What defines a Photo Detector?

A photo detector is specifically designed to sense and respond to light. It operates by converting light photons into measurable electrical signals, which can be used in various applications such as cameras, light meters, and fiber optic communication systems. This capability is what distinguishes a photo detector from other options mentioned in the question.

The other choices highlight components and devices that do not fulfill the specific function of detecting light. A source of light refers to any device that emits light but does not have the ability to sense or analyze it. Similarly, a device that emits light is not concerned with detection; it simply generates light. Lastly, a complex electronic circuit can include many functionalities, but without the specific purpose of light detection, it cannot be classified as a photo detector. Thus, the defining characteristic of a photo detector is its ability to detect and convert light into an electrical signal, making the correct answer a fundamental aspect of its function.