physicists were forced to..

As the nineteenth century gave way to the twentieth, physicists were forced to revise their notions about the nature of light. This initially modest readjustment of their world-view grew, like an avalanche triggered by a snowball rolling down a hill, to become a revolution that engulfed the whole of physics -- the quantum revolution.
The first step was the realization that electromagnetic energy cannot always be treated simply as a wave passing through space. In some circumstances, a beam of light, for example, will behave more like a stream of tiny particles (now called photons). One of the people instrumental in establishing this "wave-particle duality" of light was Einstein, who in 1905 showed how the way in which electrons are knocked out of the atoms in a metal surface by electromagnetic radiation (the photoelectric effect) can be explained neatly in terms of photons, not in terms of a pure wave of electromagnetic energy. (It was for this work, not his two theories of relativity, that Einstein received his Nobel prize.)
19世紀から20世紀へとなり、物理学者たちは光について彼らの考えの見直しを余儀なくされた。この世界観の当初の適度な見直しは、雪玉が丘を転がり落ちたことがきっかけとなった雪崩のように、物理学全体を巻き込む革新となるため大きく変化した -- 量子の革命。