Optical medium: A material through which light can pass is called an optical medium. Optical medium is sort of a transmission medium i.e., a medium of propagation.

On the basis of their behaviour towards light, different media can be classified into three categories: 

1. Transparent substances: A substance through which light can be easily transmitted, making the objects to be seen clearly is called transparent substance.
For example, air, water, glass, etc. 

2. Opaque substances: A substance which does not allow light to pass through it is called an opaque substance.
For example, wood, metal, stone, etc. 

3. Translucent substances: A substance through which light passes only partially and objects are not clearly seen is called a translucent substance.
For example, wax paper, frosted glass, clouds etc.