Learn how to use LED flash For Alerts on your iPhone 5s.
LED-Flash for Alerts
If you can’t hear the sounds that announce incoming calls and other alerts, you can have iPhone flash its LED (next to the camera lens on the back of the iPhone). This works only when iPhone is locked or asleep. Available for iPhone 4 or later.
Turn on LED-Flash for Alerts. Go to Settings > General > Accessibility > LED Flash for Alerts.