It sounds like this“The reflection is a picture on the surface of the mirror, so it is as far from you as the glass is.”
Thinking your reflection is on the glass
Why it feels right
The reflection lives in the mirror. When you look at a mirror the shiny surface is what you are looking at, and the picture seems to be painted on it, like a photograph in a frame. It is easy to assume the image is where the light bounces off, at the glass itself.
Why it’s wrong
The image seems to be behind the glass, as far behind it as you are in front. If you stand 2 m from the mirror, your reflection appears 4 m from you. Walk toward the mirror at 1 m per second and the image approaches you at 2 m per second.
A worked example
Look at your eyes in a bathroom mirror and try to focus. Your eyes do not focus on the glass; they focus on something about twice as far away as the glass. That is why a smudge on the mirror and your face are never both sharp.
The light from your face reaches the glass and bounces back to your eyes. Your brain follows those rays in straight lines back behind the mirror, and where they seem to meet is where the image seems to be: as far behind the glass as your face is in front of it. Nothing is actually there, which is why it is called a virtual image.
Stand 2 m in front of the glass and the image seems 2 m behind it, so it is 4 m from you. Walk toward the mirror at 1 m per second and the image comes toward you from the other side at 1 m per second as well, so you close the gap at 2 m per second. This is also why a small bathroom with a big mirror seems to be twice as big.
How to catch yourself
- You say the picture is on the mirror.
- You think a mirror shows you at the distance of the glass.
- You expect to be able to focus on both your reflection and a mark on the glass.
What to hold instead
A mirror makes a virtual image that seems to sit behind the glass, as far behind as you are in front of it. Your reflection is twice as far from you as the glass is, and it approaches at twice your walking speed.
Where you’ll meet it
- Setting a camera to focus on a mirror selfie.
- Putting a mirror in a small room to make it look larger.
- Wondering why you can't focus on a smudge and your reflection together.
Questions people ask
- How far away does my reflection appear to be?
- As far behind the mirror as you are in front of it. If you stand 1.5 m from the glass, your reflection looks 1.5 m behind it, so 3 m from you.
- Why can't I focus on both my face and the mirror's surface?
- Because they are at different distances. The reflection looks as far behind the glass as you are in front of it, so your eyes must focus about twice as far away to see it sharply.