The classic joystick has a light behind the Atari Fuji logo button in the top right, and a ring of LED lights around the joystick. The behavior of the lights has specific meaning.
The Atari Fuji logo button in the top right of the controller:
The ring of LED lights around the joystick will glow when the controller is powered on. When first powered on, these lights will also indicate the battery charge level.
0% to 25% of LED ring = 0 to 25% battery life
26% to 50% of LED ring = 26 to 50% battery life
51% to 75% of LED ring = 51 to 75% battery life
76% to 100% of LED ring = 76 to 100% battery life
During gameplay, the LED lights will glow brighter as the joystick is pushed or rotated to indicate direction. Game developers are able to add their own effects to activate the LED ring during gameplay.