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Measuring the size of the pixels in your eye
Posted 11/09/2018 10:44AM

As part of the study of optics, students got a chance to learn something about the inside of their eyes using just a tape measure and a pixelated image as shown.  From a sufficient distance you don't notice the pixels and you see a boy's face, give it a try yourself!  But as you move closer, at some distance you notice the pixels.  One can see in the photo AP Physics students measuring that distance for a 5mm pixelated image on the Science Center door, and their distance averaged 13 meters. 
With those two numbers, plus the lens to retina distance of 17mm, it is a simple ratio from two similar triangles to determine a corresponding small distance on the retina.  This small distance is decent estimate of the spacing of the cones on the retina, which our experiment found to be about 7 microns (a human hair is 50-100 microns in diameter).   
The center-to-center spacing of the 6 million or so cones in the human eye varies, but the average is around 3 microns.  While other effects such as diffraction are also at play, an interpretation of this experiment is that we start to discern the pixels once their image on the retina covers at least a couple cones.  It's fun to estimate the size of something small and inside your eye with just a tape measure and a simple experiment.  Plus it also helps us understand artists that use the same sort of effect to produce a piece that changes markedly with distance such as Salvador Dali's Lincoln in Dalivision, or some of the works of Chuck Close.   

By Scott Dudley

Upper School Physics Teacher

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