A light guide is a device used to direct light from a light source (commonly an LED) to a place where the light is needed. Light guides are also sometimes referred to as light pipes.
Light guides are usually made of glass or plastic, which typically have an index of refraction around 1.5. Light that is injected into the light guide within the correct range of angles becomes trapped inside the guide because of a phenomenon called total internal reflection, or TIR. Once trapped, the light remains inside the guide until it is extracted by an extraction feature, is fully absorbed by the material, or encounters a surface at less than the critical angle.
In some cases, the goal is to move the light from one end of the guide to the other. In other cases, the goal is to extract the light along the length of the light guide and send it in a specific direction. This makes the light guide appear lit. This extraction is achieved by adding components to the device like paint dots or textures (small bumps or holes) that influence the way the light is reflected, breaking the TIR condition and causing the light to exit the light guide.
Other types of light guides are used to homogenize light emerging from one or more light sources. By allowing light to travel down the length of a guide while reflecting off the sides, the light is “mixed,” and the light emerging from the end of the light guide is spatially and angularly uniform.