Such as photoelectric effect. This is a difficult problem in the early1900s. When light strikes a metal, such as zinc, electrons will fly away. This will only happen when the light comes in the form of small packages and gathers enough energy to make the election fail. A diffused wave will not produce photoelectric effect.

For example, the photoelectric effect is still a difficult problem in the early stage of 1900. When light irradiates metals such as zinc, electrons will be excited. This phenomenon only happens when the incident light is tiny particles and enough energy is concentrated to excite electrons. A distributed light wave cannot explain the photoelectric effect.

Note: This is to explain the "wave-particle duality" of light, pointing out that the particle nature of light cannot be explained by the propagation theory of light waves.