Essential oils are concentrated liquids derived from plants through various extraction methods, such as steam distillation or cold pressing. They contain the aromatic compounds and natural essences that give plants their distinct fragrances and flavors.Exposure to light can cause the oils to degrade, lose their aroma, and undergo chemical changes that can alter their therapeutic properties. So Essential oil will normally packed in dark or amber bottles with dropper caps or with inside stoppers to help block out light and protect the oils from photodegradation. In general, essential oils tend to have low to moderate viscosities compared to other liquid substances.