When light hits an object a part of light is reflected and a part enter the object. The part that is reflect is reflected and with the part that enter the object can happen two things: Or it goes nowhere and is absorbed by the object or it pass through the object, in this case we have a transparent object. When light pass through a transparent it change its direction bending in one direction, bending in oposite direction bending nothing depending on the properties of the media, this is called refraction.

Refraction only has meaning for transparent objects. With opaque objects what happens with the light then enter the object can happen two things again: or is absorbed or it bounces several times with the atoms or molecules and goes back to outside as diffuse light.

How much light is reflected and how much light eneter the object is determined by the properties of the media ouside the object and the object itself and also depend on the angle of incidence of the incoming light.

The properties of the media are defined by three parameters: dielectric constant, magnetic permeability and conductivity. For dielectric materials (not conductors) the conductivity is zero so this parameter has no effect. For most non magnetic materials the magnetic permeability is very near to one and so we can ignore this parameter and only remains the dielectric constant and then we have a number called refraction index and so we can use Snell's law for refraction and Fresnel equation to determine how much is reflect and how much is refracted in case of transparent objects or only how much is reflected.

Stupidity also evolves!