When I ask someone to look at the circles on the red and the green background, the most standard response is "Oh, I hate this one, I can't do it" and yet, it's very unusual that this test doesn't give a useful response. The difference between the two may only be slight, but like a crooked picture on the wall, it really doesn't need to be much different to be able to tell, our brains are better than we give them credit for.  

How it works:  When light passes through anything at all, some light gets split into it's component colours.  White light is actually made up from all colours of the spectrum and can be easily split – which can be seen such as when it travels through a prism (think of the album cover for Pink Floyd's Dark Side of the Moon) or through moisture – it's also how a rainbow is formed.  Red and green (although we sometimes use blue as well) are at opposite enough ends of the spectrum for the human eye to tell which has more clarity, depending on where our eye is focussed.  Simply, if we are slightly shortsighted – or light is focussed slightly behind the eye, we will see the red more clearly and if we are slightly longsighted – or light is focussed in front of the retina we will see the green more clearly.  This test therefore helps the Optician work out where the focussing point is.  

Most adults tend to overthink the test, and as the difference is only slight don't have confidence in what they see to give an answer, but almost everyone does when we push them.  Children often do better on this, as they don't worry as much about their response.  So next time you are tested, don't worry, don't overthink it, just give your first response – you'll be doing it just right.

And in answer to the title of this newsletter, and as you may have guessed – those who are just a little bit shortsighted will see Rudolph first.