Decoding Snellen Chart in 4 mins !

I will not start here with the history behind Snellen chart, since Dutch ophthalmologist introduced the Snellen chart in 1862. Rather straight to the point, the science behind Snellen Chart.

Why is the Snellen Chart posted at a distance of 6 meters or 20 ft from the patient's eye ? Easy answer for most of us, because at that distance, all rays of light are (assumed) to be entering as parallel rays of light setting at rest the involuntary action of accommodation.

But beyond this the Snellen Chart has more science to it than this simple explanation. It starts with the statement that in order for us to see two things as disctintly separate objects, the angle at which the rays of light should subtend at the nodal point of eye be 1 minute of the arc ( that is 1/60th of a degree ). This is the minimum separable distance that our eye needs to help us to identify two objects distinctively. In terms of the Snellen Chart, what this means is that, each limb of the letter ( say the letter E ) has to be separable by 1 minutes of arc (1/60th of a degree ) subtending at the nodal point of the eye.

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