Why Mirrors Reflect Images
You stand in front of a mirror and see a room that seems to continue behind the glass. Nothing is actually living in that second room, which is probably for the best. The image appears because reflected light reaches your eyes along orderly paths.
Light reflects by a simple rule
When a ray of light reaches a surface, it may be absorbed, transmitted or reflected. A mirror reflects a large share of visible light. The law of reflection says that the angle at which a ray arrives equals the angle at which it leaves, measured from an imaginary line perpendicular to the surface.
This rule applies to every tiny ray. Light from a lamp strikes your face, then rays from many points on your face travel toward the mirror. The mirror sends some of those rays into your eyes. Your brain uses their directions to judge where the light seems to come from.
A smooth surface keeps nearby rays organised, producing a clear reflection. A rough wall also reflects light, but its uneven surface sends rays in many directions. That scattered reflection lets you see the wall from different positions, but it does not form a sharp image. Polished metal and still water can create reflections for the same reason.
The image is virtual
After reflected rays enter your eyes, your brain traces them backward in straight lines. They appear to come from a point behind the mirror. The rays do not actually meet there, so the image is called virtual. You cannot project it onto a screen placed behind the mirror because no light is travelling through that imagined space.
In a flat mirror, the image appears the same distance behind the mirror as the object is in front. It is upright and the same size. Moving closer makes your image seem closer because the apparent position follows your own distance from the mirror.
People often say a mirror reverses left and right. More precisely, it reverses the direction perpendicular to its surface, front and back. Your image raises the hand opposite the one you would expect if you imagined another person turning around to face you. The mirror is not secretly swapping labels. Your comparison is doing most of the mischief.
Trace the rays yourself
A simple diagram can make mirror images much easier to understand:
- •Draw a straight line for the mirror.
- •Mark one point on the object.
- •Draw two rays from that point to the mirror.
- •Reflect each ray at an equal angle.
- •Extend the reflected rays backward with dotted lines.
The takeaway
Mirrors form images by reflecting light according to a predictable angle rule. Your eyes receive the reflected rays, and your brain traces them backward to a virtual position behind the mirror. Follow the rays, and the apparent room behind the glass stops being mysterious.