Cover up one eye with your hand - it doesn't matter which eye you cover.
Ask your friend to hold the cup out in front of you, between 30cm and one metre away. They should hold the cup at about waist height.
Reach out your hand, keeping your arm at shoulder height and try to drop the plasticine ball into the cup. Remember, you can only use one eye!
Swap which eye is covered and try dropping the ball again.
Now try with both eyes.
What's happening?
You will probably find that you're not very accurate when you only use one eye. In fact, you might miss the cup altogether!
Each of our eyes provides a picture to our brain. The two pictures are slightly different, as each of our eyes is in a slightly different position. You can test this out - close one eye and line up your thumb with some object a little distance away, such as the edge of a window. Now open that eye and close the other one without moving your hand. You'll find that your thumb isn't lined up any longer. Once you've tried that, check out this illusion to see if your eyes are really working in 3D.
Our brain cleverly combines the two pictures from our eyes into one picture, and it uses the differences between the pictures to do some quick geometry and work out how far away things are. With only one eye, we find it much more difficult to gauge distances.
Stereo vision is the reason we have two eyes.
Applications
Predators find stereo vision very helpful. It allows them to work out how far away their prey is, so they can chase, pounce on and catch it. Take a look at just about any predatory animal or bird and you'll find they have eyes at the front of their head, so that they can use stereo vision.
Prey, on the other hand, don't find stereo vision so helpful. They are more interested in spotting an animal creeping up on them, so they often have eyes on the sides of their heads. This gives them very good side, or peripheral, vision. Take a look at horses, rabbits and many fish to see these eyes at work.
In addition to being useful for catching food, stereo vision is used by humans for many different purposes. 3D movies use different strategies to trick our brain into thinking we are seeing a 3D object, when really we are looking at a flat movie screen. In a similar way, geologists, geographers and other scientists use a tool called a Stereoscope to see a 3D landscape from two aerial photographs.
Robotics engineers are also very interested in studying stereo vision, so the robots they build can 'see' in 3D and navigate around obstacles or manipulate objects.
Cover up one eye with your hand - it doesn't matter which eye you cover.
Our predators, Shayn Amber Wetherell and Kimiko Holder, find their stereo vision very useful. Kimoko helps out with the mags, Scientriffic and The Helix and Shayn Amber will assist you with all your Double Helix Science Club enquiries.
Test your stereo vision by closing one eye while you line up your thumb with an object. Then open that eye and close the other. What changes?
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