How Everything Works
How Everything Works How Everything Works

Page 7 of 9 (85 Questions and Answers)

870. How can I make 1000 nanometer light waves visible to the human eye? — DMB, Broken Aarow, OK
Although our eyes are insensitive to 1000 nanometer infrared light, there are two ways to detect it effectively. The easiest is to use an inexpensive black-and-white surveillance video camera. Many of these cameras are sensitive to a broader spectrum of light than are our eyes and they can see 1000 nanometer light. If you check around, you should be able to find one that sees the light you're interested in. The other technique is to use a phosphorescent or "glow in the dark" material. When exposed to visible light, the atoms in such a material become trapped in electronic states that can emit visible light only after a very long random wait. But exposing a phosphorescent material to infrared light can shift the states of the atoms in the material to new states that can emit light immediately. Thus exposing some phosphorescent materials to infrared light causes them to emit light promptly. You can then see these materials glow particularly brightly after storing visible light energy in them and then exposing them to infrared light. However, they'll only glow briefly before you have to "recharge" them by exposing them to more visible light.

913. How is sunlight both harmful and beneficial? - CP
Sunlight provides virtually all the energy in our world. Without it, plants wouldn't grow and we wouldn't have food or daylight. We wouldn't even have fossil fuels such as coal and petroleum because those were formed from vegetation that itself derived energy from the sun. However, sunlight also contains ultraviolet light, which can damage chemicals in biological tissue. Long exposure to ultraviolet light can age your skin or cause cancer.

953. Could you please give me a precise explanation of light scattering in relation to blue moons and red sunsets. Do dust particles, or whatever, facilitate the transmission of some wavelengths and not others? — DW
While the expression "blue moon" usually refers to the infrequent occurrence of second full moon in a calendar month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the viewer's eyes. The moon thus appeared blue.

Red sunsets are much more common and they are caused by Rayleigh scattering—the non-resonant scattering of light by particles that are much smaller than the light's wavelength. While Rayleigh scattering is rather weak, it's weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red, particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and the red sunsets are particularly intense.

954. How does a foghorn turn on and off? — M, Brant Rock, MA
Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light into their surroundings and turn the foghorn on when they see excessive reflection of that light.

999. Why are any materials transparent? — MZ, Peligna, Italy
Because light is an electromagnetic wave, it is emitted and absorbed by electric charges. For an electric charge to emit light it must move—in fact, the charge must accelerate. For an electric charge to absorb light it must also move—it must also accelerate. However, there are many materials that do not have mobile electric charges. For example, while all electric insulators have electric charges in them, those electric charges can't move long distances. The electric charges in many electric insulators can't even move enough to absorb light and the light simply passes right through them. They are transparent.

1021. What would you hypothesize the effects of black light bulbs to be on the tanning of human skin?
I would expect that certain black light sources would cause tanning with only modest burning while other black light sources would cause burning with only modest tanning. Black light—also known as ultraviolet light—consists of very energetic light particles. The particles or photons of ultraviolet light contain enough energy to break chemical bonds and rearrange molecules. When you're exposed to such energetic light, it causes damage to molecules in your skin cells and your skin may respond by darkening in the process we call "tanning." But ultraviolet light is a general term that covers a broad range of wavelengths and photon energies. Long wavelength/low energy ultraviolet light tends to cause tanning while short wavelength/high energy ultraviolet light tends to cause burning—it directly kills cells. But these differences aren't sharp and any ultraviolet light will cause some amount of skin damage.

1022. When you hold a flashlight to your hand, some of the light comes through. What light frequencies shine through people? Is it possible to see inside people? — PC
Biological tissues themselves are relatively transparent. They're not good conductors of electricity and electric insulators are typically transparent (quartz, diamond, sapphire, salt, sugar). But we also contain some pigment molecules that are highly absorbing of certain wavelengths of light. For example, the hemoglobin molecules in blood absorb green and blue light quite strongly, so that they appear red. When you look at a flashlight through your hand, the light appears red because of this absorption of green and blue light by hemoglobin. If you use a bright enough red light source and are willing to look very carefully, probably with sophisticated light sensing devices, you can probably see a little light coming through a person's body. But that light will probably have bounced several times during its passage, so that you won't be able to learn anything about what the person's internal organs look like. To get a better view of what a person's insides look like, you need light that penetrates more effectively and that doesn't bounce very often. Moreover, you must employ techniques to that block this bouncing light as much as possible so that you only see light that travels straight through the person. The light that does this isn't visible light—it's X-rays. X-rays are very high frequency, very short wavelength "light" (or rather electromagnetic waves). Tissue doesn't absorb these X-rays much at all and they can go through people to form images.

1023. When you spray water from a garden hose into the air, with the sun behind you, you see a rainbow which appears to stretch right across the sky, in the same way that rainbows form by normal rain appear. In the garden hose case, the water droplets are only a few feet in front of the observer. Is the image of a normal rainbow also only a few feet away or is it formed by droplets within the total volume of the rain shower? If this latter case is true, does the rainbow in fact form a complete circle that is cut off by the horizon? — RP, Solihull, England
A rainbow isn't an image that originates at a specific distance away from your eyes. It consists of rays of colored light that travel at particular angles away from the water droplets that produce them. You see red light coming toward you from a certain angle because at that angle, the water droplets are all sending red light toward you. In the garden hose case, the water droplets are so densely arranged that they are able to create a brilliant rainbow in only a few meters of thickness. In a typical rainstorm, sunlight must travel through hundreds or thousands of meters of raindrops to produce an intense rainbow. When you look up toward the red arc of the normal rainbow, you are seeing light directed toward your eyes by millions of water droplets, some close and others distant, that are all sending a part of the red portion of the sunlight striking them toward you and the other wavelengths of sunlight elsewhere.

You are correct that a normal rainbow is cut off abruptly by the horizon and that it would continue down below to form a full circle if the ground weren't in the way. People in airplanes sometimes see full 360° rainbows.

1082. What is infrared light? — AC, Teaneck, NJ
Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second). Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can't see infrared or ultraviolet lights because the cells of retinas aren't sensitive to these lights. Nonetheless, we can often tell when those lights are present—we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.

1083. Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible light is not absorbed and yet it slows down. What's going on? — FH, Waltham, MA
When a light wave enters matter, the light wave's electric field causes charged particles in the matter to accelerate back and forth. That's because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened—the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That's because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons' motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons' motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren't able to jiggle back and forth as far as they should and they don't slow the light wave down as well.
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