In our research into honeybee plant choices, we’d like to know how honeybees choose between different plants; we’re trying to figure out if honeybees visit plants based on the physical attributes of the plant (height, size, number of flowers), or the plant’s productivity (sugar and pollen content). And while a plant’s height, size, and number of flowers might be relatively straightforward data to collect, measuring the sugar available to a visiting bee is more difficult.
In the lab, we use refractometry every week to collect an important piece of our data. We need to determine sugar production levels in flowers on different plants; this can tell us if a bee’s decision to visit a particular plant is based on how much sugar it produces. If this affects the bee’s decision, it might indicate that bees communicate in some way about the quality of the plant; if not, it would seem that there’s no way for them to know about the sugar on a plant before they arrive at it.
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| Drawing nectar from a White Sage flower with a capillary tube |
Refractometry is a deceptively complicated name for a simple laboratory technique: we place a drop of nectar on the glass end of a tube. Then, we press down the plastic cover and look out the eyepiece on the other end. There’s a blue line lining up on tick marks inside the lens which precisely indicates the sugar percentage of the nectar. That’s all there is to it; we need the lens to be clean each time we take data, so it’s literally wash, rinse, and repeat.
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| The Lab refractometer. To measure sugar content, drop liquid onto the glass surface |
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| View through the refractometer eyepiece. This solution is 7% sugar. |
How does this work?
By way of grid lines, a refractometer really measures a fluid’s "refractive index", not sugar content. Refractive index is a way to measure light’s “phase velocity” through a material, and a higher refractive index means that the light we perceive takes longer to travel around the protons and neutrons inside. While photons still travel at the speed of light, c, they move around more slowly as they experience interference from the atoms in a material. A vacuum, with no matter, has a refractive index of 1, while translucent solids, liquids, and gases have refractive indices above one. Air is very similar to a vacuum, with a refractive index of 1.000293. This tells us that light is bent very little when traveling through our atmosphere. In comparison, diamonds have a very high refractive index of 2.42, which is one of the qualities which make them bend light from different directions and sparkle.
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| Refractometer diagram, side view. |
Light appears to change direction when it moves through different materials. The common example is when light moves through a glass of water into your eyes, the light bends where the water meets the glass. This is the guiding principle behind refractometry, because liquids with different refractive indices will bend light by different amounts. The amount the light bends can tell you just what the refractive index is.
Distilled water has a refractive index of 1.33, which increases as you add more sugar. A solution of 80% sugar water has a refractive index of 1.5. When light passes onto the droplet of nectar and into the glass, it is bent at the glass surface. Light from the plastic blue surface below the glass is also bent towards the eyepiece, and the line between blue light and white light is clearly seen through the eyepiece. That line between blue and white changes when the liquid's refractive index changes, and passes through a transparent group of tick marks which indicate exactly how much sugar is in your concentration.
Distilled water has a refractive index of 1.33, which increases as you add more sugar. A solution of 80% sugar water has a refractive index of 1.5. When light passes onto the droplet of nectar and into the glass, it is bent at the glass surface. Light from the plastic blue surface below the glass is also bent towards the eyepiece, and the line between blue light and white light is clearly seen through the eyepiece. That line between blue and white changes when the liquid's refractive index changes, and passes through a transparent group of tick marks which indicate exactly how much sugar is in your concentration.
Voilá! That’s all there is to refractometry.
We measure nectar sugar content using refractrometry, because it's an incredibly fast lab technique. Other industries like refractometry too: it's used by brewers and distillers when observing alcohol sugar content. It can be used to measure salinity near, say, mangrove communities or amounts of various organic compounds in medications. It's fast and fun - chances are, if you've been wondering about sugar in your own home, a refractometer is right for you!
Further reading:
Physics of light scattering in sugar water at various concentrations
Refractometer History
Image credits:
John Little
http://www.refractometer.pl/hand-held-refractometer
We measure nectar sugar content using refractrometry, because it's an incredibly fast lab technique. Other industries like refractometry too: it's used by brewers and distillers when observing alcohol sugar content. It can be used to measure salinity near, say, mangrove communities or amounts of various organic compounds in medications. It's fast and fun - chances are, if you've been wondering about sugar in your own home, a refractometer is right for you!
Further reading:
Physics of light scattering in sugar water at various concentrations
Refractometer History
Image credits:
John Little
http://www.refractometer.pl/hand-held-refractometer
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