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.

When light travelling through one medium, falls on the surface of another medium, the following three effects may occur at the surface of separation of the two media: 

Reflection: A part of the incident light is bounced back into the first medium. This is called reflection of light. 

Refraction: Another part of incident light is transmitted into the second medium and diverges or bends from its path at the surface of separation. This is called refraction of light. 

Absorption: The remaining part of incident light is absorbed at the surface of separation. This is called absorption of light.

(i) When light falls on a highly polished surface like mirror, reflection occurs. 

(ii) Refraction of light occurs when light falls on a transparent medium like glass or water. When there is diffrence in the density of the medium, a ray  tends to bend towards or away from the normal at the interface of media. 

When a beam of rays starting from a point source of light suffers a change in direction due to reflection or refraction, and the reflected or refracted rays actually converge or appear to diverge from another point, then the second point is called the optical image of the first point. 

For example, when we look into a plane mirror, we see our image behind the mirror. When we look at an object through a telescope, the object is seen much closer than its actual position.
This new position of the object, formed due to reflection or refraction of light, is the optical image of the object.