How much light will pass through a material or how much it will be refracted.
Different materials will allow light to more easily pass through them. Transparent liquids also refract light differently depending on the chemical composition.
Recording an essential oil’s refractive index can help fingerprint the properties of that specific oil.
Refractive Index: Cinnamon Bark 5.005 Refractive Index: Celery Seed 4.80