光子（Photon）是一种基本粒子，是电磁辐射的量子。在量子场论里是负责传递电磁力的力载子。这种作用力的效应在微观层次或宏观层次都可以很容易地观察到，因为光子的静止质量为零，它可以移动至很远距离，这也意味着它在真空中的传播速度是光速。如同其它微观粒子，光子具有波粒二象性，能够展现出波动性与粒子性。例如，它能在双缝实验里展示出波动性，也能在光电效应实验里展示出粒子性。

阿尔伯特·爱因斯坦在1905年至1917年间发展出光子的现代概念，这是为了解释一些与光的古典波动模型不相符合的实验结果。当时被普遍接受的经典电磁理论，尽管能够论述关于光是电磁波的概念，但是无法正确解释黑体辐射与光电效应等实验现象。半古典理论在麦克斯韦方程组的框架下将物质吸收光和发射光所涉及的能量量子化，而行进的光波仍采古典方法处理；如此可对黑体辐射的实验结果做出合理解释。爱因斯坦的主张与普朗克的半古典理论明显不同，他提出光本身就是量子化的概念，当时爱因斯坦称之为「光量子」（英语：light quantum）。虽然半古典理论对于量子力学的初始发展做出重大贡献，从于1923年观测到的电子对於单独光子的康普顿散射开始，更多的实验证据使爱因斯坦光量子假说得到充分证实。由于这关键发现，爱因斯坦于1921年获颁诺贝尔物理学奖。

A photon is an elementary particle, the quantum of all forms of electromagnetic radiation including light. It is the force carrier for electromagnetic force, even when static via virtual photons. The photon has zero rest mass and as a result, the interactions of this force with matter at long distance are observable at the microscopic and macroscopic levels. Like all elementary particles, photons are currently best explained by quantum mechanics but exhibit wave–particle duality, exhibiting properties of both waves and particles. For example, a single photon may be refracted by a lens and exhibit wave interference with itself, and it can behave as a particle with definite and finite measurable position and momentum. The photon's wave and quanta qualities are two observable aspects of a single phenomenon, and cannot be described by any mechanical model; a representation of this dual property of light, which assumes certain points on the wavefront to be the seat of the energy, is not possible. The quanta in a light wave cannot be spatially localized. Some defined physical parameters of a photon are listed.