Try this: Curved cards

You will need

• Paper
• Printer
• Scissors
  

OR

• Paper
• Compass (the circle-drawing type)
• Ruler
• Pencil
• Scissors

What to do

For this activity, you will need two curved cards. You can either print out an image of them, then cut it out or use a compass, pencil and ruler to make the image yourself.

If you have a printer
1. Print out this image of the two cards. (90 kb image file, opens in a new window).
2. Cut out the curved cards from the paper.

To make two cards using a compass, ruler and pencil

1. Draw a circle with a 5 cm radius.
2. Using the same centre, draw another circle with an 8cm radius. Keep your compass at this radius it will be used again in step 4.
3. Draw a line that runs through the centre of the circles.
4. Place the point of the compass where the line crosses the outer circle. Mark where the pencil end of the compass crosses the outer circle.
5. Draw a line through your mark and the centre of the circle.
6. Cut out the curved card shapes.

Once you have your cards

1.		Place them next to each other, so the curves are facing the same way. Which card looks larger?
2.	Swap the cards around. Which card looks larger now?
3.		Turn one card around. Which card looks larger now?
4.	Place one card on top of the other, to see they are both the same size.
5.	If you like, try doing the same thing with three or four cards. Does the effect still happen?

What's happening?

You should find that when you hold the cards with the curves facing the same way, one will look larger than the other. Like many optical illusions, this effect depends on how our brain interprets the information it receives from our eyes.

Our brain has as much to do with how we see things as our eyes. Our eyes receive light, but there is a large part of our brain, called the occipital lobe, responsible for analysing the signals from our eyes and constructing an image of the world around us.

The way our brains try to work out which card is larger is by looking at the sides where they are close together. Since the long side of one card is next to the short side of the other, our brains interpret that as meaning that card is larger than the other, even though we know they are the same size.

When we hold the cards so the sides that are next to each other are the same length, we can see that the cards are the same size.

During the first few years of our lives, our brains gradually learns to understand what our eyes are seeing, such as judging distances or how large objects are. Although the methods our brains use are pretty effective, there are times when they can produce false results, such as this optical illusion.

Seeing how our brains can be fooled by optical illusions help us to understand how our brains process the signals from our eyes. This particular illusion is one of my favourites because it is so simple there are no complicated lines or patterns that you often find in optical illusion. I also like it because you have a physical object you can move around, rather than just images on a page.

Applications

• Engineers who design car dashboards or aircraft cockpits need to design them so the people using them can find important information quickly and easily. Understanding how our brains process information can help provide the best layout of instruments and controls.