• Is this the left or the right eye blinking?

• No, mirrors don't invert left and right…

• …except when you use two at a right angle.

• But mirrors invert front and back.

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Physics
age: 8 – 12

# Does a mirror really invert sides?

It is common to hear students say that mirrors invert left and right. The use of a second mirror seems to correct this 'error'. However, a second look reveals that the flipping only occurs with two mirrors, not with a single one.

Learning objective

Single planar mirrors do not invert sides, but front and back.

Two mirrors at a right angle can flip the image, and thus invert sides.

Preparation

To simplify observations with two mirrors at a right angle, it is helpful to connect them with a bit of sticky tape on their back.

Hold two mirrors at a right angle and look at your mirror image while blinking with one eye. Then compare this with your mirror image in a single mirror.

1. What inverts sides: a single mirror or two mirrors at a right angle?

2. What does a single mirror invert?

Guiding Questions (if needed)

Does a single mirror invert up and down?
› No.

If you hold the two mirrors with their connected edge horizontally, do they invert up and down?
› Yes.

If you point with your finger to the right, does the image in a single mirror point to the left?
› No, it points in the same direction.

If you repeat this with two mirrors at a right angle?
› The image points in the opposite direction.

If you place two objects at different distances between you and the mirror, which appears closer to you in the mirror image?
› The one further away from the observer appears closer in the image, while the object closer to the observer appears behind the other object and further away. In the mirror image, front and back are inverted.

Findings

A single mirror does not invert sides, as you can easily see if you point with your finger to the left or right in front of a mirror – the mirror image will point into the same direction. However, since we think of the mirror image as a person in front of us, it appears as if this person in the mirror blinks with her left eye if we blink with our right eye.

A single mirror does actually invert front and back: things that are closer to the observer appear further away in the mirror.

Two mirrors, on the other hand, can indeed invert left and right, if you hold them together at a right angle. They also invert up and down as well as front and back. We only believe to see a 'normal' face in this flipped image because our faces are symmetric.