If you've ever had yellow M&Ms, bright green soda, yellow potato chips, or even mustard, chances are pretty good that you've come across the chemical tartrazine. This dye, commonly known as yellow 5, has been added to our food for years to make it look a little more eye-catching.
Here's the wild thing about yellow 5: scientists have been experimenting with this easily absorbed chemical by rubbing it into the skin of test mice in the lab. I know what you're thinking...why would we want to make mouse skin yellow? In fact, tartrazine doesn't turn skin yellow. It makes the skin...CLEAR! 😱
Yes, you read that correctly. For a period of about 10 to 20 minutes, scientists can see the inside of a mouse through its skin. We're talking about peering at blood vessels, nerves, organs and muscles simply by rubbing skin with an everyday food additive!
This amazing discovery is based on something called the index of refraction, or the way light moves as it passes through different materials.
Imagine you're fishing from a boat in the middle of a lake. You look over the side of the boat to check the water's depth, but you can't see the bottom. This is because each piece of sediment or algae floating in the water has a different index of refraction. As light enters each particle, the light travels at different speeds, which causes it to bend and scatter all over the place. Instead of seeing through the water, all you can see is a green-brown haze.
Oddly enough, skin does something very similar to murky water. Inside each cell of mouse skin (or person skin) are all sorts of molecules like proteins, lipids, and water. Light passes through each kind of molecule at a different speed, causing it to bend at different angles. Molecules like lipids bend light a lot as light passes through, while water only bends light a little bit. We can't see through our skin because the light changes speeds and bends in too many directions as it passes through the different materials.
Tartrazine increases the refraction index of water to make it close to lipids. This means that light entering water bends about the same as it bends passing through lipids. When all the materials in a liquid have a similar index of refraction, the fluid becomes see-through. If all the stuff floating in our lake had a similar index of refraction, you would be able to see the bottom. When water and lipids have the same index of refraction, you can see through skin!
Here is a visual comparing the index of refraction of different substances. Remember, the higher the refractive index, the more it bends light.
Here are some questions I'd ask my students if I brought this graph into class:
💡Based on the information on the graph, why do you predict that we cannot see carbon dioxide gas mixed with air? Air and carbon dioxide both have an index of refraction of about 1. Substances with a similar index of refraction will look transparent when they are together.
💡Which will scatter more light when dropped into a tub of water: a block of ice or a block of salt (sodium chloride)? Why do you say so? Salt will scatter more light than a block of ice because the difference between the refraction index of salt and water is greater than the difference between ice and water.
💡Cubic zirconia is a mineral used as a cheaper alternative to diamond jewelry. Use the data table to justify why cubic zirconia would be a close match to diamond. Both substances have a reasonably close index of refraction. Diamond has an index of about 2.4 and cubic zirconia has an index of about 2.2.