How do lasers work?

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How do lasers work?

Lasers use systems with excess energy to amplify light. These systems, typically atoms or atom-like structures in solids, are in excited states—they have more than their minimum amounts of energy. An excited system can get rid of its excess energy in many different ways, but certain systems tend to emit the excess energy as photons—particles of light. While an excited system will emit a photon spontaneously if you wait long enough, it can also duplicate a passing photon if that passing photon has the proper characteristics. Most importantly, the excited system must be naturally capable of emitting the passing photon spontaneously—the passing photon’s wavelength and travel path must be such that the excited system is able to duplicate it.

This duplication effect makes it possible to amplify light. When a single photon passes by a number of identical excited systems, those systems may duplicate the photon many times so that many identical photons emerge. This phenomenon is the basis for laser amplifiers. When one of the photons emitted spontaneously by the excited systems is deliberately sent back and forth through those systems with the help of mirrors, the laser amplifier becomes a laser oscillator—it both initiates and amplifies the light. The light that ultimately emerges from the laser oscillator or amplifier differs from normal light because the laser light consists of many identical photons. They all have identical wavelengths (colors) and follow identical paths through space. They also exhibit dramatic wave effects, particularly interference.

When light hits an object, how do we recognize the color?

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When light hits an object, how do we recognize the color? — CM, Levering, PA

White light is a mixture of various light waves with different wavelengths and thus different colors. When white light hits an object, some of the light waves are absorbed while others are not. The light that isn’t absorbed may pass through the object or it may be reflected in a new direction. The light that you observe coming from the object is this transmitted or reflected light. If the light that you see doesn’t include the same mixture of wavelengths that first hit the object, you won’t see this light as white. Instead, you’ll see it as colored. If the light you see contains mostly long wavelengths of light, you’ll see it as red. If the light contains mostly short wavelengths of light, you’ll see it as blue or violet. The wide range of colors that objects have comes from subtle differences in the wavelengths of light they absorb. However, when an object is illuminated with colored light, the light that it transmits or reflects may be altered. After all, it can’t transmit or reflect a light wave that never hit it in the first place. Even variations in “white” light can affect an object’s color—makeup looks different in incandescent “white” light than it does in fluorescent “white” light because those illuminations contain different mixtures of light waves.

Why does a helium balloon in a car seem to defy Newton’s laws? When you accelera…

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Why does a helium balloon in a car seem to defy Newton’s laws? When you accelerate forward suddenly, the balloon moves forward and when you brake, the balloon moves back. Is that because the air inside the car compresses when you accelerate? — CT, Charlottesville, VA

Since the air in the car is denser than the helium balloon, the air’s motion dominates the helium balloon’s motion. When your car accelerates forward, the air’s inertia tends to move it toward the back of the car-the accelerating car is trying to leave the air behind. The balloon moves forward in the car to give the air more room near the back of the car. When you stop suddenly, the air in the car continues to coast forward and accumulates at the front of the car. Again, the balloon moves backward in the car to give the air more room at the front of the car. You’ll see exactly this same effect if you watch an air bubble in a bottle of water as you drive the bottle around in a car.

How does a CD player work?

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How does a CD player work? — NL, Dearborn, MI

A CD player uses a laser beam to determine the lengths of a series of ridges inside a compact disc. Infrared light from a solid-state laser is sent through several lenses, a polarizing beam splitter, and a special polarizing device called a quarter-wave plate. It’s then focused through the clear plastic surface of the compact disc and onto the shiny aluminum layer inside the disc. Some of this light is reflected back through the player’s optical system so that it passes through the quarter-wave plate a second time before encountering the polarizing beam splitter. The two trips through the quarter-wave plate switches the light’s polarization from horizontal to vertical (or vice versa) so that instead of returning all the way to the laser, the light turns 90° at the polarizing beam splitter and is directed onto an array of photodiodes. These photodiodes measure the amount and spatial distribution of the reflected light. From this reflected light, the CD player can determine whether the laser beam is hitting a ridge or a valley on the disc’s aluminum layer. It can also determine how well focused or aligned the laser beam is with the aluminum layer and its ridges. The player carefully adjusts the laser beam to follow the ridges as the disc turns and it measures how long each ridge is. The music is digitally encoded in the ridge lengths so that by measuring those lengths, the player obtains the information it needs to reproduce the music.

Is it possible to make a black bulb that absorbs light rather than emitting it?

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Is it possible to make a black bulb that absorbs light rather than emitting it? — KD, Pflugerville, TX

Not unless you will consider a black hole to be a black bulb. For a “bulb” to absorb light that isn’t heading toward the bulb, that bulb will have to attract the light toward it. Since light has no electric charge, the only force that the bulb can exert on light is gravitational force. While a black hole’s gravity is strong enough to attract and ensnare light, it wouldn’t make a very practical bulb. However, it is possible in certain circumstances to add light to previously existing light and, in doing so, create a dark shadow that wasn’t present before. This process is called interference, where two light waves cancel one another in a particular region of space and prevent any light from reaching a certain spot. But this cancellation is difficult to achieve, except with lasers, and doesn’t occur everywhere in space—the light doesn’t vanish, it just gets redistributed. Overall, the idea of a black bulb is just not realistic.

How are luminol and fireflies related?

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How are luminol and fireflies related? — JH, Minneapolis, MN

There are a few molecules that can be chemically oxidized to produce new molecules that then spontaneously emit light. The chemical reactions that occur in these special molecules leave the resulting new molecules electronically excited—their electrons are in states that have more than the minimum allowed energies. As these energetic electrons subsequently shift to states with less energy, they release some of that energy as light.

In a firefly, the molecule that is being oxidized is called luciferin. It’s combined with oxygen and the important biological energy storage molecule ATP (adenosine triphosphate), assisted by a catalyst protein called luciferase. A series of reactions then occurs, culminating in the formation of excited decarboxyketoluciferin. This molecule emits a photon of green light and becomes normal decarboxyketoluciferin.

Luminol, a molecule used in many cold light products, is a somewhat simpler molecule that is much easier to synthesize commercially than is luciferin. When it’s oxidized with hydrogen peroxide and potassium ferrocyanide, it forms an excited molecule that emits a photon of blue light. This blue light is often shifted to green or orange with the help of a fluorescent dye. The dye absorbs the blue light and uses its energy to emit green or orange light. This material is commonly used in light sticks and glowing necklaces or toys.

Why is the sky blue? – Z

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Why is the sky blue? – Z

As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That’s because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

A reader (TAC) points out that the above explanation would seem to imply that the sky should appear violet, since violet light scatters more strongly than blue light. But the spectrum of sunlight peaks in the green—sunlight contains more green light than blue light and more blue light than violet light. The sky combines these two effects together (more green light but better scattering of violet light) and acquires an overall blue appearance.

What does the inside of a microwave oven look like? Please show illustrations.

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What does the inside of a microwave oven look like? Please show illustrations. — Dade County, FL

A microwave oven contains (1) a magnetron that produces the microwaves, (2) a high voltage direct current power supply (a high voltage transformer, a set of rectifiers, and a capacitor) that provides power to the magnetron, and (3) a computerized control system that turns the power supply and magnetron on and off. A metal pipe connects the magnetron to the cooking chamber of the oven. While there are photographs and drawings of the insides of a microwave oven in my book, I can’t reproduce them here because of copyright issues.

How does a magnetron work?

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How does a magnetron work? — MM, Czech Republic

A magnetron has a ring of resonant electromagnetic cavities around a hot central filament. Each resonant cavity acts like an electromagnetic “tuning fork”—electric charges and electromagnetic waves swing back and forth inside a resonant cavity at a particular frequency; the cavity’s resonant frequency. As electrons are “boiled” off the hot filament, a high voltage attracts them toward the walls of the resonant cavities. The resonant cavities tend to have at least small amounts of electric charge “sloshing” back and forth in them at their resonant frequencies and the electrons from the filament are attracted more strongly to the cavities’ positively charged walls than to their negatively charged walls.

However, there is also a magnetic field present in the magnetron and this field deflects the streams of electrons so that they hit the wrong walls of the resonant cavities. Instead of canceling the charge sloshing in the walls of the resonant cavities, the newly arrived electrons add to it. As electrons flow to the resonant cavities, more and more charge sloshes in the resonant cavities and these cavities accumulate huge amounts of energy. Some of this energy is tapped by a small wire loop and a microwave antenna. This antenna radiates some of the energy from the cavities into a metal channel that leads away from the magnetron. In a microwave oven, this channel leads to the cooking chamber so that energy from the resonant cavities is delivered to the food in the oven. Energy is extracted from the magnetron slowly enough that the filament and high voltage power supply can replace it and the operation continues indefinitely.

How does electricity work and is it possible to design a light bulb that will le…

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How does electricity work and is it possible to design a light bulb that will let you know when it is about to stop working? — LS, Chicago, IL

Electricity involves electric charges. While static electricity involves stationary electric charges, the electricity you are probably referring to is dynamic: electricity in which the electric charges move. Most (dynamic) electricity is the movement of electrons—tiny negatively charged particles that form the outer part of atoms. The electricity in the wires leading to and from a lamp is the flow of electrons through those wires. A lamp has two wires attached to it because the electrons flow into the lamp through one wire and out through the other wire. However, because the electricity we normally use is alternating current, the direction in which the electrons flow through those two wires reverses 120 times a second (60 full cycles of reversal, over and back, each second).

As the electrons flow through the lamp’s filament, they leave behind much of their energy. This energy is deposited in the tiny filament and the filament becomes extremely hot. It begins to emit much of its thermal energy as thermal radiation, part of which is visible light. So you can think of the electricity as a steady stream of tiny delivery trucks (the electrons), carrying energy to the lamp’s filament, and then returning to the power company to pick up some more energy. The filament sends this energy into the room as heat and light.

When a light bulb burns out, it’s because the filament has became so thin that a section of it has overheated and melted. This thinning process is caused by the slow evaporation (or actually sublimation) of tungsten atoms from the filament. A thinned filament usually fails as you turn the bulb on because that’s the time of maximum power delivery to the filament and thus maximum stress. Unfortunately, it’s very hard to tell in advance whether the filament will be able to tolerate the next attempt to turn it on. Probably the best predictor is the number of hours the bulb has been on. If you always replace a bulb after it has operated for 750 hours at full power, you’ll probably avoid most outages.