May. 20, 2011

Science Dad on Parallax and Vision

by Vince Harriman

Click to enlarge images

Beckett has been bringing home bags of seeds from school lately to share with Science Grandpa -- he'll bag a bunch of seeds while at recess at school, put them in his backpack, and deposit them on the table when he gets home. I guess Science Grandpa will be making another appearance after he and Beckett germinate all those seeds!

Until then, I've had a request from a reader in Kentucky who wants to know how parallax works. Beckett and I brought out the art supplies and got started on some simple projects that can really make sight visible.

Seeing your line of sight

I found that trying to explain lines of sight took a lot longer than just showing it so we started with a basic demonstration. We pulled out a couple big sheets of paper and a long ruler. Beckett knelt at the edge of the table and we marked on the paper how far apart his eyes are. We put a Lego block at mid page with another longer Lego block behind it. Looking down the sheet of paper, first with one eye, and then the other, Beckett could see the first Lego block 'change position' relative to the second block. Why was this happening? We took out the long ruler and traced his lines of sight for both eyes. Now it was very clear why the object was 'changing position.' But a new mystery came up -- why do we only see one object with two eyes?

It turns out that brains are fantastic at that kind of complicated thing -- parallax is the basis of binocular vision which allows certain mammals and reptiles to accurately gauge depth and distance. The brain is so good at combining two images into one that many fun optical illusions can be done that illustrate how parallax works.

Ouch! I have a hole in my hand!

Ouch! I have a hole in my hand!

To start, Beckett rolled up a piece of paper and held it next to his hand and looked through the tube with both eyes open. It appeared to him that he had a hole in his hand! When Beckett was focusing on an object beyond his hand (usually by sighting something through the tube -- a wall even) his eyes fooled his brain. The eye not looking through the tube followed the eye looking through the tube and attempted to focus on the wall beyond his hand. But, because his hand was blocking that eye's view of the wall beyond, it still took in an image of his hand. His brain combined the images coming from each eye and the result is a hand with a hole in it.

Another easy way to trick both your eyes and brain is called the 'portable snack'. Stand facing any object more than six feet away and focus on that. Slowly point your two fingers at each other and bring them together in front of your face, just under your line of sight, while continuing to look at the object beyond your fingers. As your fingers approach each other, they will 'make' a third hot dog like finger floating between your two fingers. This is because your brain is seeing two images that are not the focal point and combining them into one.

A portable snack

A portable snack

If you look at the first photo in this post, you will see Beckett's sight lines and an x where we placed the Lego. From Beckett's point of view, when you focus on this point, your eyes are both pointing at it. Now imagine your fingers coming together in front of that point -- you can easily see that when your fingers get close together, they will overlap the sight lines. Focusing on the 'x' (or a distant object) then placing something in front of the distant object creates two images. Overlapping the sight lines will cause the two images to overlap. Your brain will try to combine the images even though they don't make sense. Fingers don't magically turn into hot dogs and holes don't appear and disappear in hands.

Monocular vision (cows and some birds and many fish for example) have eyes on opposite sides of their heads -- their brains have two distinct fields of vision that may or may not overlap. This allows them to see a much wider range -- better for seeing predators and dangers. What their brains do with the two images is much different from what our brains (and similarly binocular animals) do.

Here is today's science question: are animals with monocular vision capable of seeing optical illusions the same way animals and humans see them? We found a bunch of great websites with optical illusions:

And don't forget to go back to our Measuring and Measurement experiment to take another look at the parsec and how parallax is used to measure vast distances in space.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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