The app works by detecting high-energy photons, which are abundant in cosmic ray-induced particle showers, plus a type of particle called a muon. Photons are particles of light, which cellphone cameras are already built to detect. Camera equipment is tuned for photons in the visible light spectrum, but they still work for the higher-energy photons present in particle showers, according to a paper the California team wrote. (The paper hasn't yet been peer-reviewed.) In addition, the team wrote, photodiodes in cameras will also interact with muons, which are charged particles that easily pass through buildings, phone cases, and other obstacles.