How do dyes work in regards to the light being reflected?

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So am apple is red because it absorbs all other colours/light on the spectrum and reflects the red back to your eye. Same with anything of any colour as far as I’ve seen.

So how do dyes work when you change them from one colour to another? What are you altering in the fabric that allows it to now absorb the colour it was and reflect a different colour?

In the same vein, what is bleach doing when it removes colour?

In: Physics

2 Answers

Anonymous 0 Comments

A lot of this relates to chemical structure. Basically, molecules can absorb energy from light at specific wavelengths, depending on said structure. We largely do not see this, unless the wavelengths they absorb are in the visible spectrum. — Relatedly, UV is dangerous because it is high energy *and* absorbed by very important DNA molecules in the body, causing damage. This absorption profile just depends on the molecule, and may be somewhat unique, but follow trends based on arrangement / electron arrangement / surrounding chemical environment / etc.

Changing dye colors usually means altering their chemical structure, or introducing them to something that changed their chemical context, thus changing the wavelengths they absorb/miss.

Bleach is a strong chemical, to my knowledge usually a strong oxidizer or base, so it’ll provoke a good deal of chemical change–either by directly disrupting the coloring molecule or by disrupting bonds and allowing it to be removed. After all, bases are essentially the complement to acids, and can be just as destructive.

Anonymous 0 Comments

Dyes don’t usually change the color being *reflected* to a completely different color. You can’t take a bright blue scarf and add dye to make it yellow, for example. If you tried to add yellow dye, you’d get green.

That’s because pigments add to the colors being *absorbed*. So the dye actually removes or prevents light from reaching your eyes. The blue scarf absorbed all the wavelengths except the ones necessary to make the color blue. When you added the yellow dye, you took away more colors so now only green gets to your eye.

You can’t usually take something black and dye it, right? That’s because it is already absorbing all of the colors and reflecting back none (or very little).

But you can take something white and turn it red. The red dye absorbs all the light but red.

This is why mixing colors of light and colors of pigment work differently, by the way. When you mix light of different colors, it’s additive, but mixing colors of pigment is reductive.

When you understand that the pigment is adding to the absorption, it becomes easier to envision what’s happening. The scarf fibers have pigment molecules scattered among them that absorb light. Add more dyes and you get more pigment molecules absorbing more (and different) wavelengths of light. The original molecules that absorbed everything but blue are still there, but now there’s also molecules absorbing everything but yellow. The light that remains after those have both absorbed their fill is green.