March 24, 1995
One Minute Papers - Questions and Answers
How do polarizing materials work?
The sheet polarizers that are used in sunglasses or in the demonstrations in class contain molecules that absorb electromagnetic waves of only one polarization. These molecules form long chains that interact with electromagnetic waves only when the electric fields push charge along the lengths of the molecules. In the polarizing sheets, the molecules are all oriented along the same direction so that they all absorb light of the same polarization. The other polarization of light passes through the sheets virtually unscathed. When unpolarized (randomly polarized) light enters one of these sheets, any waves that are polarized along the molecules are absorbed while any that are polarized across the molecules are permitted to pass. About half the light makes it through and that half is polarized across the molecules. If this remaining light is sent through a second polarizing sheet, turned 90 degrees so that the molecules of the second sheet are aligned with the polarization of the light leaving the first sheet, then the remaining light will be absorbed in the second sheet and essentially no light will emerge from the pair of sheets. This arrangement, two polarizers turn 90 degrees with respect to one another, is called "crossed polarizers". It is a useful arrangement for observing materials that rotate polarization by distorting the electric and magnetic fields. If a distorting material is placed between the two crossed polarizers, light from the first polarizer may be altered by the material and thus be able to pass through the second polarizer.
Why do you think you see water on a road ahead of you when it's not really there?
On a sunny day, heat from the pavement can create a layer of very hot air at the surface of the road. Since hot air is less dense than cold air, its index of refraction is slightly less than that of cold air, too. As light from the sky enters this layer of low-index air, that light is bent. Light from the sky far out in front of you is turned upward so that you see the sky "reflected" from the road's surface (actually bent upward by the air above the road's surface). You interpret this sky light as coming from a pool of water on the road. But as you approach the road and look down at it, you see that the road is dry and black.
How do shadows form?
Light from the sun travels in straight lines (apart from some wave effects called diffraction, that are unimportant in this case). As sunlight passes objects, those objects absorb or scatter the sunlight, leaving regions of space that no longer contain any electromagnetic waves. Regions of space behind the objects contain no sunlight and do not appear illuminated. We perceive those dark, unilluminated regions as shadows.
Can you see out of sunglasses which shade both horizontally and vertically polarized light?
No. Such sunglasses would absorb all light and would appear black. Polarizing sunglasses are designed to absorb only horizontally polarized light; the light associated with glare. There is no reason to absorb vertically polarized light.
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.
Does the rainbow go all the way to the ground?
Yes, it forms an arc that extends to the ground. However, any hills or valleys may obscure its visibility or its sunlight, so you often see it truncated or in shadow.
How come I never find the pot of gold at the base of the rainbow?
The people who invented that tale were well aware of the impossibility of reaching the rainbow itself. Knowing that the rainbow moves with you, they were free to promise anything about what lies at the base of the rainbow.
Does a mirage operate under the same principle as the puddles on a road?
Not exactly. A puddle contains water, which reflects light directly. Light from the blue sky travels toward the puddle and illuminates it. As the light enters the water, with its higher refractive index, part of the light reflects. You see this light when you look at the surface of a puddle. But a mirage involves refraction (bending) of light. As light from the blue sky enters a regions of hot air near the surface, that light bends upward. You again see light from the sky, but bent upward by the air rather than being reflected upward by a surface of water. Since the two appear similar, you interpret the shimmering blue light of a mirage as coming from a pool of water. But it is just hot air.
What is Brewster's angle?
When light reflects from a horizontal surface at an angle, the reflected light tends to be polarized horizontally. At a specific angle, called "Brewster's angle", the light is completely horizontally polarized. Any vertically polarized light that hit the surface at that angle is allowed to enter the surface without reflection. Since reflections from horizontal surfaces are mostly horizontally polarized, glare is mostly horizontally polarized. Polarizing sunglasses deliberately block horizontally polarized light to reduce glare.
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.
How do oil spills/spots (i.e. in parking lots and streets) create rainbows?
A thin layer of oil on water creates interference effects, just like those seen in a thin soap film. Sunlight reflects from both the top and the bottom of the oil layer and these two reflections can interfere with one another. If the blue/green wavelengths of light interfere destructively on their way to your eye, you will see the oil layer as red. If the green/red wavelengths of light interfere destructively, you will see the oil layer as blue. How you see the oil layer depends on its thickness and the angles of the light.
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 is not possible to chemically treat the glass to make it absorb only one polarization of light because that treatment would have to carefully oriented to 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.
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.
What causes the colors in the aurora borealis?
These colors come from atomic fluorescence on particles high above the earth's surface. As charged particles from the sun's "solar wind" spiral through the earth's magnetic field toward its poles, they collide with one another and with atoms in the earth's upper atmosphere. The energy released by these collisions excites the atoms, which emit light.
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.
What color is the sun as viewed from outside our atmosphere?
The sun appears bluer when viewed from outside our atmosphere. The earth's atmosphere scatters a substantial fraction of the violet and ultraviolet light in sunlight, leaving a reddened sun disk to our view. Without that scattering, the sun's disk will appear to contain more blue and ultraviolet light.
Why do regular light bulbs have different effects on plants than fluorescent lights?
Regular (incandescent) light bulbs create light with a hot filament. This light is relatively reddish and contains very little blue, violet, or ultraviolet light. Since it comes from a hot, thermal source, this light covers all the wavelengths from infrared to the green and blue range of the spectrum continuously and smoothly, although its intensity peaks in the red and orange range of the spectrum. Fluorescent lights, on the other hand, create light through the fluorescence of atoms, molecules, and solids. The light is not created by hot materials so it contains certain regions of the spectrum, often including blue and violet light. Depending on the exact make-up of the fluorescent lamp, this light may include wavelengths that are particularly important to a plant's metabolic processes.
Why do different sunglasses appear darker than others?
Polarizing sunglasses block half the light (stopping horizontally polarized light and passing only vertically polarized light). But sunglasses of all types contain chemicals that absorb light of both polarizations. The darkness of the sunglasses depends on which chemicals are used and how much of those chemicals they contain. Some sunglasses are also coated with thin metallic layers that reflect a fraction of the light that strikes them. These semi-transparent mirrors can change the transmission of the sunglasses dramatically so that those sunglasses may transmit 50% of the light or 0.01% of the light. The manufacturer can choose.
Why, if white doesn't absorb heat, do I get very hot when I wear a white shirt?
A white shirt doesn't absorb visible light (or at least very much visible light), but it may absorb lots of infrared light. Since much of the sun's light and heat are in the form of invisible infrared light, that infrared absorption can be very important. There are many materials that appear white to your eye that do absorb strongly in the infrared and thus get very hot in sunlight.
Why do fine mists of water create rainbows?
Fine mists of water are basically spherical water droplets in air and these can produce rainbows in exactly the same manner as raindrops do in natural rainbows.
Is refraction the idea behind eye glasses? If so, how?
Yes, refraction is used in eye glasses. By carefully sculpting the front and back surfaces of a sheet of glass or plastic, the light passing through that sheet can be bent in remarkable ways. We will look at image formation in the section on Cameras.
What causes a magnifying glass within a ray of sun to burn such a small, specific spot? Is it the shape of the glass?
The magnifying glass is a lens, a carefully shaped piece of glass that can refract sunlight to create an image. When you burn wood with a magnifying glass, you are creating an image of the sun on the wood. This tiny image, a circle that looks just like the sun itself, only much smaller, is so bright and contains so much thermal radiation, that it overheats the wood that it strikes and causes that wood to burn.