AcademyGeometric Optics
Academy
Plane-Surface Reflection
Level 1 - Physics topic page in Geometric Optics.
Principle
Plane mirrors form virtual upright images by preserving the angle a ray makes with the normal.
Notation
\(\theta_i\)
angle of incidence measured from the normal
\(\mathrm{rad}\)
\(\theta_r\)
angle of reflection measured from the normal
\(\mathrm{rad}\)
\(s\)
object distance from mirror
\(\mathrm{m}\)
\(s'\)
image distance from mirror
\(\mathrm{m}\)
\(m\)
lateral magnification
1
Method
Derivation 1: Reflection law
The incident ray, reflected ray, and surface normal lie in one plane. A plane mirror reverses the normal component of the ray direction and preserves the tangential component.
Angle equality
\[\theta_r=\theta_i\]
Plane-mirror image
\[s'=-s\]
Magnification
\[m=-\frac{s'}{s}=+1\]
The negative image distance means the image is virtual and appears behind the mirror.
Rules
Reflection law
\[\theta_r=\theta_i\]
Plane image
\[s'=-s\]
Plane magnification
\[m=+1\]
Examples
Question
A ray strikes a plane mirror at
\[35^\circ\]
to the normal. Find the reflection angle.Answer
\[\theta_r=\theta_i=35^\circ\]
Checks
- Measure angles from the normal, not from the mirror surface.
- A plane-mirror image is virtual, upright, and same size.
- Ray paths are reversible.