Among the iPhone's many features is the ability to easily control the backlight, or screen brightness. There are many situations where the screen might be too bright– like checking your text messages during a movie–or too dim, like outside on a sunny day. Either way, Apple makes it easy to set your iPhone's screen to a brightness perfect for you and your situation.
Tap the Settings icon on the home page of the iPhone.
Tap the "Brightness" tab. This will open the settings for the brightness on your iPhone to allow you to change the brightness of your screen.
Tap and hold your finger on the slider. Slide it to the left to decrease the brightness of the screen, or slide it to the right to increase the brightness of the screen. You can also turn on the auto-brightness setting so that the screen will adjust itself based on the brightness of the room you're in.