For simplicity, however, in geometrical optics the passage of light may be represented by rays showing the direction of travel. The amount of light reflected at the boundary of two substances depends on the ratio of their refractive indexes. If a ray arrives at an angle, in a substance with a higher refractive index it is bent toward the perpendicular, and in a substance with a lower refractive index it is bent away from the perpendicular. At an angle of incidence known as the critical angle, the refracted ray makes an angle of 90 with the perpendicular and travels along the boundary between the two substances. If the angle of incidence is increased beyond the critical angle, the ray will be totally reflected back.
Fiber optics is an application of such reflection.
The moving points of a transverse wave describe the vibration of the electric field, or vector , of the light wave. If the electric vectors of a group of transverse waves are at random angles, in all directions, the light is said to be unpolarized. Conversely, if the electric vectors are all along the same plane, the light is said to be polarized. Some materials can introduce phase differences between two sets of polarized waves, to separate them or to analyze their states of polarization. When two light beams cross, they may interfere, interacting in a way that affects the resultant intensity.