A lead sulfide cell used for flame detection is sensitive to which type of light?

The correct choice is infrared light, as lead sulfide (PbS) cells, also known as lead sulfide photoconductive cells, are specifically designed to be sensitive to infrared radiation. These cells operate on the principle of photoconductivity, where the resistance changes with the absorption of infrared light. In many applications, particularly in flame detection systems, the heat emitted from flames is primarily in the infrared spectrum. The ability of the lead sulfide cell to detect this infrared radiation enables it to function effectively in sensing the presence of flames, as most flames emit radiation in this wavelength range.

In contrast, visible light, ultraviolet light, and X-ray light do not fall within the sensitivity range for lead sulfide cells. Visible light detection generally involves different types of sensors that respond to light within the visible spectrum. Ultraviolet light detection requires sensors that are specifically designed for higher energy wavelengths. X-ray detection is even more specialized, relying on materials and technologies that can interact with X-rays, which have a significantly different interaction with matter compared to infrared radiation. Therefore, understanding the specific function and capabilities of lead sulfide cells clarifies why they are optimized for detecting infrared light over the other types listed.