Europium and terbium are two rare earth elements that share a colorful similarity: they emit bright red and green light, respectively, when exposed to ultraviolet light. In the image above, there are five thin polymer films embedded with different concentrations of europium and terbium. The far-left film contains primarily terbium, hence the bright green light, while the far-right film contains primarily the red light-emitting europium. … Continue reading In a Ghostly Mirror Rainbow