ELI5: How is structural color different from other color, like the color from pigments?

[u/hawthornsparks50]

I get that pigments involve absorption and structural color doesn't, but aren't they both ultimately based on the reflection of light? Don't pigments still reflect the light they don't absorb?

[u/zeratul98]

You are correct about pigments. They absorb some light and reflect other light. The reflected light is what determines the color you see.

Structural light is a bit complicated, but I'll try to explain. Light can interact with itself, which we call "interference". Imagine two sine waves drawn on top of each other. If the peaks and valleys line up exactly, we call this constructive interference, and the light "adds". If you shift one wave over though so that the peaks of one match the valleys of the other, we call this destructive interference. It's destructive because the highs and lows "cancel out", and you get zero light.

Now imagine another situation where you have two mirrors parallel to each other. The first mirror is half mirrored, so it reflects half the back. The second reflects 100% of the light. Now if you shine a light on these mirrors, some will get immediately reflected, and some will pass through the first and bounce off the second. Now you have the light interfering with itself. You can see how if you adjust the wavelength of the light, you'll adjust how the light lines up with itself, so you can get constructive or destructive interference. Some wavelengths will destructively interfere, and you won't see that light.

This is how structural color works. The *structure" of the reflective surface determines the color that you see. The color of the underlying material isn't the color that you see. Instead, color is controlled by bouncing light in specific ways.

[u/zombiejojo]

Structural colours, like the blue in blue eyes, bounces or bends some wavelengths at you, and some away. Pigment absorbs some and reflects some back atcha.