The Law of Reflection
When light hits a mirror, it bounces off. The law of reflection states:
Angle of incidence = Angle of reflection
Both angles are measured from the normal — an imaginary line perpendicular to the mirror surface.
Plane (Flat) Mirrors
A plane mirror produces an image that is:
- Virtual (cannot be projected onto a screen — rays don't really meet)
- Upright (same way up as the object)
- Same size as the object
- As far behind the mirror as the object is in front
- Laterally inverted (left and right are swapped)
When you look in a plane mirror, your image appears to be behind the mirror. This is a virtual image — you can't project it onto a screen.
Curved Mirrors
Concave Mirrors (Converging)
Curved inward — like the inside of a bowl. Parallel rays reflect and converge at a focal point.
Uses: make-up/shaving mirrors (magnified image), headlights, satellite dishes, telescopes.
| Object position | Image |
|---|---|
| Beyond focal point | Real, inverted |
| Inside focal point | Virtual, magnified, upright |
Convex Mirrors (Diverging)
Curved outward. Parallel rays reflect and diverge (spread out). Image is always:
- Virtual, upright, smaller than object
- Covers a wide field of view
Uses: car wing mirrors, security mirrors in shops.
Real world: Car wing mirrors (convex)
Car wing mirrors use convex mirrors so the driver gets a wide-angle view of the road behind. The image is smaller and virtual, but covers a much larger area than a flat mirror of the same size — helping drivers spot cyclists and pedestrians in their blind spot.
Real world: Dentist's mirror (concave)
Dentists hold a small concave mirror inside patients' mouths. When the mirror is held close (inside the focal length), it produces a virtual, upright, magnified image of a tooth — making tiny cracks or cavities much easier to spot.
Real world: Bathroom mirror (plane)
The flat mirror in your bathroom is a plane mirror. Your reflection appears to stand as far behind the mirror as you stand in front of it, is the same size as you, and is laterally inverted (left and right swapped) — which is why text looks backwards.
Practice Problems
A person stands 2 m from a plane mirror. Where is their image and how far is it from the person?
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Why are convex mirrors used as car wing mirrors rather than plane mirrors?
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State the law of reflection.