Color: A Journey into the Atomic and Quantum World

Introduction: Why is Ruby Red and Emerald Green?

Saying that a ruby is red and an emerald is green sounds easy: light passes through each material, and voilà! But reality is much trickier. Both contain the same impurity ion, chromium, which makes us wonder: how do they end up looking so different?

The answer lies in how electrons interact with light and the subtle differences in crystal structures. Color depends not only on which wavelengths are absorbed or transmitted but also on the atomic arrangement and the quantum rules that restrict energy levels like the rungs of a quantum ladder.

The Energy Ladder: Where Color Begins

Atoms occupy specific energy levels. An electron can absorb or emit light only if the photon’s energy matches the difference between two levels.

Ruby: Chromium ions absorb violet and yellow-green light, leaving red visible.

Emerald: The crystal structure alters energy levels, blocking red and enhancing green-blue light.

How Light Interacts with Matter: The 7 Causes of Color (there are at least 7 more reasons)

1. Electronic Transitions; E.g., Ruby and emerald, where crystal fields adjust energy levels and visible spectra.

2. Blackbody Radiation; E.g., An incandescent filament shifts from red to white as temperature increases.

3. Rayleigh Scattering; E.g., The sky appears blue because air molecules scatter blue light more effectively than red.

4. Color Centers; E.g., Fluorite turns purple as electrons get trapped in crystal defects.

5. Geometric Optics and Interference; E.g., Soap bubbles and butterfly wings exhibit iridescence through light-wave interference.

6. Metals and Semiconductors; Gold and silver: Free electrons reflect light, producing characteristic shine. Semiconductors: Band gaps define color. E.g., Cadmium sulfide (CdS) appears yellow.

7. Pigments and Dyes; Organic pigments rely on conjugated bonds to keep energy within the visible spectrum, Copper phthalocyanine: A deep blue used in high-quality printing, Quinacridone: Bright red, with excellent light and heat resistance.

Conclusion: Science, Art, and the Illusion of Control

While we obsess over optimizing every process and ink drop, how many supplier contracts have we left on autopilot? Understanding the causes of color allows us to choose inks wisely, but it also reminds us that color is both science and art—and an excellent way to save time, money, and avoid unpleasant surprises.

Bon appétit!