By design, you can use an optocoupler to transmit a signal between two completely separate circuits. When two circuits do not have a common electrical connection, then one speaks of electrically isolated circuits.
To use light to transmit signals has several advantages. Forstly, light is fast and can be transmitted over long distances (fiber optic links are basically optocoupler). Also light is not affected by magnetic fields.
In this first part you get to know the optocoupler. In simple circuits, you discover the properties and recognize how and where optocouplers are used.
The second part (Go on...) describes applications and various types of optocouplers.
Let's go...and have fun...