How do window tints (for your car windows) work? Are they just polarized materia…

How do window tints (for your car windows) work? Are they just polarized materials?

Some of them may be polarized materials, blocking horizontally polarized light, but most are simply absorbing materials that are embedded directly in the glass during its manufacture. Chemically tinted glass just darkens the sky be absorbing some of the light passing through the glass, regardless of polarization. It’s not possible to chemically treat the glass to make it absorb only one polarization of light because that treatment would have to carefully align its molecules. In the plastic polarizing sheets, there is an alignment process (usually stretching in one direction) that lines up all the absorbing molecules.

What makes the clouds white – or having colors at sunset and why is the sky gray…

What makes the clouds white – or having colors at sunset and why is the sky gray on a cloudy day?

The water droplets in clouds are quite large; large enough to be good antennas for all colors of light. As light passes by those droplets, some of it scatters (is absorbed by the antenna/water droplets and is reemitted by the antenna/water droplets). Since there is no color preference in this scattering from large droplets, the scattered light has the same color as the light that illuminated the cloud. In the daytime, the sunlight is white so the clouds appear white. But at sunrise or sunset, the sun’s light is mostly red (the blue light has been scattered away by the atmosphere before it reached the clouds) so the clouds appear red, too. If the clouds are very thick, they may absorb enough light (or scatter enough upward into space) to appear gray rather than white. Another way to see why the clouds are white is to realized that light reflects from every surface of the water droplets. As the light works its way through the random maze of droplets, it reflects here and there and eventually finds itself traveling in millions of random directions. When you look at a cloud, you see light coming toward you from countless droplets, traveling in countless different directions. You interpret this type of light, having the sun’s spectrum of wavelengths but coming uniformly from a broad swath of space, as being white. These two views of how light travels in a cloud (absorption and reemission from droplets or reflections from droplet surfaces) turn out to be exactly equivalent to one another. They are not different physical phenomena, but rather two different ways to describe the same physical phenomena.

As long as the sun is to our back, shouldn’t the rainbow stay visible; instead o…

As long as the sun is to our back, shouldn’t the rainbow stay visible; instead of disappearing when we approach it?

If the sky were uniformly filled with water droplets and uniformly illuminated with sunlight, then you would always see the rainbow, no matter where you moved. However it would always appear out in the distance. The light that reaches your eyes as the rainbow comes from a broad range of distances, but it appears to come from pretty far away. As you walked toward this perceived rainbow, you would begin to see light from other raindrops, still farther away. You could never actually “reach” the rainbow. It would just move about with you; always appearing to be in the distance.

Why does purple bend more in a prism than, say, red?

Why does purple bend more in a prism than, say, red?

Purple (or violet) light travels slower in most materials than does red light. That occurs because violet light is higher in frequency than red light and gives the charged particles that it jiggles about less time to move up and down. With very little time to move, these charged particles barely notice that they are parts of atoms and molecules and respond easily to the passing electromagnetic wave. But when red light pushes and pulls on charged particles, there is more time for them to find the limits of their freedom. These charged particles are not able to move so easily when pushed on by a passing wave of red light so they do not interact with that passing wave as well as with one of violet light. Thus red light passes by with less effect and it behaves more like it would in empty space. Violet light, which interacts relatively strongly with the atoms it passes, slows down more than red light. Since red light travels more quickly than violet light, it bends less in passing through a prism. Violet light slows down more and bends more than red light.

How does light cancel in destructive interference?

How does light cancel in destructive interference?

When two identical waves (usually two halves of the same wave) arrive together out of phase, the electric field in one wave (or half-wave) is up at the same moment that the electric field of the other wave (or half-wave) is down. These two electric fields add together and create a total electric field that is neither up nor down. An electric charge at this location in space will experience no forces so there is no electric field (one wave pushes that charge up while the other wave pushes that charge down). With no electric field around, there is no light to be absorbed. If two waves coming toward you interfere destructively, you will see no light. You might worry about conservation of energy; where did the light and its energy go? It went somewhere else. Any time there is destructive interference at one point in space, there will always be some other point in space at which there is constructive interference. Thus when you look at a soap film and see no red light, you can be sure that the red light has gone somewhere else. In the case of the soap film, when you see no red light in the reflection from the film, that red light has been transmitted by the film and is visible on the opposite side of the film.