# Course:Optical properties

The optical properties of gemstones are important to understand as it is the core of many instruments which enable us to distinguish between gemstones.
Throughout this small course you will see references to things as refraction, reflection, total internal reflection and polarization (to name a few). All of these are optical properties a gem might possess. Some of these you might already understand, but we will start from the bottom up just in case.
Make sure you understand a topic in full before moving on, otherwise you would not be able to follow the text and you will find yourself lost very soon.

Topics that will follow:

• Reflection
• Refraction
• Polarization
• Polariscope
• Dispersion
• Diffraction
• Spectroscope
• Birefringence (double refraction)
• Dichroscope
• Total Internal Reflection
• Refractometer

Take your time when studying. A week per topic is a minimum.

## Some prerequisites

Before we move on, we must introduce a few basic principles.
Although this is far away from being correct, it should provide you a basic understanding of the concepts.

### The normal

 A "normal" is an imaginary line that we draw (either physically in a diagram, or in our mind) that is perpendicular to a plane or a surface. That means, it is at a 90° angle to that plane (or surface). In the animation on the left, you will see a cross-section of a brilliant. When you start the animation, a black line will appear that is at 90° to the table (the table is the horizontal plane). After a few seconds another line will appear, but this time on one of the slanted planes. That line is perpendicular to that slanted plane. Note that this second line is not running parallel to the slanted lower line on the left, but at 90° to the plane it is on. Both of these lines are named a "normal" and we use those normals to calculate angles etc. which you will learn lateron. For now it is only important that you know what a normal is. You can pause the animation at any time to have a closer look.

### Incidence

"Incidence" means the arrival of something at a surface. In our case that "something" is a ray of light.

 When light travels from its source (like the sun or a lightbulb) to an object, we call that light "incident light". At some point it will reach the surface of that object at a certain place and we call that point of impact, the "point of incidence". It is at that point of incidence where we draw the normal. The angle the incidence light makes with the normal is termed "the angle of incidence", which will be discussed in later chapters. We usually follow only one ray of light when discussing this kind of theory, in reality there are many of these rays. In the animation on the left, the red line represents the incident light and it strikes the horizontal plane at the point of incidence. Then the normal is drawn in. You can pause the animation at any time to have a closer look.

### Optical density

Optical density refers to a closer (denser) packing of atoms in a medium compared to another medium.

 Maybe the above statement about optical density sounds intimidating, but it really is not that hard to understand. Run the animation on the left for an illustration of the explanation that follows. Imagine you are riding on your bicycle on a paved road. There you are paddling away at a constant rate. All of a sudden there is a bush of high grass in the middle of the road. If you keep paddling at the same rate (you apply the same amount of energy), you can image that you will travel slower through the grass as you were on the road. In other words, you feel a resistance due to the closer packing (higher density) of the grass compared to the usual packing (density) of air. When you get out of the grass, you will pick up speed again. The same thing happens when light travels from air into an object with closer packed atoms, like glass or a gemstone. The light will slow down. Although resistance is not a scientifical correct way to explain this, you can regard optical density as a resistance for now.