An example of an optical property is the index of refraction. The index of refraction is the change of angle in the light that passes through the material.

How can we observe the Optical properties of a material?

Index of refraction is one property intrinsic to a transparent material.  We can see the difference in index be comparing refraction among different materials. Diamond has a high index while glass has a low index. Both will cause light refraction, but to a different degree. This can be seen using the example of a prism.