If you can see something with one eye closed then when seen with both eyes why doesn’t it appear double?

Have you ever wondered how we see the world? At first, the answer seems pretty obvious; through our eyes! But like most things in life, it’s not that simple. When humans began to think about the workings of the eye, it didn’t come as a surprise that the optics of the eye is a very complex subject. Today, we are going to ask ourselves something that is obvious yet often unheard of; if we can see a single image with one eye closed, then how come we don’t see two similar images with both eyes open?

This question makes sense if we think of eyes as the parts of our bodies that are solely responsible for our vision. However, that reasoning is flawed because the eyes aren’t the only components of our body that enable us to see. The actual part of the body responsible for how we see the world is the human brain. The brain is the control center of our vision. It controls things the movements of the eyes and interprets the light rays that constantly get bombarded into our eyes.

Fun Fact: Our eyes see the world upside-down. It’s the special ability of the brain to go through several processes to create the image right-side-up that we actually see. People with rare disorders of the brain and vision see the world upside-down.

Our human brain is adapted to give us the best results possible. When we consider how it would feel like to see everything around us duplicated, at all moments, we are pretty much forced to thank our brains for allowing us to see the world in a single image. To truly appreciate this function of our vision, we need to talk about dimensions.

When we look at something in a photograph, let’s say, the side view of a car, we are able to tell two things about it; how tall it is and how long it is. These two properties are called the height and length of an object, respectively. Like everything, they are two of the three dimensions of an object. That is why a normal image or video is called a 2-D (2-dimensional) image or video. What we can’t tell while looking at the image is the width of the car. This property of the image is also called the depth of the image, and it is the 3rd dimension of any object we see around us.

Now, in order to see the world around us so that we can understand what we are looking at, we need to see everything from a 3-D perspective. For instance, we can try an experiment. Let us hold a ball with our arm stretched straight in front of our nose, and look at it with our left eye closed. Our right eye will mostly see the right portion of the ball and some of the middle part. Now, if we close our right eye and look from our left one, we will mostly see the left side of the ball and some of the middle side. And, if we open both of our eyes, we can actually see both the right and left portions of the ball, and the middle one. In this way, two separate images have been combined to form a single image.

This is our brain’s way of helping us see the world around us conveniently. The part of the brain that is involved in the fusion of the two images is called the “corpus callosum”. 

Try closing your one eye and have someone toss a ball at you. You would find that it is much harder to catch the ball as you can’t really tell how close the ball is to you. With two eyes, however, you can easily tell when the ball is close enough for you to catch it.

This ability of the brain to make us see a compound image instead of two images is an extremely important factor in giving us depth perception. It means that with one eye, we would look at a thing, and it would appear to be in 2-D. But with two eyes showing a combined image, we see everything in 3-D! Each of our eyes sees an image with a slightly different angle, that tells us exactly how far or close something is.