AP® Physics 2: Algebra-Based · Unit 13 · Topic 13.2
Plane Mirrors
A plane mirror forms a virtual image by reflecting diverging rays that only appear to come from a point behind the glass, the same distance back as the object is in front.
A flat mirror doesn't swap your head and feet because gravity defines up and down for you, not the mirror. What actually flips is front and back: the mirror reverses the direction pointing into the glass. Your right hand's reflection looks like a left hand because reflection reverses that front-back axis, and your brain, used to seeing other people face to face, reads that reversal as a left-right swap. Nothing physical about left and right is special here; it's a trick of perspective.
To find where the image sits, trace rays. Light leaves a point on your face, hits the mirror, and reflects following the law of reflection: angle in equals angle out. Several of these reflected rays spread out heading toward your eye, and your brain assumes light always travels in straight lines, so it extends those rays backward, straight through the mirror. They all meet at one point behind the glass, exactly as far behind as the object is in front. That's the image location, but no actual light is there. Because the rays only appear to meet, the image is virtual, upright, and the same size as the object.
This geometry is why a full-length mirror only needs to be half as tall as you are: light from your feet and from the top of your head each need just a short piece of mirror, located partway up, to bounce into your eye. The same ray-tracing habit, reflect then extend backward, carries straight into curved mirrors next, where the surface bends rays enough to make images that shrink, grow, flip, or even become real.
Key terms
- Virtual image
- An image where light rays only appear to diverge from a point, so it cannot be projected onto a screen.
- Law of reflection
- The angle of incidence equals the angle of reflection, both measured from the normal to the surface.
- Normal
- An imaginary line perpendicular to a surface at the point where a ray strikes it.
- Ray tracing
- Drawing straight lines to represent light paths in order to locate where an image forms.
Watch it explained
- How it worksHow do reflected rays trick your eye into placing an image behind the glass?
- Worked exampleFind the shortest full-length mirror a 1.8 m tall person needs to see all of themselves.
- Common mistakeWhy isn't the image in a flat mirror something you could catch on a screen?
- Going furtherWhat happens to the number of images when you stand between two mirrors facing each other?
Check yourself
End of topic.