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.
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.
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.
The spectrum of light from an incandescent bulb is what is known as a blackbody thermal spectrum—the light produced by a hot object. A blackbody spectrum is relatively featureless—you can't even tell what material is producing the light; only what temperature it has. All the wavelengths of light are present in thermal radiation and their intensities vary smoothly with wavelength. For the filament temperature of a normal incandescent bulb, the reds are brighter than the greens and the blues are rather weak.
A fluorescent bulb pieces together white light out of several separate colored lights. The spectrum of light from a fluorescent lamp is not simple or featureless—many wavelengths are essentially missing and the intensities of the remaining wavelengths don't vary smoothly with wavelength. Viewed through a spectroscope, the light from a fluorescent light has many bright bands of color interspersed with relatively dark bands.
I think that most black lights are gas discharge lamps that resemble normal fluorescent lamps. However, while a normal fluorescent light uses fluorescent phosphors to convert the ultraviolet light produced by its mercury discharge into visible light, a black light allows that ultraviolet light to emerge from the lamp unchanged. The ultraviolet light from a mercury discharge has too short a wavelength to be useful or safe as artistic black light, so other gases are likely to be used. The lamps are probably filtered so that they emit relatively little visible light or short wavelength ultraviolet light.
All objects in our universe have wave-like characteristics that manifest themselves in certain circumstances. These wave-like characteristics become more significant as objects become smaller. Their wave-like characteristics allow small particles to have ill-defined locations. To understand what I mean by "ill-defined locations", consider a wave on the surface of a lake. There is no one point at which this wave is located—it is located over a region of the water's surface. Waves don't have well defined locations. Similarly, if you observe an electron, which is really a wave, there is no one point at which that electron is located—it is located over a region of space. Because of the detailed relationships between wavelength, frequency, and energy, the smaller the region of space in which the electron-wave can be found, the higher its energy must be. Thus an electron that is localized at all—that is known to be within a certain region of space—must have a certain minimum energy, even if it is stationary. This minimum energy is called zero point energy and it is a consequence of trying to localize the particle within a certain region of space. Since the zero point energy is a base level and can't be reduced, you can't use zero point energy to do anything useful. It's just there.
A picture camera uses a lens to form a real image of a distant scene on the surface of a sheet of film. The lens bends rays of light so that all the light from a certain spot on the scene that passes through the lens comes together to a single point on the film. You can see this real image formation process with a magnifying glass. Just go into a darkened room with one window on a sunny day and hold the magnifying glass a few inches away from the wall opposite the window. You should see an inverted image of the window and the scene outside it projected on the wall. If you don't move the lens toward or away from the wall until that image forms. Everything else about a camera is just helping that lens form its image on the film in a controlled fashion. The camera's shutter limits the amount of time that light has to form this image. The focus controls make sure that light from the object you are interested in forms a sharp image on the film and doesn't appear blurry.
The earth's atmosphere has poor optical properties that seriously diminish the resolving powers of even the finest earth-based telescopes. You can see these optical problems by watching the warm air rise above a radiator or hot pavement on a summer day. The little swirls and eddies of heated air distort the scenery beyond them. Earth-based telescopes have to look at the stars through several miles of swirling, inhomogeneous atmosphere and they struggle to compensate for the imaging problems this air causes. Most world-class telescopes are located on mountaintops, far from lighted urban centers and away from humidity and clouds. But even the sky above these mountaintop observatories causes problems. By putting Hubble in space, they got rid of all atmospheric problems—air turbulence, clouds, and nearby lighting. They also made it possible for Hubble to operate around the clock by eliminating the blue sky that blinds telescopes during the day.
A typical analog-to-digital converter (ADC) uses a process called "successive approximation" to find a binary number that accurately represents the voltage on an input wire. It samples the voltage on the input wire at one moment in time and then gradually constructs a binary number representing that voltage. The ADC tries various binary numbers and uses a digital-to-analog converter to form a voltage from each number. It compares the two voltages, the original and its approximation, to determine how close its current guess is to the correct value. With each successive approximation, it adds a bit a precision to its measurement so that after 16 approximations, it has a 16 bit number that accurately represents the voltage on the input wire.
For applications requiring even faster measurements, there are flash ADCs. These devices synthesize the entire range of possible voltages and then compare the input voltage directly with the complete collection of possible voltages. Since 8 binary bits can represent 256 possible numbers, an 8 bit flash ADC synthesizes 255 different voltages and makes 255 voltage comparisons simultaneously. It instantly determines where among the various voltages the input voltage falls and it reports this value in billionths of a second.
Common window glass is made by melting a mixture of quartz sand (silicon dioxide), soda (sodium oxide), and lime (calcium oxide). The quartz is the network forming material that forms the basic structure of the glass. The soda makes it much easier to melt and work with—along with making the glass weaker and more temperature sensitive. The lime prevents the soda-rich glass from dissolving in water.