A key concept in optics is the existence of an interface. An interface exists whenever there is a change in the index of refraction. For example, a glass sculpture sitting in a room, in other words, in air; two optical materials with different indexes. At this interface some of the light is reflected and some is transmitted into the glass. Light that is transmitted into the glass can be absorbed or scattered depending on the wavelength of the light.
Light's interacts with an optical material can be described purely through the materials index. Two different chemical compounds with the same index would behave identically toward light. The index of refraction is in general a complex number, in the mathematical sense, that it has a real and imaginary part. The reflection and transmission of light from an interface can be described by the real part of the index. The absorption of light by a material is described by the imaginary part.
A certain percentage of light is always reflected off an interface because of this change in the real part of the index. To quantitatively describe how much light, the angle of incidence relative to the normal direction needs to be known. The normal direction is the direction perpendicular to the surface at the point where the light is incident. For instance, for light falling on a glass window, at 90 degrees to the plane of the glass window, the normal direction, the equation is ((index of glass - index of air) / (index of glass + index of air))2 or numerically, using the index of glass as 1.5; ((1.5 - 1) / (1.5 + 1))2 = .04 or 4 percent. This is the familiar experience of looking through a window at night and seeing yourself as if you were looking into a mirror. You are aware of that 4 percent reflecting back into your eyes because there is even less light that is coming in the window from the outside and your eyes become adapted to the low light conditions.
Snells Law quantitatively describes the angle of bending of light. When light travels through medium, i (for the incident material) with real index ni into another material, t (for the transmitted material) with real index nt the equation is,
ni x sin(thetai) = nt x sin(thetat)
The two angles, thetai and thetat are measured from the normal direction as described above. This bending of light is seen everyday, as when we stick a spoon into a glass of water, the spoon appears to be bend. Another example is when when hot air rises off the surface of a highway in summer and you see a far off object down the road appear to wobble or ripple. Hot air being less dense will have a slightly different index of refraction than cooler air and the streams of rising hot air are moving and rippling.
The following diagram is an example of a ray of light, red line, traveling through air, index = 1, at an 60 degree angle from the normal and bending, green line, as it travels through glass, index = 1.5. The angle of refraction is 35.2 degrees from the normal.