When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich.
Chemistry and physics are branches of science that both study matter. The difference between the two lies in their scope and approach. Chemists and physicists are trained differently, and they have different professional roles, even when working in a team.
Chemistry is also part of physical science. It could be said that chemistry is the bridge between physics and biology. Chemistry is rather like applied physics: when you take physics principles and build them up to the level of molecules and compounds, you can explain why chemistry works.