1164. How does an ear thermometer work so quickly? — SN, West Covina, California
An ear thermometer examines the spectrum of thermal radiation emitted by the inner surfaces of a person's ear. All objects emit thermal electromagnetic radiation and that radiation is characteristic of their temperatures—the hotter an object is, the brighter its thermal radiation and the more that radiation shifts toward shorter wavelengths. The thermal radiation from a person's ear is in the invisible infrared portion of the light spectrum, which is why you can't see people glowing. But the ear thermometer can see this infrared light and it uses the light to determine the ear's temperature. The thermometer's thermal radiation sensor is very fast, which accounts for the speed of the measurement.