By the 1920s, the photoelectric effect and the atomic spectra (plus the blackbody radiation which is not in the H2 syllabus) had already provided solid evidence that light consists of particles. Yet the double-slit interference pattern remained as persuasive argument that light consists of waves.
To a classical physicist, these two descriptions of light are mutually exclusive. So (at least) one of them must be wrong. To some quantum physicist in the 1920s, these two descriptions are complementary to each other: whether an object behaves as a particle or as a wave depends on your choice of apparatus and experiments for looking at it. This doctrine is known as the wave-particle duality.