News

What does polarization of light mean?

To understand the polarization of light, we need to have a proper image of electromagnetic waves.

A stationary charge has an electric field around it, which can be imagined as lines of force radiating from the charge in straight lines in all directions. If the charge is suddenly moved a short distance, and the lines of force are still connected to the charge at this time, the displacement will propagate along the lines of force at the speed of light. In this way, the vibrating charge produces waves that propagate along the lines of force, much like the ripples on a taut rope. The above image of electromagnetic waves is very illustrative of the fact that the electric vector component of such a wave is in a plane determined by the propagation direction and the direction of vibration of the charge. Light with all electric vector components in the same vibration plane is called linearly polarized light. The angle between the vibration plane and a certain direction (such as the vertical direction) is called the polarization angle. Most light sources emit unpolarized light, and the electric vector component of this light wave has no definite direction. Linearly polarized light can be produced by passing unpolarized light through a polarizing filter, such as a suitable crystal or crystal array (polarizer) that can select the electric vector component of the light wave in a plane.

Linear polarized light has a particularly interesting property, namely, the intensity of light reflected from the surface of a mirror medium such as glass depends on the angle between the plane of vibration of the light and the plane of incidence, which is the plane determined by the incident light and the surface normal. When the plane of vibration of the light coincides with the plane of incidence, and the angle of incidence of the light is just such that the refracted light and the reflected light are at right angles, the intensity of the reflected light is equal to zero, that is, there is no reflection. This incident angle is called the Brewster angle. The value of the Brewster angle can be obtained from the law of reflection and refraction. It is equal to tan-1(ng/n1). When light enters glass from air (n=1.5), the Brewster angle is 56°19'. When non-polarized light is incident at the Brewster angle, the reflected light is linearly polarized in a plane at right angles to the plane of incidence. When calculating the intensity of light reflected from a mirror twice or more times, polarization must be taken into account, so the calculation process is quite complicated.

The polarization effect during mirror reflection can be used to reduce glare and improve the contrast of the illuminated scene. If the incident light is polarized in the vertical plane, the specular reflections from horizontal surfaces such as polished tabletops and glossy paper will be reduced. This effect can also be used in the design of lamps. One design of lamps is to place some parallel reflecting planes under the light source so that the light transmitted at (or close to) the Brewster angle lacks the component polarized in the horizontal plane.

In stressed solids and some crystals, it has been found that the refractive index is related to the polarization angle of the light relative to the stress direction or an axis in the crystal. This has been applied to crystal analysis and measuring stress in transparent materials.

PREVIOUS：Absorption and scattering of light: the inter NEXT：Diffuse reflection and transmission, the cosi