We detect an object with the help of light. When photons hit the retina our eyes find the presence of the source of the light (object), the source being the original source of detected light or reflected light's source. But how the exact position of the source (object) is detected? Photons just reach our eyes, but how we are able to locate an object?
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$\begingroup$ @Ankit you can identify position many ways without seeing an object. Echolocation and proprioception are two examples. $\endgroup$– electronpusherCommented Oct 1, 2020 at 9:22
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1$\begingroup$ Why bring up photons? $\endgroup$– my2ctsCommented Oct 1, 2020 at 9:27
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2 Answers
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The easiest way would be to not just look with one eye, but with two. The image of an object is projected on the retina of each eye, but slightly displaced and the brain can from this compute the distance. It's basically a distance measurement by parallax, i.e. measuring the angle under which the source is seen from two points with a known distance (as is done also in astronomy when measuring the distance to some stars).
There is also the slight possibility that the brain can measure distances somewhat by defocusing the object (the eye's lens is pulsating). The brain could than assess a distance by the relative blur of an object.
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It's called imaging. A lens in your eye converts direction into position information, that is, an image on your retina. With the information of two slightly displaced images the visual cortex can convert the images into a 3D representation of your field of view. Photons are relevant only for very low intensity light or for the operation of the detector molecules in the retina.
