beam

A light beam or beam of light is a directional projection of light energy radiating from a light source. Sunlight forms a light beam (a sunbeam) when filtered through media such as clouds, foliage [树叶], or windows.

From the side, a beam of light is only visible if part of the light is scattered by objects: tiny particles like dust, water droplets (mist, fog, rain), hail, snow, or smoke, or larger objects such as birds. If there are many objects in the light path, then it appears as a continuous beam, but if there are only a few objects, then the light is visible as a few individual bright points.

A laser differs from other sources of light in that it emits light which is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar. 激光与其他光源的不同之处在于它发出的光是相干的。空间相干性允许激光聚焦到一个较紧的点，从而实现激光切割和光刻等应用。空间相干性还允许激光束在远距离内保持窄距离（准直），从而实现激光指针和激光雷达等应用。

• Coherent light is light which comes from a very very small light source.
• If a group is coherent, its members are connected or united because they share common aims, qualities, or beliefs.
• 为啥不把coherent翻译成一致性呢？

The word laser started as an acronym for "light amplification [放大] by stimulated emission of radiation". A laser that produces light by itself is technically an optical oscillator [振荡器] rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation", would have been more correct.

波长乘频率=速度。红光比紫光波长长，紫光比红光频率高。Ultraviolet=紫外; infrared=红外. ultra:超; infra:低。

X-ray wavelengths are shorter than those of UV [ultraviolet] rays and typically longer than those of gamma rays.

Cathode rays (electron beam or e-beam) are streams of electrons observed in discharge tubes. Cathode rays are not electromagnetic wave because they do not have electric and magnetic components perpendicular to each other. 阴极射线（电子束或电子束）是在放电管中观察到的电子流。阴极射线不是电磁波，因为它们没有正交(相互垂直)的电磁成分。

• broad in the beam = 臀部相当丰满
• off beam = incorrect or mistaken