Author: Lou Bloomfield

How would I go about making a camera that’s more than just a pinhole camera?

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How would I go about making a camera that’s more than just a pinhole camera? — JL, Longview, WA

While a pinhole will project the image of a scene on a piece of film, it doesn’t collect very much light. That’s why a pinhole camera requires very long exposures. A better camera makes use of a converging lens. If you hold a magnifying glass several inches away from a white sheet of paper, you will see that it forms a real image of anything on the other side of it—particularly bright things such as light bulbs or well-lighted windows. A typical camera uses a converging lens that’s not unlike a magnifying glass to form an image of this sort. You could use a magnifying glass to build a camera, but I’d suggest that you start with a camera and rebuild it yourself. Go to a company that processes film and see if they will give you any used disposable cameras. These cameras are of essentially no value to them and they either discard them or recycle them. If you ask around, you should find a photo shop that will give you a couple. You can then disassemble them. You’ll find a very nice lens, a shutter system, a film advance mechanism, and so on. You can use a toothpick or small screwdriver to turn the exposure dial backward so that the camera behaves as though it still has film left. You can then “advance the (non-existent) film” by turning the film sensing gears in the back of the camera with your fingers until the shutter cocks. Finally, you can press the shutter release and watch the shutter open the lens to light. Disposable cameras are great because if you break something in your experimenting, you can just throw away your mistake.

What path does sunlight follow for you to see a mirage?

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What path does sunlight follow for you to see a mirage? — XF

The first step in explaining a mirage is to understand why the sky is blue, or why it has any color at all. If it weren’t for the earth’s atmosphere, the sky would be black and dotted with stars. That’s how the moon’s sky appears. But the earth’s atmosphere deflects some of the sunlight that passes through it, particularly short-wavelength light such as blue and violet, and this scattered light (Rayleigh scattering) gives the sky its bluish cast. When you look at the blue sky, you’re seeing particles of light that have been scattered away from their original paths into new paths so that they reach your eyes from all directions.

The blue light from the sky normally travels directly toward your eyes so that you see it coming from the sky. But when there is a layer of very hot air near the ground in the distance, some of the blue light from the sky in front of you bends upward toward your eyes. This light was traveling toward the ground in front of you at a very shallow angle but it didn’t hit the ground. Instead, its entry into the hot air layer bent it upward so that it arced away from the ground and toward your eyes. When you look at the ground far in front of you, you see this deflected light from the blue sky turned up at you by the air and it looks as though it has reflected from a layer of water in front of you. This bending of light that occurs when light goes from higher-density cold air to lower-density hot air is called refraction, the same effect that bends light as light enters a camera lens or a raindrop or a glass of water. Whenever light changes speeds, it can experience refraction and light speeds up in going from cold air to hot air. In this case, the light bends upward, missing the ground and eventually reaching your eyes.

How does an overhead projector work?

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How does an overhead projector work? — SR, Hartford, CT

An overhead projector uses a converging lens and a mirror to project a real image of your transparency onto a screen. A lamp brightly illuminates the transparency and a special surface under the transparency (actually a Fresnel lens) directs the light from the transparency through the projector’s main lens. This lens bends the light rays in such a way that all of the rays spreading outward from one point on the transparency bend back together and merge to one point on the screen. For example, if you make a green dot on the transparency, light rays spread outward from that green dot and some of them pass through the main lens. The lens bends these rays back together so that they form a single green dot on the screen. There is a single point on the screen for the light rays from each point on the transparency.

The pattern of light that forms on the screen is called a real image because it looks just like the original object—in this case the transparency—and it’s real, meaning that you can touch it with your hand. Real images are usually upside-down and backward, but the overhead projector uses its mirror to flip the image over so that it appears right side up. Because of this vertical flip, the side-to-side reversal is a good thing—the right side of the transparency becomes the left side of the screen image (as viewed by the same person) and the screen image is readable.

What is the relationship between gravitational force and electromagnetic force?

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What is the relationship between gravitational force and electromagnetic force? — TPC, Foster, OK

As yet, there is no direct relationship between those two forces. Our best current understanding of gravitational forces is as disturbances in the structure of space itself while our best current understanding of electromagnetic forces involves the exchanges of particles known as virtual photons. However, physicists are trying to develop a quantum theory of gravity that would identify gravitational forces with the exchange of particles known as gravitons. How closely such a quantum theory of gravity would resemble the current quantum theory of electromagnetic forces (a theory called quantum electrodynamics) is uncertain. It’s also uncertain whether those two quantum theories will be able to merge together into a single more complete theory. Only time will tell.

I read a recent article about the FCC requiring all TV stations to switch to dig…

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I read a recent article about the FCC requiring all TV stations to switch to digital signals instead of analog ones by 2006. How are digital signals different from analog signals, and will they work with our current TV’s? — JP

Current video signals use continuous physical quantities to represent the brightness and color of the spots on a television screen. For example, the current in a video cable can take any value and that value is used to represent the brightness and color of the spots. This use of a continuous physical quantity (such as current) to represent a continuous physical quantity (such as brightness) is called analog representation.

In a digital video signal, a physical quantity first represents numbers and then these numbers represent the brightness and color of the spots. The physical quantity representing the numbers doesn’t have to be continuous. For example, a current that’s on could represent the number 1 while a current that’s off could represent the number 0. A certain pattern of on and off currents could represent larger numbers and these numbers could then represent brightness and color. This use of a continuous or non-continuous physical quantity (such as magnetization, charge, or current) to represent numbers and then these numbers to represent a continuous physical quantity (such as brightness) is called digital representation.

One advantage of digital representation is that it’s relatively immune to noise. In analog representation, any disturbance in the continuous physical quantity representing the information leads directly to a disturbance in the recovered information. For example, if the strength of a radio wave is representing brightness and color on your television (the current technique), then any disturbance of the radio wave leads directly to a damaged image on your television. But in digital representation, small changes in the physical quantity that’s carrying the information won’t change the numbers that are obtained from that physical quantity and will thus have absolutely no effect on the recovered information. For example, if the strength of a radio wave is representing numbers in digital format, using binary (base two) encoding, then a small disturbance of the radio wave will not affect the binary numbers that are recovered from the radio wave. To see why that’s true, imagine representing the number 1 as a powerful radio wave and a 0 as no radio wave at all. It’s pretty easy to tell a powerful radio wave from an absent one so that, even if there is some radio interference around, it’s unlikely to confuse the receiver. Moreover, even if noise does occasionally confuse the receiver about a number or two, the digital scheme can include redundant information that allows the receiver to identify errors and to fix them! That’s why a compact disk is so immune to noise—even if there is a flaw or dirty spot on the disk, there is enough redundant digital information to reproduce the music flawlessly.

The other advantage to digital representation is that digital compression techniques become possible. A typical video signal contains lots of unnecessary and duplicated information. For example, when two people are standing in a room and the only things that are changing with time are the images of those two people, there is really no reason to keep sending an image of the room itself from the broadcast station to your home. Digital compression can identify redundant information and remove it from the transmission. In doing so, it can use the communication channel more efficiently.

By adopting a digital transmission scheme, the FCC has recognized that broadcasters will be able to send much clearer, more detailed images using digital representations than with the current analog representations, while still occupying the same portions of the electromagnetic spectrum. However, there is a cost—current televisions will not work directly with these new digital signals. To fix that shortcoming, there will be inexpensive converters that receive the new digital signals and recreate the analog signals needed for current televisions. This conversion will allow older televisions to keep working, but the new digital televisions will be designed to make better use of the enhanced details in the transmissions. The new transmissions will contain about 4 times the detail of current transmissions so that the images will be sharper as well as more immune to noise than the current transmissions.

Why does water freeze at very low pressure? I saw an experiment in which a small…

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Why does water freeze at very low pressure? I saw an experiment in which a small amount of water first boiled and then froze solid when exposed to a vacuum. — BLG, Old Bridge, NJ

Water molecules are always leaving the surface of liquid water and when they do, they carry away more than their fair share of the water’s thermal energy. Placing the water in a vacuum speeds this process because (1) it prevents those gaseous water molecules from returning to the liquid water, in which case they would return the thermal energy, and (2) it makes it possible for bubbles of water vapor to remain stable inside the liquid water even at low temperature, so that the water can boil. Overall, the main effect of putting the water in a vacuum is that its molecules leave rapidly and don’t return. Since each leaving water molecule takes away more than its fair share of thermal energy, the water molecules that remain behind become cooler and cooler. You experience this effect when evaporating water from your skin makes you feel cold. In the present case, this cooling is so effective that the remaining water cools all the way to water’s freezing point and the water begins to crystallize into ice. Water molecules continue to leave the surface of ice, a process called sublimation, so that even the ice gradually gets colder in the vacuum.

What are atoms made of?

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What are atoms made of? — Fifth Grade Class, Knifley, KY

My answer to that question depends on the level of detail you’re interested in. As an example of what I mean by that statement, imagine describing what a simple house is made of. At the coarsest level, you might say that it consists of a floor, a ceiling, four walls, and a roof. At a greater level of detail, you might say that it consists of many boards, some tarpaper, and lots of nails. At a still finer level of detail, you might say that it consists of atoms and molecules, and… you get the point. So it is with atoms. I’ll answer the question at a fairly coarse level of detail, one that’s familiar to many people, and then say a word or two about the next level of detail.

The principal constituents of an atom are protons, neutrons, and electrons. These are three most important subatomic particles; the main building blocks of matter in the same way that wood, bricks, and steel are the major building blocks of houses. Each of these particles has a mass—the measure of their inertia—and two of them, electrons and protons, are electrically charged. Each electron has one unit of negative charge while each proton has one unit of positive charge. Because an atom is normally electrically neutral—its positive and negative charges must balance—it has an equal number of electrons and protons. The number of neutrons in an atom is somewhat flexible.

These particles, electrons, protons, and neutrons, are held together by several types of forces. The protons and neutrons, which are relatively massive, stick to one another at the center of the atom and form a dense object called the atomic nucleus. The particles in the nucleus are held together by the “nuclear” force, which binds together protons and neutrons that are touching one another. This nuclear force is quite strong and is able to overcome the strongly repulsive electromagnetic forces that the protons in the nucleus exert on one another—like electric charges repel one another and the protons are all positively charged. The electrons circulate around the atom’s nucleus, held in place by the strongly attractive electromagnetic forces that protons exert on electrons—opposite electric charges attract one another and the electrons are negatively charged while the protons are positively charged.

The electrons do most of the circulating around the nucleus, rather than the other way around, because they are much less massive than the nucleus. As with the planets around the sun, the less massive objects tend to orbit the more massive objects. At a basic level, you can view an atom as a tiny solar system with its neutrons and protons at the center and its electrons orbiting around this central nucleus. Quantum physics dramatically complicates this picture, but it’s a helpful picture nonetheless.

At the next level of detail, the protons and neutrons themselves have structure—they are built out of yet smaller particles known as quarks. The particles also stick to one another by tossing particles back and forth—particles including photons and gluons. But that is a whole new story.

If one accepts the existence of black holes, would it be plausible to assume tha…

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If one accepts the existence of black holes, would it be plausible to assume that a “white hole” exists on the opposite end due to captured light by the black hole?

I think not. Depending on your frame of reference, the passage of material into a simple black hole—one that isn’t spinning very fast and that doesn’t have a great deal of electric charge in it—has one of two results. If you are traveling with the material, things proceed more or less normally as you pass the point of no return—the so-called “event horizon” from which even light can’t escape. You accompany the material all the way to the center of the black hole—its “singularity”—and are crushed to infinite density. If instead of traveling with the material, you remain outside the black hole looking in toward it, you see the material approach the event horizon but without ever quite entering its surface. In fact, all of the material that went into forming the black hole in the first place, plus all the material that has fallen into the black hole since its formation, appear to reside forever on the event horizon surface. In effect, the material never quite gets to the black hole. Since the material never quite gets to the black hole, there is no need for it to reemerge elsewhere from a “white hole.”

However, there are more complicated black holes—ones involving angular momentum and electric charge—that have more complicated structures. In falling into one of these black holes, it is apparently possible to miss the singularity. There is some discussion of such material reemerging from the “other end” of one of this black holes but I believe that there are serious problems with such two-ended interpretations of the equations governing such black holes.

What is sonar?

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What is sonar? — BK, Australia

Sonar stands for “sound navigation ranging” and involves the bouncing of sound waves from objects to determine where those objects are. It’s based on the reflection of sound waves from objects. Whenever a wave of any sort moves from one medium to another and experiences a change in speed (or more generally, a change in impedance), part of that wave reflects. Because sound travels much faster in solids than it does in air, some sound reflects when it moves from air to rock—which is why you hear echoes when you yell at a mountain! But even more subtle changes in the speed of sound will cause modest reflections. Thus a sophisticated sound generator and receiver can detect objects immersed in water or buried in the ground. Another form of sonar is used in medical imaging—ultrasonic imaging.

What type of laser is in a laser printer?

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What type of laser is in a laser printer? — DFC, Asheville, NC

A laser printer uses a single diode laser that’s scanned across the surface of the photoconductor drum by a rapidly turning, multifaceted mirror. These diode lasers are very similar the ones used in laser pointers or supermarket barcode readers. The multifaceted mirrors are typically octagonal prisms that are aluminized to make them highly reflecting and spun by a motor. The laser beam bounces off the spinning mirror and its reflection sweeps across the photoconductor. Modulating the current supplying power to the diode laser causes its brightness to fluctuate so that it writes information on the surface of the photoconductor.