Figure 14 - Classical physics failed to explain black body radiation.

The term “modern physics”, taken literally, means of course, the sum total of knowledge under the head of present-day physics. In this sense, the physics of 1890 is still modern; very few statements made in a good physics text of 1890 would need to be deleted today as untrue. The principle changes required would be in a few generalizations, perhaps, to which exceptions have since been discovered, and in certain speculative theories, such as that concerning the ether, which any good physicist of 1890 would have recognized to be open to possible doubt.

On the other hand, since 1890, there have been enormous advances in physics, and some of these advances have brought into question, or have directly contradicted, certain theories that had seemed to be strongly supported by the experimental evidence.

For example, few, if any physicists in 1890 questioned the wave theory of light. Its triumphs over the old corpuscular theory seemed to be final and complete, particularly after the brilliant experiments of Hertz, in 1887, which demonstrated, beyond doubt, the fundamental soundness of Maxwell's electromagnetic theory of light. And yet, by an irony of fate which makes the story of modern physics full of the most interesting and dramatic situations, these very experiments of Hertz brought to light a new phenomenon—the photoelectric effect—which played an important part in establishing the quantum theory. The latter theory, in many of its aspects, is diametrically opposite to the wave theory of light; indeed, the reconciliation of these two theories, each based on incontrovertible evidence, was one of the great problems of the first quarter of the twentieth century [10, http://en.wikipedia.org/wiki/History_of_physics].