Friday Illusion: Mystery of the colour-changing banana

Sandrine Ceurstemont, editor, New Scientist

You might expect to see a colour change after combining two different hues. But a demo involving a banana shows that it's not always the case.

In this video, courtesy of Al Seckel from Eye Wonder Books, a banana image turns green when covered with a bluish filter that matches its shape. However when a larger filter of the same colour, which encompasses the whole piece of paper, is placed over the top, no colour change is perceived.

Do you know what causes the illusion? Let us know in the comments below and the first correct answer will win a New Scientist goodie bag.

Last week, we showed you a vanishing train that appeared to defy space and time. Although we received many creative answers, most commenters correctly explained the trick: the train loops under the table then makes its way back up, giving the impression that it disappears for a period. Lionel Corporation, the US toy manufacturer, used this demo to demonstrate how magnetraction - turning the wheels into permanent magnets - would allow a locomotive to do a tight turn while keeping the train on the tracks.

Seckel remains perplexed at why many people don't find the solution obvious, despite it being a simple trick.

If you enjoyed this trick, there are many more mind-boggling illusions in Seckel's new digital book, Hear Eye Fooled You! The world's best optical and auditory illusions and the science behind them. You may also like to check out our previous Friday Illusions, to see, for example, how colours can emerge from a monochrome disc or ghostly images change shape.

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