Left and Right

Your left hand is a mirror image of your right hand and your left foot is a mirror image of your right foot.

When two objects are mirror images of each other, we often call one the left and the other the right. Your hands and feet are good examples of objects which have left and right versions. Even though your left foot and your right foot are the same shape, your left shoe doesn't feel comfortable on your right foot, and your right shoe doesn't feel comfortable on your left foot. They're different.

Hands and feet aren't the only objects which come in left and right versions. Many molecules come in left and right versions. Chemists call these molecules stereoisomers. Left and right versions of molecules have different properties. For example, most of the sugars in your body are right-handed, and most of the amino acids (proteins) are left-handed. Left-handed sugars taste the same, but your body can't digest them. So, scientists are working on making left-handed diet sugar.

An example of a molecule which has a stereoisomer is limonene. Limonene is found in orange peel and lemon peel. Oranges have the left-handed molecule; lemons have the right-handed molecule. Just like your right foot fits only in your right shoe, the right-handed limonene molecule fits only in certain smell receptors in your nose, and the left-handed limonene molecule fits only in other receptors. That's why oranges and lemons smell different.

Whether an object is left-handed or right-handed is called its chirality. Notice that when you have two objects with different chirality, like a left hand and a right hand, you cannot lay one on top of the other in such a way that the outlines match. (This is called superimposing one on the other.) However, if you pick one up and flip it over, you can superimpose it on the other. A two-dimensional object has to pass through the third dimension to change its chirality.

Does your reflection in the mirror look exactly like you? Almost, but not quite. Your reflection's left hand is your right hand; your reflection's left eye looks just like your right eye.

Suppose you were to put a mirror in the middle of your body. Would half of you, together with a reflection of that half , look like all of you. Again, the answer is almost, but not quite. That is because people are nearly reflectionally symmetric. The left half of you is very much like the right half of you reversed. (There will be small differences. If you have a freckle on your left cheek, you probably don't have one just like it on your right cheek.)

Many natural objects are reflectionally symmetric, like a dog's face.

Look at these objects. Where would you put a mirror to get a reflection that looks like the other half? Watch out! One of them isn't reflectionally symmetric.

Mirror Writing

Do letters and numerals have mirror symmetry? Most letters and numerals look different when reflected in a mirror, though some don't. For example, if you put a mirror beside the letter i,

the reflection looks just the same.

However, if you put a mirror next to the letter G, the reflection looks quite different.

You can also put the mirror under the letters.

Also, some letters and numerals can be divided in half so that one half is the reflection of the other. Mirror writing is hard it read. This makes it useful for writing secret codes. See if you can read these messages, then hold up a mirror and read the reflections.