How do polarisers work

By using a polarizer, you can help bring out the colors and textures of foliage. A circular polarizer reduces or removes the reflections to see what is on the other side. Something that cannot be done later in any editing software. Taking a photo of a house or car? Most paints are shinny and reflect more light than needed, which in turn lightens or washes out their color.

A Circular Polarizer reduces or removes the reflections for better color. It will not be completely polarized vertically, because only a small fraction of the incident light is reflected, and so a significant amount of horizontally polarized light is refracted.

Polarization by scattering. Unpolarized light scattering from air molecules shakes their electrons perpendicular to the direction of the original ray. The scattered light therefore has a polarization perpendicular to the original direction and none parallel to the original direction. If you hold your Polaroid sunglasses in front of you and rotate them while looking at blue sky, you will see the sky get bright and dim. This is a clear indication that light scattered by air is partially polarized.

Figure 11 helps illustrate how this happens. Since light is a transverse EM wave, it vibrates the electrons of air molecules perpendicular to the direction it is traveling. The electrons then radiate like small antennae. Since they are oscillating perpendicular to the direction of the light ray, they produce EM radiation that is polarized perpendicular to the direction of the ray. When viewing the light along a line perpendicular to the original ray, as in Figure 11, there can be no polarization in the scattered light parallel to the original ray, because that would require the original ray to be a longitudinal wave.

Along other directions, a component of the other polarization can be projected along the line of sight, and the scattered light will only be partially polarized. Furthermore, multiple scattering can bring light to your eyes from other directions and can contain different polarizations.

Photographs of the sky can be darkened by polarizing filters, a trick used by many photographers to make clouds brighter by contrast. Scattering from other particles, such as smoke or dust, can also polarize light. Detecting polarization in scattered EM waves can be a useful analytical tool in determining the scattering source. There is a range of optical effects used in sunglasses. Besides being Polaroid, other sunglasses have colored pigments embedded in them, while others use non-reflective or even reflective coatings.

A recent development is photochromic lenses, which darken in the sunlight and become clear indoors. Photochromic lenses are embedded with organic microcrystalline molecules that change their properties when exposed to UV in sunlight, but become clear in artificial lighting with no UV. Find Polaroid sunglasses and rotate one while holding the other still and look at different surfaces and objects. Explain your observations. What is the difference in angle from when you see a maximum intensity to when you see a minimum intensity?

Find a reflective glass surface and do the same. At what angle does the glass need to be oriented to give minimum glare? 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. Furthermore, this property can be turned off by the application of a voltage, as illustrated in Figure 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. The light travels to the front screen through millions of tiny units called pixels picture elements.

One of these is shown in Figure 12 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. 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 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. Optical activity is the ability of some substances to rotate the plane of polarization of light passing through them.

The rotation is detected with a polarizing filter or analyzer. 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 It is apparent that the effect depends on wavelength as well as stress.

The wavelength dependence is sometimes also used for artistic purposes. Optical stress analysis of a plastic lens placed between crossed polarizers. Another interesting phenomenon associated with polarized light is the ability of some crystals to split an unpolarized beam of light into two.

Such crystals are said to be birefringent see Figure The Polarizing filter allows light only to pass through it on a single axis parallel to the chain molecule pattern.

Imagine putting a quarter into a parking meter, the single plane slot only allows the quarter to pass through when turned to the allotted angle. Since typical light vibrates at all angles, only the light traveling at an angle within the threshold on the single plane of the Polarizer makes it through. The Quarter Wave Plate turns the single plane-polarized light, back into circularly polarized light which allows modern cameras to accurately focus and meter the light.

Then the resulting light enters your lens and contacts your camera sensor recording the image. Do all the transparent materials react the same way? What could polarized lenses be used for? Details Activity Length 20 mins. Polarizing sunglasses work the same way. Objectives Explain how polarizing filters work and their use in our daily lives. Part 2: Explaining how a polarizer works Hold up the cooling rack. This represents a magnified version of a polarizing filter.

Hold the rack so that the bars are horizontal. Only light that is wiggling horizontally will get through. Repeat with the bars oriented vertically. Use two cooling racks to show that if the polarizers are in opposite directions, no light will get through the pair.

Part 3: Exploration Hand out 2 pieces of polarizing filter to each student or small group. Sandwich the filters together, then hold them up and look through them. Rotate one of the filters against the other while looking through them.