Which device shines light at the end of a fiber for testing purposes?

The device that shines light at the end of a fiber for testing purposes is a light source. In fiber optic testing, a light source is used to inject light into the fiber optic cable, which allows technicians to analyze the transmission characteristics of the optical fibers being tested. By doing this, they can assess parameters like loss, integrity, and overall performance of the fiber link.

In contrast, while a power meter is an essential instrument for measuring the power of the light signal after it has traveled through the fiber, it does not shine light into the fiber itself. A visual fault locator is designed to identify issues such as breaks or bends in the fiber by emitting a visible light through the fiber, which is useful but serves a different testing purpose than a standard light source. The receive cable does not shine light; instead, it typically connects to transmitters or receivers for data transmission rather than being a testing device.

Thus, the light source is specifically intended for introducing light into the fiber, making it the correct choice for this question.