Why does a mirror reverse an image? The mirror doesn’t reverse the image. It shows the image face on. It is our minds that make us think it is reversed. The mirror doesn’t flip an image left to right, it flips the image front to back.
A mirror is a piece of glass with a reflective material as a backing. The material that is used is usually aluminium or silver because they have a very high reflectivity. Silver reflects 95% of the light that hits it. The mirror doesn’t necessarily need the glass layer, but it protects the reflective silver layer at the back. A mirror would actually be more reflective if it didn’t have the glass layer because some of the light waves won’t make it all the way through the mirror, both on the way in and on the way back out again. However, glass is mostly transparent, very smooth, doesn’t scratch easily, doesn’t damage easily, and doesn’t tarnish. If you just had the layer of silver, it would be damaged very quickly and would tarnish.
When you want to see an image in a mirror, you must have light, of course. Light comes from the sun (or any other source of light) and hits whatever we are holding in front of the mirror. That material absorbs some of the light and reflects some of the light. The wavelength of the light it reflects is the color that we see. When that light bounces off the object, it scatters in every direction. Some of it hits the mirror and reflects off the silver layer at the back. This is the light that enters our eyes and makes up the image we see in the mirror. So, why is the image we see flipped left to right? If you write something on a piece of paper and hold it up, the writing will be backwards. Why is that?
There are two reasons. The first reason is the way the light hits the mirror, and the second reason is our brains. If we are standing in front of a mirror and lift our right hand, the mirror image of us lifts their left hand, or at least, that is what it looks like. This is because the light hits our bodies and bounces straight back, going into the mirror. From there, it is reflected back into our eyes. The light from our right hand as we lift bounces off and goes in a straight line into the mirror and comes directly back to our eyes. It looks like our left arm in the reflection, but it is the light from our right arm. The image hasn’t been flipped left to right, it has been flipped from to back. What does that mean? You are looking at your reflection in the mirror as though you could see through yourself from behind. That doesn’t seem to make sense. Imagine this. If you write the word “hello” on a piece of paper and look at it in a mirror, you would see the word back to front. But it hasn’t been flipped. Hold up the same paper in front of a light and look at the paper from the back. You will see the same reversed text. This is what a mirror does. It allows you to look at something from behind. It flips the font and the back.
So, why do we think that the mirror has flipped left and right? This basically comes down to our brain and what it is used to. Our brain is very used to meeting people. We meet people all day and every day. Our brain is very good at recognizing people and very good at analyzing them. If you meet a person and they lift their left arm, you automatically know that it is their left arm. You don’t have to think about it. You just know it. Our brain is far more used to meeting real people than it is seeing reflections. When we see a reflection, our brain just assumes it is a real person. We then have to mentally think that it is a reflection, and that makes us feel that it is flipped.
People have been able to look at their reflections in polished metal mirrors for thousands of years. 6,000 year old polished obsidian mirrors have been found. Glass mirrors have also been around for a very long time, but the modern style of plate glass mirrors have only existed since the 17th century. And this is what I learned today.
Sources
https://www.wtamu.edu/~cbaird/sq/2013/01/05/why-do-mirrors-flip-left-to-right-and-not-up-to-down
https://www.wonderopolis.org/wonder/how-do-mirrors-work
https://science.howstuffworks.com/innovation/everyday-innovations/mirror2.htm
https://www.furniturelibrary.com/mirror-glass-darkly
Photo by Andrea Piacquadio: https://www.pexels.com/photo/photo-of-woman-looking-at-the-mirror-774866/