattered light, no image |
Plane Mirror Images
An image in a plane (flat) mirror is:
- Virtual (behind the mirror)
- Same size as the object
- Same distance behind the mirror as the object is in front
A ray hits a mirror at 35° to the normal. What is the angle of reflection?
Real world: Periscope in a submarine
A periscope uses two flat mirrors set at 45° to let a submarine crew see above the water surface without surfacing. Light hits the top mirror at 45° (angle of incidence = 45°), reflects downward at 45° (angle of reflection = 45°), then hits the bottom mirror and reflects horizontally into the observer's eye — all thanks to the law of reflection.
Real world: Solar cooker with parabolic mirror
A solar cooker uses a curved mirror made of many small flat sections. Each flat section reflects sunlight according to the law of reflection, and the mirror's curved shape is designed so all reflected rays converge at one point — the cooking pot. Temperatures at the focal point can exceed 200 °C.
Real world: Cat's eyes on the road
Cats' eyes (road studs) are small reflectors embedded in road surfaces. Each reflector consists of two small mirrors angled so that car headlight beams reflect straight back toward the driver, regardless of the exact angle of approach — a direct application of the law of reflection for safety in the dark.
Practice Problems
A ray strikes a mirror at 50° to the surface. What is the angle of incidence?
Show answer
You stand 0.8 m in front of a mirror. How far behind the mirror does your image appear?