Fig. 2. The wavelength of light can be measured from peak to peak or from any other position as desired (upper illustration). Light is constantly vibrating in a plane normal (90°) to the direction of propagation (middle figure). If a cross section of a ray of light is made, unpolarized light is found vibrating in all planes normal to the direction of propagation (A). Partially polarized light has a vibration in one plane that is greater than that in another (B). Plane polarized light is vibrating in only two directions normal to the direction of propagation (C).