"Why is this solution blue?"
Toma held the test tube to the light.
Rei answered. "Determined by wavelength of absorbed light."
"Absorption?"
Milia explained. "This molecule absorbs red and orange light. So blue remains."
Kana asked curiously. "Why absorb only specific colors?"
"Because electron excitation energy matches light energy," Rei drew diagram.
"Excitation energy?"
"Energy needed for electron to jump from low to high energy level."
Milia continued. "When that energy matches specific wavelength of light, absorption occurs."
Toma thought. "So different molecules absorb different colors?"
"Exactly. That's the source of color diversity."
Kana wrote in notebook. "But how is energy determined?"
Rei explained. "Length of conjugated system. Structure with alternating double bonds."
"Conjugated system?"
"Continuous carbon-carbon double bonds. Electrons delocalize."
Milia showed molecular model. "Here, this part is conjugated."
"Longer conjugated system, longer absorption wavelength."
Toma understood. "Long absorbs red?"
"Yes. Short absorbs ultraviolet. Invisible to eye."
Kana asked. "Carotenoids?"
Milia smiled. "Good example. Long conjugated system, absorbs blue and appears orange."
"Carrot color."
"Yes. Also precursor to vitamin A."
Rei gave another example. "Chlorophyll absorbs red and blue. So green."
"Plant color," Toma said.
"Exactly. Selecting optimal wavelengths for photosynthesis."
Kana thought. "Blood's red?"
"Hemoglobin," Milia answered. "Heme group containing iron absorbs specific wavelengths."
"When oxygen binds, color changes," Rei added.
"Arterial blood is bright red, venous blood is dark red."
Toma was impressed. "Color shows oxygen presence."
"Yes. Spectroscopy is also used for diagnosis."
Kana wrote on whiteboard. "Color = Light not absorbed"
"Perfect summary," Rei acknowledged.
Milia added another solution to test tube. "Change pH and..."
Color changed from blue to red.
"Wow!" Kana was surprised.
"Molecular structure changed, absorption wavelength shifted," Milia explained.
"Litmus paper principle," Toma said.
"Yes. pH indicators change color through structural change."
Rei added. "Temperature, concentration, solvent... color changes with environment. Color tells molecular state."
Kana said quietly. "Color isn't just decoration."
"It's information," Milia nodded. "Tells us what molecule is doing."
Toma lined up test tubes. "Like rainbow."
"Each has physical reason," Rei said.
"Electrons, photons, energy levels... all cooperate to create color."
Kana was impressed. "World is colored by quantum mechanics."
"Poetic but accurate," Milia smiled.
Rei finally added. "Seeing color is dialogue with molecules. Through light, knowing electron states."
Toma murmured. "Blue sky, sunset, same principle?"
"Scattering and absorption differ, but both are light-matter interaction."
Kana closed notebook. "Deep science behind colors."
"Always," Rei said quietly. "Beauty and truth are inseparable."
The four gazed at colorful test tubes. Each color tells a story.