Is light a wave or a stream of particles? To classical physicists in the 1920s, these two descriptions of light are mutually exclusive. So one of them (if not both) must be wrong.
On one hand, the photoelectric effect and the atomic spectra (plus the blackbody radiation which is not in the H2 syllabus) provide convincing evidence that light consists of particles called photons. On the other hand, the double-slit interference pattern provides persuasive evidence that light consists of waves. So is light a wave or a stream of particles? We have a hung verdict!
To overcome the impasse, it was proposed by leading quantum physicist of the time that we view these two descriptions as complementary to each other: whether light behaves as a particle or as a wave depends on our choice of apparatus and experiments for looking at it. This doctrine is known as the wave-particle duality.