What defines Passive Sensors in remote sensing?

Passive sensors are defined by their ability to detect energy that is naturally emitted or reflected from the terrain they observe. This energy primarily comes from sunlight, which is reflected off the Earth's surface during the day or can include thermal radiation emitted by the Earth at night. Since these sensors do not generate their own energy for illumination but instead rely on external sources, they capture the natural state of the environment.

The distinction made by the correct answer highlights that passive sensors harness existing energy rather than creating it artificially. For instance, in daytime applications, passive sensors effectively utilize sunlight to detect features on the Earth's surface, measuring different wavelengths of light based on characteristics such as color, temperature, and reflectivity.

The other choices do not align with the principles of passive sensing. Artificial light is a characteristic of active sensors, where they emit light and measure the energy that is reflected back. Chemical energy is irrelevant in the context of passive sensing, and while passive sensors can operate at night when relying on thermal emissions, they are not limited to this condition, making the statement about nighttime conditions inaccurate.