• Kim Rasmussen

What is refraction?

Updated: Apr 11

Summer showers may be good for gardens and crops, but they are also great news if you want to spot a rainbow. Having the sun behind you and at an angle of less than 42° above the horizon and moisture-filled air in front creates the perfect conditions for rainbows to appear. [1] And whilst they may not harbour a pot of gold, rainbows do bring us an excellent example of refraction in action.

Quite simply, rainbows form when light bends as it passes through water droplets. That bending is called refraction. As sunlight moves from the air and into the water droplet, all of the separate colours of the light bend in a slightly different way, thereby creating rainbows. We mostly think of refraction in terms of light. Perhaps that’s something to do with school projects involving light being refracted through prisms, but other waveforms, including water or sound, can also be refracted.

So why does light refract? “Quite simply, because light travels at different speeds in different materials, and it follows the route which takes the shortest time. And when it crosses from one material to another, this route is no longer a straight line".

The speed of light in a material (or the amount that light bends) is described by a property called the refractive index. Air, for example, has a refractive index of about 1, whereas water has a refractive index of about 1.33. This means that the speed of light in water is about 75% of that in air.

What is important from our point of view at Occuity is that the refractive index is a property of the material. So, if the refractive index of pure water is 1.33, then if you dissolve some sugar in it, the refractive index increases. And the more you put in, the more it increases. If you can measure the refractive index, you can measure the concentration.

Fluid in the eye contains sugar, just as blood does and measuring eye sugar level gives the same result as measuring blood sugar level. We can therefore measure eye sugar level by measuring the refractive index.

The challenge with this is that, until recently, no one has been able to measure the refractive index of a material without taking a sample. However, Occuity has come up with a novel method where the eye's refractive index can be measured simply by shining a beam of light into it. Using this approach, we plan to develop a device that can measure blood sugar levels without invasive finger sticks or any need to take blood samples.

This article was written with the assistance of James Reynolds, who is Occuity’s Chief Optical Engineer with over 30 years’ experience in R&D within a wide range of industries. To meet more of our team, click here.

[1] https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/rainbows/how-are-rainbows-formed

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