Why Are Blue Lights Harder to See?

25 April 2017

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blue fairy lights

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Question

While walking my dog in the evenings during the holiday season this year I noticed I was unable to focus on and clearly resolve the dark blue LEDs in Christmas tree lights or fairy lights. For all the other colours I could resolve the shape and even the texture of the individual light bulbs, but with these deep royal blue LEDs all I could see was a hazy blur. Why is that?

Answer

Izzie Clarke put this question to George Dobre from the University of Kent.

George - the cells in our eyes that process colour are called cones. It’s easy to remember - cones for colour, and they can be found at the back of our eye called the retina. These cones really detect just three colours - red, green and blue. We then build other colours based roughly on the amount of red, green, and blue the cones report to the brain. However, the blue cones are a small minority: only about 10% of the total.

Izzie - So each cone is set to tell the brain about mainly one of these three colours, but as more cones register red and green light, blue is at a slight disadvantage. But why does blue light appear blurry?

George - To be able to see a tiny dot of colour, like a blue LED in detail, the eye needs to be able to focus that LED light on just a small number of cones. The smaller this area can be, the more detail we can see, but for a blue dot that’s difficult to achieve. The eye automatically adapts to see red and green with the sharpest focus, which leaves the blue unfocused and fuzzy. This is called chromatic aberration.

Izzie - The human eye has evolved towards a compromise. We still see sharp images most of the time, except for when we look at tiny blue dots or lines, and that’s because it’s a relatively rare occurrence in nature. That’s the anatomy of physiology part covered. Come on George - hit me with some physics...

George - The physics part of the answer has a lot in common with the question why is the sky blue? Which also applies to these blue fairy lights being at the short wavelength end of the visible spectrum when blue light hits air molecules it scatters much more than red or green. In contrast, red light at the long wavelength end of the spectrum tends to scatter less continuing along in mostly a straight line, which is why we get red sunsets. The consequence is that blue light is focussed to a spot on the retina that is a little bit bigger than that for red and green.

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