We can see why mirrors fool us into thinking left and right are swapped, though: it looks like a second person is standing in the mirror, looking back at us. When you raise your right hand, the mirror person appears to raise her left hand. However, what's really happening is that the mirror person is still raising her right hand, just that the front of your hand is the back of hers, the front of your head is the back of hers, and so forth. If the mirror really flipped left and right, the mirror person would be facing the same way you are: you'd be seeing the back of her head instead of her face!
Concave Mirrors for Makeup and Telescopes
Ordinary bathroom mirrors are flat, but there a kind of mirror that flips left and right as well as front-to-back - but it also reverses up and down, too. This type of mirror is a concave mirror: one like the inside of a polished metal bowl or the cupped part of a soup spoon. Again, you've probably played with making faces into a shiny metal spoon: one side gives you an upside-down reflection. (I'll talk about the other side of the spoon in a little bit - that's a third kind of mirror.) A spoon is kind of an odd shape, since reflecting your image isn't their main purpose, but many makeup and shaving mirrors are closer to being ideal concave mirrors.The upside-down and backward image you see will always appear smaller than you are, but it will also seem to be closer to the mirror than you are. Unless the mirror is nearly flat, your face will appear to be distorted: a big protruding nose and smaller ears fading in the distance. If you sway left, your image will sway right; if you duck down, your image will bob up. That's how we know the image is truly reversed, unlike the flat mirror! A big concave mirror can be a bit headache-inducing (at least if you're like me): the image looks very strange compared to the image in your bathroom mirror. That's because it's what is known as a real image: it's on the same side of the mirror as your face, so your eyes have a lot of trouble focusing on it. In fact, if you put a piece of paper at the right location, you can actually project the image from a concave mirror onto it.



microwaves instead of visible light, which is why they don't look like mirrors at first glance. Again, the purpose is to focus the signal from the satellite. Big radio telescopes are also mirrors: the biggest mirror in the world is the Arecibo radio telescope in Puerto Rico: that one is 1000 feet (305 meters) across!
Objects In Mirror Are
Closer Than They Appear
As a quick aside: if you have trouble remembering the difference between "convex" and "concave", here's a mnemonic. Concave includes the word "cave": that's a
mirror that bows inward. Convex rhymes with "flex": that's a mirror that bows outward. At least that's how I remember which is which!
The passenger-side mirror of a car bears the message "Objects in mirror are closer than they appear". (Hopefully the object is not a tyrannosaurus.) That mirror is convex, and it's designed to give a wider view of the side and rear of the car than can be done with a flat mirror. The price of the wider field of vision is that objects do end up looking farther away than they really are. You also see convex mirrors in shops, so that the staff can look down aisles out of their direct vision, and in a famous self-portrait by M.C. Escher.
Reflections
We've come a long way in a short time from a basic flat bathroom mirror: we've seen why normal mirrors don't flip left and right, but why concave mirrors do. We connectedmakeup mirrors to the biggest telescopes in the world, and shop mirrors to cars. Even better, you probably have all these types of mirror easily accessible, especially if you're willing to goof around with spoons. Try them out, see how they work, and the next time someone tells you that mirrors reverse left-to-right, you can help get them facing back the correct way.
Matthew Francis, Double X Science Physics Editor
@DrMRFrancis
Sorry, but I don't quite buy it. Say I am wearing a shirt with my name over the left pocket. In the mirror I see someone with a reversed name over what would be their right pocket. I wave my right hand, this person waves what would be their left hand. Move to a pair of mirrors joined at a right angle. I see a person with the unreversed name over what would be the left pocket. Wave the right hand and that person waves what would be their right hand.
ReplyDeleteI see what you are getting at, but it is really contrary to the nature of mirrors. What you are thinking about is a TV screen attached to a camera behind the viewer.
What I'd really like to know is how a mirror reflects accurately all the colors visible in it while simultaneously appearing to be entirely silvery.
ReplyDeleteFor your objection: imagine if you're transparent. Someone standing behind you would see the lettering on your shirt appear backwards. Reversing front and back in a mirror image has that same effect.
ReplyDeleteI do understand your objection, but again: if a mirror really reverses left and right, it would also reverse up and down since there's nothing special about "left and right". Compare with what happens in a concave mirror, where reversal really does happen.
For your second comment: a mirror isn't perfectly reflective, so not all the light reflects back to us. It bounces the colors back to us, but some still get scattered or absorbed, which leaves the silvery impression you note.
I hope this helps!
Tnx! The transparent me makes it transparent to me.
ReplyDeleteNow that I thought of it, I should have used that in the post! Maybe for the second edition. :)
ReplyDeleteYour picture explaining concave mirrors doesn't match your description. The picture shows what happens to a reflection in a mirror that is only concave in the up-down direction (top and bottom are switched, but left and right aren't.)
ReplyDeleteYes, you're right: for the ORLY? Owl example, I considered what's called a cylindrical mirror, where only the top and bottom are swapped, but the left and right are not. That was for reasons of simplicity.
ReplyDelete