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In this movie, we show the image of a grey flag in a blue mirror line and the image of this image in a green mirror line. The angle between the mirror lines is 45˚.
The grey flag moves a few units across the page and as it does so the images move too. You might want to pause the movie to examine the reflections more closely and to ask:
• What single transformation maps the object flag onto the second image?
• Does this transformation stay the same when the grey flag moves?

Afte a while, the grey flag stops moving and it is then rotated through 90˚ about A, the point of intersection of the mirror lines.

It is worth asking students what they think has happened.
• Do they realise the rotation is through 90˚?
• Do they realise that the centre is at A ?

It is important to stress that all we have here is a demonstration.
Can we prove that the two reflections are equivalent to a 90˚ rotation about A ?

On the next page we see what happens when the grey flag stays in one place but the mirror lines move.