Why does red food coloring look green when it dries?

[Happy Scientist]

While experimenting with an idea for a demonstration in surface tension, I ran across something interesting. I placed a drop of red food coloring on a glass slide, and let it dry. Once dry, the residue was a light green in color, but when held up to let light pass through it, it looked red again. I tried this again, using green food coloring. Once again, the colors reversed. The residue looked red when light reflected from it, but green with transmitted light.

After quite a bit of searching, I came across a few articles talking about something similar with fluorescence in chlorophyll, but I am not sure this is the same thing.
http://plantsinaction.science.uq.edu.au/content/125-chlorophyll-fluorescence

Anyone have any thoughts on this?

 

[fresh_42]

The usual cause of color change of food is oxidation. But I'm not sure if this can explain this complementary effect you described. However, I would try to find out what the colors are made of and how these chemicals react with air / oxygen. Citric acid can slow down oxidation, which means it would be interesting to know, what happens if you add some of it before drying.

 

[Happy Scientist]

That was my first thought too, but if you redissolve it, it goes back to its original color. It also still looks its original color when light shines through it. It is only when light reflects from it that the color changes so dramatically.

For the red, the ingredients are: WATER, PROPYLENE GLYCOL, FD&C REDS 40 AND 3, AND 0.1% PROPYLPARABEN (PRESERVATIVE).

For the green: WATER, PROPYLENE GLYCOL, FD&C YELLOW 5, FD&C BLUE 1, AND PROPYLPARABEN (PRESERVATIVE).

I will take some photos tomorrow in sunlight, and post them.

 

[NTL2009]

Actually, I have wondered why all translucent colors don't appear this way.

Consider a red tail light lens cover on a car. It filters the broad spectrum of the filament bulb, passing the red spectrum, so the light from the bulb appears red to us. So doesn't that mean it is reflecting the other colors? So when we look directly at the lens, shouldn't it appear blue-green to us?

 

[pixel]

So doesn't that mean it is reflecting the other colors?

It is mostly absorbing the other colors (wavelengths).

 

[NTL2009]

It is mostly absorbing the other colors (wavelengths).

So absorbing rather than reflecting? I think that makes sense to me.

Is that essentially the difference between a translucent object and an opaque one? We can't see through an opaque one, as the other light is being reflected? There's maybe other causes/conditions, but is that part of it? Thanks.

 

[pixel]

Is that essentially the difference between a translucent object and an opaque one? We can't see through an opaque one, as the other light is being reflected? There's maybe other causes/conditions, but is that part of it? Thanks.

There are several processes that may be present in any given situation: transmission, reflection, absorption and scattering. For an opaque coated mirror, it is mostly reflection from the metal coating. For a covering layer of paint, it is mostly absorption and scattering, with some reflection. Depends on the specifics of the material.

 

[quackyneudle]

There are multiple effects going on here, like pixel said. For this situation, we are mostly concerned with absorption, transmission, and reflection. Since you're probably looking at this under white light scattering doesn't really matter.

In solution, the dye molecules are dispersed and as you are viewing them, the dominant characteristic is absorption and transmission. In this situation, they look the color that they do because they absorb one color and transmit the complement. This is kind of like a resonant electronic excitation. In the solid state the situation is different. There is now a surface with a solid/air interface where reflection can occur, and in general surfaces can very different optical properties from the bulk as well. When you hold the solid up to the light it still behaves normally, because now you are looking at absorption/transmission like you were in the liquid. If you look at the surface at an angle, you are mostly seeing the surface contribution, which for organics often behave how you describe. This is due the the details of the optical dielectric properties of the material and the electronic structure that's typical of organics.