Is it normal for my head lights to dim slightly and then come back to normal while my car is stopped at a light?
This is a normal characteristic and is caused by variations in the electrical system created by the onboard computer. The computer controls the output of the electrical charging system to improve fuel economy and minimize vehicle vibration.
While the engine is at idle, the computer monitors the electrical usage of the car and then controls the electrical charging system so that it charges just enough to keep up with demand. This change reduces the load on the engine and, in turn, increases fuel economy and reduces vehicle vibration. As electrical accessories turn on and off, the computer changes the rate of electrical charge in response to the changes in demand for electricity. As the computer responds to the addition of electrical load, you may notice a momentary dimming of the lights. For example, the headlights or instrument panel lights may briefly dim as the air conditioning cycles on.