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.
It's a model train like no other: when it enters a tunnel, it seems to defy the properties of space and time. But of course, what you're seeing is just an illusion. Do you know what the trick involves?
Think you're living in the moment? You could actually be experiencing another time.
A brain trick called the flash-lag illusion shows how we don't always perceive the present. This version, created by Eiji Watanabe from the National Institute for Basic Biology in Okazaki, Japan, presents a moving cube occasionally accompanied by a flashing twin. When the second box appears, it's really lined up with the moving cube yet it seems to lag behind. A second example uses a gear animation to show how a flashing piston looks out of sync with another that's shifting up and down.
A head-scratching illusion comes in at number 10 in our best videos of 2012 countdown.
Try to wrap your head around this illusion featuring a roof that appears to defy gravity by attracting balls to its peak. Once the house is rotated to uncover the secret, you'll be surprised to see how dramatically your brain was tricked.
You might not expect a mind-bending vortex to emerge from an old quilt. But about 100 years ago, a housewife armed with a needle and some red and white scraps of cloth crafted a unique pattern that doubles as a brain trick.
There are no editing tricks involved and it's not CGI. In this video, filmed in one continuous shot, a banana vanishes before your eyes. Of course, the disappearance could be explained by some sleight of hand, but what's even more mysterious is the reflection that seems to linger behind.
Now Georgie Powell from Cardiff University in the UK and colleagues have come up with a twist on this classic illusion - one that causes phantom images to shrink or grow. While fixing your eyes on a dot in the centre, an arrangement of coloured circles is presented (see video above). Then, when the image is replaced with an outline of the pattern, it appears to be filled in with the complementary colours of the original image even when the circles change size.
The illusion shows that contour edges influence ghost images,
an effect that can also occur with "real" colours when surrounded by edges that they don't fill in completely.
"However, we have shown that after-image signals might be even more constrained and enhanced by edges than signals arising from real objects," says Powell.
She suspects it's because the brain somehow "knows" that the after-image is an illusion: by providing an edge it gives the colour a shape, which makes it feel more real.
Powell came across the illusion while investigating different factors that can either cause or suppress an after-image, such as the brightness of edges and other aspects of a scene.
Think an object can't be in two places at once? This animation shows how the perceived location of a dot is influenced by what's happening around it.
In this video, a flashing dot is surrounded by two diamonds that shift across the screen. When they move horizontally, the dot seems to shift sideways and slightly upwards. In a second version, in which the corners of the diamonds are obscured, the dot appears to move diagonally. In fact, the dot never changes place.