The speed of light is a vacuum is defined as "c". The speed of light in a more dense medium is always less than “c”. As light passes through a medium its energy results in a non-resonant vibration prior to emitting the light. Some of the light energy is typically lost from the light rays and absorbed by the denser medium. This is depicted in the following figure.

Transparency

A transparent object is one that allows light to pass through it. Transparent objects can selectively allow specific light waves to pass, while other light waves are absorbed. A transparent object that absorbs ROYBIV will emit only G wave lengths and the emitted light will appear green. A transparent object that absorbs ROYIV will transmit GB and appear greenish-blue or cyan in color. The energy from the absorbed light waves is retained by the transparent object and the energy is converted to heat.