Morning sun floods into assistant professor Liheng Cai's office in the University of Virginia's Wilsdorf Hall. The photons of infrared radiation carrying heat energy, streaming through his window after their 8-minute, 20-second journey from the sun, are welcome in the winter; they keep Cai warm as he prepares to teach his new class, Advanced Polymers. But what about those summer days when he'd rather stay cool? Cai is developing adaptive photonic polymers — materials that could be applied to windows and, at the flick of a switch, reflect the invisible infrared wavelengths that toast things up while letting in all other visible light waves, enabling him to keep chill while maintaining his epic view of the UVA School of Engineering.