A neat option of the iPhone 4S, related to the virtual assistant Siri, is that you can have an option to raise the phone to your ear in order to speak commands and ask Siri questions. It turns out it uses an additional infrared sensor.
iFixit discovered that the Apple iPhone 4S uses an additional infrared sensor housed at the top of the phone, near the proximity sensor. This IR-sensor emits a beam invisible to the human eye, in order to determine whether it is close to your ear or not. The beam is fired whenever the screen is on and, even though it uses little battery, if you don’t really use the option, turn it off to save some millivolts.