The Golden Rule: When a light ray hits a plane mirror, it follows the Law of Reflection:

θ incidence​  =  θ reflection​

• Incident Ray: The light ray traveling toward the mirror.

• Reflected Ray: The light ray bouncing off the mirror.

• The Normal: An imaginary dotted line drawn perpendicular (90∘) to the mirror surface.

When you look into a plane mirror, the image formed is always consistent. Use the acronym S.A.L.T. to remember the properties:

• S (Size): Same Size. The image is the same size as the object (Magnification = 1).

• A (Attitude): Upright. The image is not upside down, but laterally inverted (left and right are reversed). 

• L (Location): Same Distance. The image appears to be at the same distance behind the mirror as the object is in front of it.

• T (Type): Virtual. The image is formed by extending rays backward. Light does not actually pass through the image location, so it cannot be projected onto a screen.

Even though we cannot see light rays with our eyes, here is how the image is geometrically constructed:

1. Light Travels to the Mirror Imagine light rays traveling from the object (Green Arrow) and hitting the vertical mirror surface.

2. Light Bounces to the Eye These rays reflect off the mirror and travel to the observer's eye. The angle of incidence equals the angle of Reflection (θi = θr).

3. The Brain "Back-Traces" Our brain assumes light always travels in a straight line.

• It traces the reflected rays backward through the mirror (shown as dotted lines in ray diagrams).

• The point where these imaginary lines meet is where the Virtual Image appears.