What does the 'red and green' test tell the optician?
This test is known as the 'duochrome test'; it's used to estimate if you're short or long sighted.
This 'red and green' test works on the principle that the wavelengths of red and green light are focused slightly differently by the eye, red being closer to the retina with short sight (i.e. clearer) and green focused closer to the retina in long sight.
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