How can our eyes differentiate distance?

[kashiark]

All photons that we see hit our eyes, so how do our eyes know where they came from?

 

[jamesb-uk]

Because light from the left side of us hits the right side of our eyes and vice versa, as the light has to pass through the pupil. Not all photons coming from a specific place hit every part of our retinas. The image formed is laterally and vertically inverted. The image is then inverted by our brains. Our brains distinguish which side the light is coming from (as many things will be seen in both eyes) by using (partly) the optic chiaism, where the thwo optic nerves cross. However, only half of each nerve crosses to the other side of the brain, so everything we see on the left side of us is processed in the same place etc, so we can process it better.

Our eyes can determine how far away something is by seeing a very slight difference in the images produced by the left and right sides.

 

[Moonbear]

Yes, it's binocular vision that gives us depth perception.

Here are two little demonstrations you can try to help understand how it works.

First, just look at a fixed object...anything will do, even your computer screen. Now, close one eye, then quickly switch back and forth between which eye is closed. You'll see that the object appears to move a little. Your brain processes the difference in what each eye sees to determine depth.

Second, just to really convince yourself of this, find a friend to help out for another demonstration. Close your eyes and have your friend place an object in the middle of the room somewhere...out in the open, preferably not near objects you are already familiar with. When they tell you they've placed it, only open one eye. Now, keeping the other eye closed, try to walk to and touch the object. You should notice that with only one eye, you have a much harder time determining distance to the object (depth) than you normally would with both eyes open.

 

[kashiark]

Oh ok thanks moonbear that makes sense.