Light, Sound and Waves

Why lateral inversion?

Physics Narrative for 11-14 Supporting Physics Teaching

More thinking about lateral inversion

We advise you to read the following passage whilst in front of a mirror!

The ray diagram shows quite clearly that the image is laterally inverted, with right appearing as left. But in class, every now and again, you will get this question:

Why does the mirror swap right and left but not up and down – how does it know which way up things are?

The problem is in saying that the image in the mirror swaps right and left: It doesn't. If a person raises their right arm, the image in the mirror raises an arm that is on the right of the image. That is all very simple and there is no lateral inversion. Then we get thinking about things and it seems to us that the person in the mirror has swapped right for left. They seem to be raising their left arm. The only way we could look like that is if we turn round. Right and left are not actually reversed in the image; it is just our interpretation of the image.

In fact what the mirror does is swap front for back. The back of our heads are furthest from the mirror and end up furthest behind it in the image. Our faces start closest and end up closest. The mirror just turns us back to front.

Here is something that sometimes convinces a class. First, find a large mirror which is safe to stand on without breaking and be sure you are wearing clothes which allow you to stand on the mirror (trousers!).

When the class see the image it is clear this time top and bottom have been swapped, not left and right. As before, in fact, the mirror has turned us back to front.

And here is the final test to show that it is not really about left and right. Get a pupil to lie in front of a biggish mirror. The pupil will tell you that left and right have been swapped. The other pupils will tell you that up and down have been swapped (their up and down). Both are just describing the event from their own point of view – the mirror has simply turned the pupil back to front.

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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