Now to continue; We want to arrange our new pencil axis so that it is perpendicular to the vertical axis (note: the vertical axis still exists but there is no pencil along the vertical axis. What we have to do is pretend or imagine the axis being there). An axis that is perpendicular to the vertical is called a horizontal axis probably because it is often coincident with the horizon. On the ocean the horizon looks like

Notice that the water has a refection of the Sun thus it acts like a mirror. We’ll discuss this later.

Anyway retape your pencil to the rectangle with your name so that it is in the horizontal position with the pointy end pointing to your left when you read your name normally.When you’re done it should look something like

Now, like you did before, stand in front of a mirror holding your rectangle so you can read your name. Next, keeping the pointed end of the pencil pointed to your left rotate the pencil until you can read your name in the mirror. When you’re done you will see something like

in the mirror. Since we did a rotation this new pencil axis is called a “horizontal rotation axis”.

What’s interesting is that in this case the letters in the word are no longer backwards. Instead they are upside down! What we see is that the mirror image we see depends on which axis is used to rotate the rectangle. Vertical rotations produce images that appear backwards but are right side up while horizontal rotations produce just the opposite. That is, horizontal rotations produce images that are upside down but are not backwards.

Normally things are rotated either explicitly or implicitly about a vertical axis but not always. A good example of implicit horizontal rotation is the reflection from a pond or lake such as shown at right. Notice that the reflection in the lake is upside down but has the correct left to right order. This doesn’t look strange to us because our human brains seem to be programmed to process this image without detecting anything strange.

Even if we rotated the image so that it becomes an implicit vertical rotation we don’t seem to have any mental problems in interrupting the reflected image. For example look at the image at right. This is just the original picture rotated by 90 degrees backwards so that the real (rotated) mountain is on the left and the mirror reflection is on the right. It may look unusual but we have no trouble figuring out what’s going on but notice that the additional rotation makes this an implicit vertical rotation. This is a general property. There is more than one way to get from here to there. Notice that the image is now “right side up” instead of upside down and that just like the words in your name the image is now backwards. You may want to stop and think about this for awhile. Maybe even practice some more experiments before going on to the next step.

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Tags: Physics, Science

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