Which statement describes a lead sulfide cell's sensitivity to light?

Which statement describes a lead sulfide cell's sensitivity to light?

• A. Infrared rays
• B. Visible light
• C. Ultraviolet rays
• D. X-rays

Answer: (A)

Lead sulfide has a very small bandgap, so it responds to photons with relatively low energy. That means infrared photons can excite electrons across the gap, increasing conductivity and producing a detectable signal. At room temperature, the bandgap of PbS corresponds to wavelengths around a few micrometers, placing its strong photoresponse squarely in the infrared region. Visible light has much higher-energy photons and is less efficiently detected by PbS, while ultraviolet and X-ray photons are even higher in energy and are not the material’s usual domain of sensitivity. So the best description of a lead sulfide cell’s light sensitivity is infrared rays.

Lead sulfide has a very small bandgap, so it responds to photons with relatively low energy. That means infrared photons can excite electrons across the gap, increasing conductivity and producing a detectable signal. At room temperature, the bandgap of PbS corresponds to wavelengths around a few micrometers, placing its strong photoresponse squarely in the infrared region. Visible light has much higher-energy photons and is less efficiently detected by PbS, while ultraviolet and X-ray photons are even higher in energy and are not the material’s usual domain of sensitivity. So the best description of a lead sulfide cell’s light sensitivity is infrared rays.