The opposite thing happens as the car leaves the sand and speeds up, the car will turn away from the wheel that hits the road first. As one of the front wheels hits the sand before the other, that wheel will slow down first and the whole car will turn towards that wheel. As the car moves from the road to the sand it will slow down. To work out the way every light ray turns when it hits the round glass, you can imagine each ray as a car driving from the road onto something more difficult like sand. This change of direction is called refraction and it happens because the light slows down as it enters the glass and speeds up again as it leaves. Instead of spreading out in straight lines, the light changes direction both when it enters and leaves the glass of water. However, when we add our glass of water it gets a bit more complicated. Some of the rays reach your eyes, that’s how your brain sees where the tip of the arrow is: You can imagine light spreading out from the tip of the arrow, travelling in straight lines called rays. Without the glass of water, we see both arrows as they are, pointing left to right. Optical illusions are images or pictures where we “see” something that is confusing to our brains or different to the way it really is, just like our reversing arrow. Have you used a magnifying glass to change the way things look?.When you’re drinking through a straw, has it ever looked like the top and bottom half don’t meet up?.

To perform the trick, all you need to do is pour water into the glass and watch their amazed faces as the arrow reverses before their eyes.Get you family in position, in person or online.Once you’ve worked out where that is, mark the position, pour the water back into the jug and put the empty glass back into position.Move the glass towards and away from your chair/webcam until you find the spot where the bottom arrow is reversed and the same size as the top one.Put the glass in front of your arrows, and pour water into your glass until the level of the water is above one arrow but below the other.Find a way to stand up your paper – fold the sides, lean it against a wall, box or book (etc.).Draw two short horizontal arrows, both pointing the same way, on your piece of paper, they should be about as long as your glass is wide.

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