Where is blue sky

This behavior is used in the proposed experiment. Some materials are intrinsically isotropic, but others can change their structure under a load condition, becoming anisotropic and then displaying birefringence. These materials, under illumination of white light and observed through a polarizer, present colored patterns, which are related to the stress distribution inside the material. The experiment is described in this section. First, the materials with birefringence are presented.

Following, the experimental procedure. As a polarizer for the experiment, it can be used a polarizer filter of photographic cameras or a polarized sunglass lens, or any other polarizer of the selective absorption type. The structure of is kind of material can be sensible to stress.

To verify this, polarized white light and a polarizer should be used. The following procedure was performed: these materials were illuminated with the polarized white light e.

Then, the transmitted light was registered with a digital camera. Figure 7 a shows an image of a transparent protractor without the polarizer.

It is possible to see the white light from the monitor and the scale. In Figure 7 b , the same passing through a polarizer. A colored pattern appears. This pattern is related to the distribution of stress inside the protractor, modifying the direction of polarization of the light from the monitor.

The pattern is not present in the entire extension, and its shape is not uniform, but it appears that the right side is symmetric to the corresponding left side. This piece was used to do the majority of the experiments because it is easy to focus on the scale. This technique does not depend on whether the direction of the polarizer is orthogonal to the direction of light polarization.

In Figure 8 the colored pattern appears with the polarizer direction parallel to light polarization. In this case, the transmitted light is rotated, part of it is absorbed by the polarizer and a colorful pattern also appears. It is interesting to note that the pattern at the left side looks complementary to the pattern to the right side, and the colored fringes appear in places of one half size where does not appear in another half.

This results in a different pattern of the one in Figure 7 b. Figure 9 a shows an image of a transparent plate, where it is possible to see the white light from the monitor.

If a polarizer is put between the material and the digital camera Fig. Its shape is different from the one in Figure 7 b. At the top of the piece, there is a small defect where are the most colored fringes. No symmetry is presented in this pattern. The experiment consists of illuminating a birefringent material with the blue skylight, and adding a polarizer between the piece and the observer's eye.

The experiment were done in several days, at different hours and in two different places. The experiment also experienced weather changes several times during the period. The analysis was done using only the half top of the piece because it was handled in the other half.

The majority of the images were captured in open air. Figure 10 presents five images of the protractor Fig. On one side of the piece, the colored pattern appears more prominently. Figure 10 a shows the image at the north direction, the colored pattern does not appear because the low polarization level since the Sun is near the visual field of the camera. However the colored pattern appears in Figures 10 b , 10 c , 10 d and 10 e , with no significant difference between them. In the south direction and in the zenith the pattern presented the best contrast.

The shape of the colored pattern is similar to the one present in Figure 8. From the background of each picture is possible to see the sky on a sunny day.

Another important thing to know about light is that it travels in a straight line unless something gets in the way to.

The blue and violet waves, however, are just the right size to hit and bounce off of the molecules of gas in the atmosphere. This causes the blue and violet waves to be separated from the rest of the light and become scattered in every direction for all to see.

The other wavelengths stick together as a group, and therefore remain white. They are still mixed together, unscattered by the atmosphere, so they still appear white. The scattered violet and blue light dominates the sky, making it appear blue. What happens to the violet? Some of the violet light is absorbed by the upper atmosphere. Also, our eyes are not as sensitive to violet as they are to blue.

All this scattering mixes the colors together again so we see more white and less blue. As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

Sometimes the whole western sky seems to glow. The sky appears red because small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to go through the atmosphere.

For example, Mars has a very thin atmosphere made mostly of carbon dioxide and filled with fine dust particles.

During the daytime, the Martian sky takes on an orange or reddish color. But as the Sun sets, the sky around the Sun begins to take on a blue-gray tone. The top image shows the orange-colored Martian sky during the daytime and the bottom image shows the blue-tinted sky at sunset.

Notice that when the person looking down from the top sees a bright beam, the person looking in from the side will see a dim beam, and vice versa. You can also hold the polarizing filter between your eyes and the tank and rotate the filter to make the beam look bright or dim.

The filter and the scattering polarize the light. When the two polarizations are aligned, the beam will be bright; when they are at right angles, the beam will be dim.

Scattering polarizes light because light is a transverse wave. The direction of the transverse oscillation of the electric field is called the direction of polarization of light. The beam of light contains photons of light that are polarized in all directions—horizontally, vertically, and all angles in between.

Consider only the vertically polarized light passing through the tank. This light can scatter to the side and remain vertically polarized, but it cannot scatter upward! To retain the characteristic of a transverse wave after scattering, only the vertically polarized light can be scattered sideways, and only the horizontally polarized light can be scattered upward.

This is shown in the diagram below click to enlarge. Discover why your favorite cat videos don't leak out of optical fibers. Attribution: Exploratorium Teacher Institute.