Crystal photography polarized light

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Crystal photography polarized light: Crystals viewed and photographed through

Cross-Polarized Crystal Photography. Text and photography copyright Thomas Webster Webster Front Page. About Us. Website design and graphics copyrighted Reasonable Expectations Productions Cross Polarization is a technique in which you use a pair of polarizers whose pass axes are set to be perpendicular to each other. For example if one polarizer is set to block horizontally polarized light and the other is set to block vertically polarized light, no light would make it through the two polarizers.

Ordinarily putting your sample between a pair of crossed polarizers would result in a completely dark image. However what we actually get looks like this. Alum crystal viewed at 30x in cross polarized light. Notice how everything around the crystal is dark, the crossed polarizers are indeed blocking light from passing through to the camera.

Crystal photography polarized light: How to photograph crystals

But why do we still see light which has passed through the alum crystal? Not only is this light getting through, we are able to see many more details in the crystal including areas which are not colored in red and blue. The reason we see this light is because the crystal is rotating the polarization of the light passing through it. Even though the light entering the crystal may have been horizontally polarized after passing through the first linear polarizer, it can leave the crystal polarized along some other axis.

Some of this light will now be vertically polarized and able to pass through the second crossed polarizer. How much the polarization of the light is rotated depends on several factors including both the thickness of the crystal and the color of the light. This property of the crystal is called Birefringence, which also has a nice description in Wikipedia.

Not all materials are birefringent, most are not. Crystalline materials however can be birefringent and the property is related to arrangement of atoms in the crystal itself. These axes are perpendicular to each other and result in a totally dark field when viewed through the eyepieces without the insertion of birefringent crystals.

Crystal photography polarized light: Mindblowing Colours using Polarized Light.

Figure 1 a illustrates an anisotropic birefringent crystal that has the long optical axis oriented parallel to the direction of the polarizer. In this case, light passing through the polarizer, and subsequently through the crystal, is vibrating in a plane that is parallel to the direction of the polarizer. There is no contribution from light passing through the analyzer because of the single direction of light vibration -- parallel to the polarizer resulting in the crystal being very dark and almost invisible.

In Figure 1 a the crystal is not totally extinct as it would be between crossed polarizers but passes a small portion of red light. This was done for illustration purposes only to allow visitors to note the position of the crystal. Microscopists classically refer to this orientation as being a position of extinction for the crystal. This observation is important in determining the refractive indices of anisotropic materials with a polarizing microscope.

By removing the analyzer in a crossed polarizing microscope, the single permitted direction of vibration of light passing through the polarizer interacts with only one electrical component in the birefringent material crystal. This allows segregation of a single refractive index for measurement. The remaining refractive index of the material can then be measured by rotation of the polarizer by 90 degrees.

Crystal photography polarized light: Viewing microscopic crystals under crossed-polarized

When two polarizers are layered such that they transmit light of perpendicular orientations, together they will become opaque. This is called cross polarization. The birefringence of crystals double refraction allows the entering light to produce certain colors when polarized.