Politicalpolarization

Many crystals and solutions rotate the plane of polarization of light passing through them. Such substances are said to be optically active. Examples include sugar water, insulin, and collagen (see Figure 2.3.20). In addition to depending on the type of substance, the amount and direction of rotation depends on a number of factors. Among these is the concentration of the substance, the distance the light travels through it, and the wavelength of light. Optical activity is due to the asymmetric shape of molecules in the substance, such as being helical. Measurements of the rotation of polarized light passing through substances can thus be used to measure concentrations, a standard technique for sugars. It can also give information on the shapes of molecules, such as proteins, and factors that affect their shapes, such as temperature and pH.

The Sun and many other light sources produce waves in which E (and B, though it is not shown) are not preferentially oriented – they exist in every direction perpendicular to the direction of propagation (see Figure 2.3.11). Such light is said to be unpolarized because it is composed of many waves with all possible directions of polarization.

In contrast, light that is plane polarized (also called linearly polarized) has E oriented in one specific direction in space (Figure 2.3.12).  The polarization direction is defined by the orientation of E (as opposed to B).

Figure 2.3.16 illustrates how the component of the electric field parallel to the long molecules is absorbed. An electromagnetic wave is composed of oscillating electric and magnetic fields. The electric field is strong compared with the magnetic field and is more effective in exerting force on charges in the molecules. The most affected charged particles are the electrons in the molecules, since electron masses are small. If the electron is forced to oscillate, it can absorb energy from the EM wave. This reduces the fields in the wave and, hence, reduces its intensity. In long molecules, electrons can more easily oscillate parallel to the molecule than in the perpendicular direction. The electrons are bound to the molecule and are more restricted in their movement perpendicular to the molecule. Thus, the electrons can absorb EM waves that have a component of their electric field parallel to the molecule. The electrons are much less responsive to electric fields perpendicular to the molecule and will allow those fields to pass. Thus the axis of the polarizing filter is perpendicular to the length of the molecule.

Polarizing filters have a polarization axis that acts as a slit. This slit passes electromagnetic waves (often visible light) that have an electric field parallel to the axis. This is accomplished with long molecules aligned perpendicular to the axis as shown in Figure 2.3.15.

Polaroid sunglasses are familiar to most of us. They have a special ability to cut the glare of light reflected from water or glass. Polaroids have this ability because of a wave characteristic of light called polarization. What is polarization? How is it produced? What are some of its uses? The answers to these questions are related to the wave character of light.

Circularly polarizedlight

In flat screen LCD televisions, there is a large light at the back of the TV. The light travels to the front screen through millions of tiny units called pixels (picture elements). One of these is shown in Figure 2.3.19 (a) and (b). Each unit has three cells, with red, blue, or green filters, each controlled independently. When the voltage across a liquid crystal is switched off, the liquid crystal passes the light through the particular filter. One can vary the picture contrast by varying the strength of the voltage applied to the liquid crystal.

polarization极化

The colour of the objects that we see is largely due to the interaction of the object with the incident light. The colour of the object is not within the object itself. Rather, it is in the light that shines upon it. When the light of a certain frequency falls on the object, the light frequencies that are absorbed never make it to our eyes. Only the frequencies of incident light that are reflected or transmitted contributes to the colour of the object. So if an object absorbs all the frequencies of the light wave except the frequency associated with green, then the object appears green.

Photonpolarization

2. One common historical belief was that coloured objects in nature produce small particles (perhaps light particles) that subsequently reach our eyes. Different objects produce different coloured particles, thus contributing to their different appearance. Is this belief accurate or not? Justify your answer.

Polarizers are composed of long molecules aligned in one direction. Thinking of the molecules as many slits, analogous to those for the oscillating ropes, we can understand why only light with a specific polarization can get through. The axis of a polarizing filter is the direction along which the filter passes the electric field of an EM wave (see Figure 2.3.13).

Another interesting phenomenon associated with polarized light is the ability of some minerals and other crystals to split an unpolarized beam of light into two polarized beams (Figure 2.3.22). Such crystals are said to be birefringent.

Electricpolarization

Contrary to the absorption of light, transmission and reflection of light occur when the natural frequency of the vibration of electrons do not match the frequencies of the incident light. In such cases, when the light wave strikes the objects, the electrons of the object begin to vibrate. The electrons vibrate for a brief period of time with small amplitude after which the energy is re-emitted as a light wave. If the object under consideration is transparent, the vibrations of the electrons are passed on to the neighbouring atoms and are re-emitted from the opposite side of the object. In such cases, the light is said to be transmitted. If the object is opaque, the vibrations of the electrons are not passed in bulk, rather the electrons of the atoms vibrate for a short period of time and are re-emitted as a reflected light wave from the surface of the object.

Only the component of the EM wave parallel to the axis of a filter is passed. Let us call the angle between the direction of polarization and the axis of a filter θ. If the electric field has an amplitude E, then the transmitted part of the wave has an amplitude E cos θ (see Figure 2.3.18). Since the intensity of a wave is proportional to its amplitude squared, the intensity I of the transmitted wave is related to the incident wave by I = I0 cos2 θ, where I0 is the intensity of the polarized wave before passing through the filter.

Linearpolarization

This view presumes that the appearance of an object is independent of the colours of light which illuminate the object. It must be noted that the same objects appear to be of different colour under a different light. Hence, the colour of an object is strictly not due to the object’s ability to produce a colour. The only role the object plays in determining its appearance is by absorbing certain wavelengths of the incident light.

When the room lights are turned off, any object present in the room appears black. The colour of the object depends on the frequency of the light that is reflected in the observer’s eye. Without light, there is no reflected light as a result of which the green shirt appears black.

Watch the first 6 minutes of the video below to see a practical overview of plane polarized light, using crossed polarizers, and how a third polarizer (which is how many minerals act) can be used to increase light output from crossed polarizers.

Figure 2.3.17 shows the effect of two polarizing filters on originally unpolarized light. The first filter polarizes the light along its axis. When the axes of the first and second filters are aligned (parallel), then all of the polarized light passed by the first filter is also passed by the second. If the second polarizing filter is rotated, only the component of the light parallel to the second filter’s axis is passed. When the axes are perpendicular, no light is passed by the second.

While you are undoubtedly aware of liquid crystal displays (LCDs) found in watches, calculators, computer screens, cellphones, flat screen televisions, and other myriad places, you may not be aware that they are based on polarization. Liquid crystals are so named because their molecules can be aligned even though they are in a liquid. Liquid crystals have the property that they can rotate the polarization of light passing through them by 90 degrees. Furthermore, this property can be turned off by the application of a voltage, as illustrated in Figure 2.3.19. It is possible to manipulate this characteristic quickly and in small well-defined regions to create the contrast patterns we see in so many LCD devices.

Glass and plastic become optically active when stressed; the greater the stress, the greater the effect. Optical stress analysis on complicated shapes can be performed by making plastic models of them and observing them through crossed filters, as seen in Figure 2.3.21. It is apparent that the effect depends on wavelength as well as stress. The wavelength dependence is sometimes also used for artistic purposes.

Polarization of light

Each of the separated rays has a specific polarization. One behaves normally and is called the ordinary ray (o or ω), whereas the other does not obey Snell’s law and is called the extraordinary ray (e or ε). Birefringent crystals can be used to produce polarized beams from unpolarized light. Some birefringent materials preferentially absorb one of the polarizations. These materials are called dichroic and can produce polarization by this preferential absorption. This is fundamentally how polarizing filters and other polarizers work. We will use the property of birefringence to help us identify and distinguish minerals in thin section!

Electromagnetic waves are transverse waves consisting of varying electric and magnetic fields that oscillate perpendicular to the direction of propagation and perpendicular to each other.

polarization中文

Light is the only electromagnetic radiation that is visible to the human eye. It is made of little packets of energy known as photons. When a light wave is an incident on an object, a number of things can happen. It is either absorbed, transmitted or reflected. The manner in which visible light interacts with an object depends on the frequency of light and the nature of the atoms of the object. In this article, we will discuss how and why the light of certain frequencies are selectively absorbed, reflected and transmitted.

To examine this further, consider the transverse waves in the ropes shown in Figure 2.3.13. The oscillations in one rope are in a vertical plane and are said to be vertically polarized. Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves. For EM waves, the direction of the electric field vector E is analogous to the disturbances on the ropes (Figure 2.3.14).

Analytical Methods in Geosciences Copyright © by Elizabeth Johnson and Juhong Christie Liu is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

At specific frequencies, the electrons of the atom tend to vibrate. When a light wave of the same natural frequency is incident on the atom, the electrons of that atom will be set into vibrational motion. During vibration, electrons interact with neighbouring atoms in such a manner it converts the vibrational energy into thermal energy. Hence, we can conclude that the selective absorption of light occurs when the frequency of the light matches the frequency at which the electrons in the atoms vibrate. Different atoms and molecules possess different natural frequencies of vibration, hence they will selectively absorb different frequencies of visible light.