The classical interpretation of Young’s double slit experiment is that, before and after passing through the diaphragm of the double slit, the light behaves as waves. Feynman called the double slit experiment “the only mystery [of quantum mechanics]. We have shown for the first time in the classical wave experiments: (1) Particle patterns (non-interference patterns) indicating the particle nature of light are near the diaphragm, and Interference patterns are near the screen; (2) Particle patterns evolve to Transition patterns (non-interference patterns) and finally, evolve to Interference patterns. We proposed Postulates of lens for utilizing lens in wave experiments. In this article, we experimentally suggest that Particle patterns, Transition patterns and Interference patterns simultaneously emerging on the screen in the double slit experiments show that the light beams are photons. The photon chamber shows that the photons distribute as waves in the wave experiments. We suggest that these phenomena and the physical mechanism of them are new mystery.