iFixit decided to answer that question and promptly disassembled an iPhone 4S to see how it ticks.
The answer? There is an infrared sensor that is always activated on the device.
During our iPhone 4S teardown, iFixit buddy Markus noted that the new iPhone had a rather unusual-looking black component next to the ambient light sensor….
Now that the teardown is wrapped up, we’ve re-opened the mystery and made a neat discovery about the 4S: that black component is an infrared LED, and the little bugger almost always wants to know if you’re nearby.
Users don’t have to worry about health risks of the infrared LED. The amount of light emitted by the sensor isn’t nearly strong enough to damage eyesight (though we still wouldn’t recommend staring at the sensor — or any source of light, really — for a long period of time). This also doesn’t mean your iPhone now works as a flashlight. Infrared light has a wavelength longer than visual light for humans, meaning there’s no way you’d detect the sensor unless, as iFixit points out, you are a snake, fish, mosquito or cheap digital camera.
SEE ALSO: Siri vs. Furby: When Two Worlds Collide
The iPhone has always had some sort of proximity sensor, such as for when the screen shuts down during a call to avoid accidentally pressing buttons with your cheek. In older models of the iPhone, these sensors would only activate when a call was made. Siri’s “raise to talk” feature, however, requires the sensors be on whenever the screen is active. Presumably these sensors can also differentiate between your face and, say, your pocket, otherwise Siri’s going to be doing a lot of eavesdropping.