法兰克-赫兹实验是一个由德国物理学家詹姆斯·法兰克和古斯塔夫·赫兹完成的著名物理实验。这实验首先直接地证实了波耳模型离散能级概念的正确性。 1914 年，他们发表了这令人信服的实验结果，对于现代量子力学被科学界接受做出重大的贡献。由于他们「发现那些支配原子和电子碰撞的定律」，法兰克和赫兹共同得到1925年诺贝尔物理学奖。

在尼尔斯·波耳创建的波耳模型里，电子是绕着原子核运行于离散能级的轨道。法兰克-赫兹实验显示出，原子的确只能够吸收（受激）特定数量的能量（量子），因此证实了波耳原子的能级是离散的。

The Franck–Hertz experiment was the first electrical measurement to clearly show the quantum nature of atoms, and thus "transformed our understanding of the world". It was presented on April 24, 1914 to the German Physical Society in a paper by James Franck and Gustav Hertz. Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.