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Episode 4: Reversing Arrow

The perfect demonstration for showing off over a webcam, after seeing our reversing arrow your family won’t know if they’re coming or going…

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Read the full transcript of Episode 4: Reversing Arrow

Before you start

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.

What you’ll need

  • A glass or jar with straight sides
  • Paper
  • A marker pen
  • Water in a jug or bottle for pouring

What to do

Before setting up this demonstration it’s important to know that it only works if you’re looking at it from the right place. Either set up a chair for your family member, or if you’re doing it online position the camera on your laptop or phone, so you know where the demo will be viewed from. Now you’re ready to get started.

Make sure the arrows you draw on your piece of paper are one above the other.
  1. 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 
  2. Find a way to stand up your paper – fold the sides, lean it against a wall, box or book (etc.)
  3. 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.
  4. 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.
  5. 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. You're now set up.
  6. Get you family in position, in person or online.
  7. 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.

What to talk about

  • When you’re drinking through a straw, has it ever looked like the top and bottom half don’t meet up?
  • Have you used a magnifying glass to change the way things look?

What’s going on?

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.

Without the glass of water, we see both arrows as they are, pointing left to right. You can imagine light spreading out from the tip of the arrow, travelling in straight lines called rays. Some of the rays reach your eyes, that’s how your brain sees where the tip of the arrow is:

Do Try This at Home - Episode 4: Reversing Arrow

However, when we add our glass of water it gets a bit more complicated. Instead of spreading out in straight lines, the light changes direction both when it enters and leaves the glass of water. 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.

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. As the car moves from the road to the sand it will slow down. 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. 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. The path the light takes changes in a similar way.

We can sketch out the path that some of the rays take as they travel from the paper, through the glass of water and out the other side:

Do Try This at Home - Episode 4: Reversing Arrow

Our optical illusion is that our round glass of water ends up changing the path of the rays of light enough for them to cross over and spread back out. Now, for the web cam or anyone looking at it, it looks like the rays of light are spreading out from an arrow pointing right to left instead. So we are tricked into seeing the tip of the arrow in a different place.

What next?

Investigate together with your family. Discover how what you see changes if:

  • You use a bigger glass or one with different shaped sides.
  • You move the glass towards and away from the arrows. 

To make yourself part of the experiment, sit somewhere different to look at the illusion from a different place.

Once you’ve worked out what happens with something simple like an arrow, you can try out different shapes, letters and pictures to see what happens! Can you predict how they will change when you look at them though your water?

Did you know?

Lots of different optical illusions are caused by refraction, from mirages in the desert to swimming pools looking shallower then they really are.


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