In effect, all Einstein did was to apply the equation E=hv to the electromagnetic radiation, instead of to the little oscillators inside the atom. He said that light is not a continuous wave, as scientists had thought for a hundred years, but instead comes in definite packets, or quanta. All the light of a particular frequency v, which means of a particular color, comes in packets that have the same energy E. Every time one of these light quanta hits an electron, it gives it the same amount of energy and therefore the same velocity. More intense light simply means that there are more light quanta (now we call them photons) that all have the same energy, but changing the color of the light changes its frequency, and so changes the amount of energy that is carried by each photon.

In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing through the optics in a camera. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances

lovelegend寫到:
我應該修正我的說話, 光並不是 wave 也不是粒子, 而是量子...
論點出處是愛因斯坦在 1905 年 Annalen der Physik (一本科學雜誌)上發表的三大論文之一 "On a heuristic viewpoint concerning the production and transformation of light" (愛因斯坦亦是憑這篇論文而得到 1921 年的諾貝爾物理獎的)...

當然我沒有看過那篇論文 , 我看的是一本介紹 Quantum Physics 的書, John Gribbin 的 In Search of Schrödinger's Cat - Quantum Physics and Reality:

而書中說到愛因斯坦這篇論文的部份是在第 47 頁:

In effect, all Einstein did was to apply the equation E=hv to the electromagnetic radiation, instead of to the little oscillators inside the atom. He said that light is not a continuous wave, as scientists had thought for a hundred years, but instead comes in definite packets, or quanta. All the light of a particular frequency v, which means of a particular color, comes in packets that have the same energy E. Every time one of these light quanta hits an electron, it gives it the same amount of energy and therefore the same velocity. More intense light simply means that there are more light quanta (now we call them photons) that all have the same energy, but changing the color of the light changes its frequency, and so changes the amount of energy that is carried by each photon.

If one were to ask the question "What is light, really?" there can be no simple answer. There is no familiar object or macroscopic modle to employ as an analogy. But understanding need not be based on analogy. A consistent and unambiguous theoretcial explanation of all optical phenomena is furnished jointly by Maxwell's electromagnetic theory and the quantum theory.

(題外話一下, 這本書亦提到了愛因斯坦和 Erwin Schrödinger 在量子物理學上的爭拗 (所以書名也用到了 Schrödinger's cat 假說), Schrödinger 說 "Nothing is real unless it is observed", 但愛因斯坦很不以為然, 並說 "God does not play dice"... 這個爭拗一直不能解決, 直到 1982 年 University of Paris-South 進行了一些實驗, 最終證實 Schrödinger 是對的...
P.S. 其實 Schrödinger 也很不太能接受自己的理論, 所以他說過一句話: "I don't like it, and I'm sorry I ever had anything to do with it" )