The color of an object depends on the physics of the object in its environment, the physics of light in its environment, and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the color of the light leaving their surfaces if it travels through the vacuum of space at speed c and does not pass through a physical medium such as a prism. The perceived color normally depends on the spectrum of the incident illumination, the wave velocity, the reflectance properties of the surface, and potentially on the angles of illumination and viewing. Some objects not only reflect light, but also transmit light or emit light themselves, which also contributes to the color. A viewer's perception of the object's color depends not only on the spectrum of the light leaving its surface, but also on a host of contextual cues, so that color differences between objects can be discerned mostly independent of the lighting spectrum, viewing angle, etc. This effect is known as color constancy.