Goodday Nairalanders,you know those small dots thats closed to your smartphones,yeah do you know what they are .they are called proximity sensor and its very useful in smartphones these days ,

Large Touch screen displays consume a lot of power. Hence, it is wise to switch them off when not in use like when talking to someone/ when the phone is in the pocket.
In both the cases, your phone is held near some object (like your ear or the wall of your pocket). In such cases, the proximity will generate a message to your phones processor telling it to switch off your screen.
Android and IOS have an API for proximity sensor. So, you can also use it in your custom program. May be say a flash light that switches of if you put your phone in the pocket.

As far as the hardware goes, the proximity sensor consists of a simple IR transmitter-receiver pair. The transmitter is an LED and emits infrared light. If you place an object close to the transmitter the light will bounce back.This light is picked up by the receiver.

Older smartphones uses IR sensors,(infarad ray).thats why you usually see red light in those dots,those are IR sensors ,modern phones are now using capacitive sensors,
Effects of a bad proximity sensor
The phone wont be able to respond to stimuli like when you place the phone in your ear its screen wont shut off.

NOTE;proximity sensor can be disable in mobe phones
If you want to know how nust ask in the comments bellow .