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

Understand one thing that human eye does not perform the task of seeing. Eyes are only lenses which capture the light reflected from objects and it is sent to the brain. Afterward the brain itself sees the sight. Brain mingles with each other different light-rays captured at different angles with two eyes, so the joint sight is constructed. And only then depth is identified. This “depth” means third dimension of the sight! Length and width are the first two dimensions of sight, but depth is only ascertained when the brain gets two images of the sight taken from different angles. Again, these angles must not be any “xyz” degree. The two lenses need to be at a precise distance from each other. This precise distance is 2.5”. The Nature has placed this exact distance between two human eyes. Because of this a tree nearby appears closer than a mountain far away. Along with length and width depth is also identified.
This is the reason while shooting a 3D film a scene is taken with two different cameras placed at a precise distance from each other. While watching the film the right eye-glass captures and sends in the scene made for the right eye, and the left eye-glass does the same with the scene made for the left eye. This is how instead of flat screen it seems like seeing the exact natural scene.

Additional Reading:
Visual perception (Wikipedia)
3-D film (Wikipedia)

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