31:T5b0,md`Jeppe Hein’s 2018 artwork *360° Illusion V* consists of two mirrors mounted at 90° to each other, with the whole ensemble rotating slowly. It’s currently part of the Space Shifters exhibition at the Hayward Gallery in London.

One curious effect of this arrangement is that the reflected image rotates twice as fast as the mirror is rotating, so that the image is oriented normally when the mirror is horizontal, and upside down when the mirror is vertical. I found this rather surprising. Thinking about it, I realised the effect that surprised me is essentially a two-dimensional effect. The image you see in this double-mirror is like the image you see in an ordinary mirror, but two-dimensionally reflected across the join between the two mirrors. If a two-dimensional image is reflected in a line, and the line of reflection is rotated, the reflected image rotates at twice the speed the line of reflection does. The animation below demonstrates that two-dimensional effect. The small image in the corner is the original, shown with the line of reflection, and the large image is the result of reflecting in that line. Click to pause the animation, and click again to resume. When you pause the animation, you’ll see the small image reflected across the red line so that it matches the large one.`32:T60e,html`

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