Sugar Eye:

How does a refractometer work?

by Bryan Cruz


I was at the Botanic Garden when I ran into a refractometer. It looks super interesting and I wanted to know how it worked. I later then found out that it measures the amount of sugar in foods or substances and it reveals the answer when you point the daylight plate (a piece on the refractometer) to the sun with the substance already on it. My question is how do the mechanisms inside the refractometer make it work, and why do we have to point it to the sun to find the amount of sugar in the substance?


First off to find my answer we need to know the easy things. There are different kinds of refractometers but mine in particular is called the “Brix refractometer”. By now you should know that is measures the amount of glucose or a more common name “sugar” in any substance. The way the refractometer finds the amount of glucose in certain substances is pretty intriguing. The way my refractometer works is it uses a prism and a light source to illuminate the sample that is placed on the “daylight plate”.


Glucose molecule

Refraction is the bending of light. It happens with sound, water and other waves. The bending of refraction makes it possible for us to have magnifying glasses, prisms and rainbows. Without refraction we wouldn’t be able to focus light onto our retina. Also different liquids affect the speed of light and the angle it bends at depending on the substance. This is explains why the refractometer works. When plain water is on the daylight plate the light refracts at a certain angle. The refractometer reads that angle and tells you there is zero sugar in the water. If you put juice on the daylight plate the light that hits the daylight plate refracts at a different angle because juice has glucose. The more glucose there is the bigger the angle gets. You can notice when you look through the refractometer and the number increases. The increasing number means there is more glucose.