Why is it that when you stand in front of a flat mirror, your image is reversed …

Why is it that when you stand in front of a flat mirror, your image is reversed horizontally (left-right) but remains the same vertically (up-down)? — CC, Martinsville, NJ

A mirror doesn’t really flip your image horizontally or vertically. After all, the image of your head is still on top and the image of your left hand is still on the left. What the mirror does flip is which way your image is facing. For example, if you were facing north, then your image is facing south. This front-back reversal makes your image fundamentally different from you in the same way a left shoe is fundamentally different from a right shoe. No matter how you arrange those two shoes, they’ll always be reversed in one direction. Similarly, no matter how you arrange yourself and your image, they’ll always be reversed in one direction.

While you’re looking at your image, the reversed direction is the forward-backward direction. But it’s natural to imagine yourself in the place of your image. To do this you imagine turning around to face in the direction that your image is facing. When you turn in this manner, you mentally eliminate the forward-backward reversal but introduce a new reversal in its place: a left-right reversal. If you were to imagine standing on your head instead, you would still eliminate the forward-backward reversal but would now introduce an up-down reversal. Since it’s hard to imagine standing on your head in order to face in the direction your image is facing, you tend to think only about turning around. It’s this imagined turning around that leads you to say that your image is reversed horizontally.

What holds the atoms in a molecule together?

What holds the atoms in a molecule together?

The atoms in a molecule are usually held together by the sharing or exchange of some of their electrons. When two atoms share a pair of electrons, they form a covalent bond that lowers the overall energy of the atoms and sticks the atoms together. About half of this energy reduction comes from an increase in the negatively charged electron density between the atoms’ positively charged nuclei and about half comes from a quantum mechanical effect—giving the two electrons more room to move gives them longer wavelengths and lowers their kinetic energies.

When two atoms exchange an electron, they form an ionic bond that again lowers the overall energy of the atoms and sticks them together. Although moving the electron from one atom to the other requires some energy, the two atomic ions that are formed by the transfer have opposite charges and attract one another strongly. The reduction in energy that accompanies their attraction can easily exceed the energy needed to transfer the electron so that the two atoms become permanently stuck to one another.

Recently, my doctor attached a small clip to my index finger that allowed a mach…

Recently, my doctor attached a small clip to my index finger that allowed a machine to not only measure my pulse rate but my blood gasses too. No needles were involved. How does this work? — CM, New York, New York

The red blood cells in your blood contain large amounts of a complicated and brightly colored molecule known as hemoglobin. This molecule’s ability to bind and later release oxygen molecules is what allows blood to carry oxygen efficiently throughout your body.

Each hemoglobin molecule contains four heme groups, the iron-containing structures that actually form the reversible bond with oxygen molecules and that also give the hemoglobin its color. However, this color depends on the oxidization state of the heme group—red when the heme group is binding oxygen and blue-purple when the heme group is alone. That color difference explains why someone who is holding their breath may “turn blue”—their hemoglobin is lacking in oxygen. The clip you wore was analyzing the color of your blood to determine the extent of oxygenation in its hemoglobin. It measured your pulse rate by looking for periodic fluctuations in the opacity of your finger, brought on by changes in your finger’s blood content with each heartbeat.

I know that the medium of electromagnetic waves is a photon. What is a photon? …

I know that the medium of electromagnetic waves is a photon. What is a photon? What is it made of? — ON, Istanbul, Turkey

First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as they change with time and they travel together through empty space. An electromagnetic wave of this sort carries energy with it because electric and magnetic fields both contain energy. That much was well understood by the end of the 19th century, but something new was discovered at the beginning of the 20th century: an electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of energy, units that are commonly called “quanta.” An electromagnetic wave that carries only one quanta of energy is called a “photon.”

The amount of energy that a photon carries depends on the frequency of that photon—the higher the frequency, the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules, which is why they provide our eyes with such useful information about the objects around us—we see how this visible light is interacting with the world around us.

In science, we learned that a color’s energy depends on its wavelength

In science, we learned that a color’s energy depends on its wavelength—that violet light with its short wavelength has more energy than red light with its long wavelength. But in art, we learned that red, orange, and yellow are warm and blue and violet are cool. Is that because of how the people feel about the colors, like fire is red and water is blue? — ON, Istanbul, Turkey

Both of your observations are correct: short wavelength light, such as violet, carries more energy per particle (per “photon”) than long wavelength light, such as red, and red light does appear “warmer” than blue light. But the latter observation is one of feelings and psychology, rather than of physics. It is ironic that colors we associate with cold and low thermal energies are actually associated with higher energy light particles than are colors we associate with heat and high thermal energies.

Given a certain chemical structure, can it be determined which spectrum of light…

Given a certain chemical structure, can it be determined which spectrum of light that molecule will absorb? Are there any known compounds that charge their color or intensity when exposed to electric fields? – GS

While it is possible in principle to calculate the exact spectrum of light that a molecule will absorb, in practice it is normally extremely difficult. It’s a matter of complexity—the quantum mechanical equations describing a molecule’s electromagnetic structure are easy to write down but extraordinarily difficult to solve, even in approximation. One of the great challenges of atomic and molecular physics and physical chemistry is determining the full quantum mechanical structure of atoms and molecules through calculation alone. Except with small atoms and molecules, it’s awfully hard but not impossible. As computers get faster and approximation schemes get better, the calculated spectra of molecules get closer to their experimental values.

As for compounds that change their optical properties while in electric fields, the answer is yes—all compounds exhibit such changes, although they may be undetectably small. However, I can’t think of any isolated molecules that change dramatically in normal fields. Still, electric fields can alter the “selection rules”—the symmetry-based laws that often control which optical transitions can or cannot occur. It’s possible that a modest electric field will turn on or off import optical transitions in some molecules so that they exhibit large color changes in small fields. Still, I can’t think of any useful examples.

Why are metal-halide lamps so efficient?

Why are metal-halide lamps so efficient?

Metal-halide lamps are actually high-pressure mercury lamps with small amounts of metal-halides added to improve the color balance. Light in such a lamp is created by an electric arc—electricity is passing through a gas in the lamp and causing violent collisions within the gas. These collisions transfer energy to the mercury and other gaseous atoms in the lamp and these atoms usually emit that energy as light. Overall, an electric current passes through the lamp and gives up most of its energy as light and heat in the gas. As you’ve noted, the lamp is relatively efficient, meaning that it produces more light and less heat than ordinary incandescent or halogen lamps. However, metal-halide lamps aren’t quite as energy efficient as fluorescent lamps.

What makes a metal-halide lamp so efficient is that there are relatively few ways for the lamp to waste energy as heat. While collisionally excited mercury atoms normally emit most of their stored energy as ultraviolet light—the basis for fluorescent lamps—they can’t do this in a high-pressure environment. A phenomenon called “radiation trapping” makes it almost impossible for this ultraviolet light to escape from a dense vapor of mercury, so a high-pressure mercury lamp emits mostly visible light. Even without the metal-halides, a high-pressure mercury lamp emits a brilliant blue-white glow. The metal-halides boost the reds and other colors in the lamp to make its light “warmer” and more like sunlight.

Next time you watch one of these lamps warm up, observe how its colors change. When it first starts up, its pressure is low and it emits mostly invisible ultraviolet light (which is absorbed by the lamp’s glass envelope). But as the lamp heats up and its pressure increases, the rich, white light gradually develops. Incidentally, if the power to a hot lamp is interrupted, the lamp has to cool down before it can restart because it only starts well at low pressures.

I have heard that there is a substantial cost to starting a fluorescent light fi…

I have heard that there is a substantial cost to starting a fluorescent light fixture. When entering and exiting a room frequently, is it better to leave a fluorescent light turned on, or to turn it off when leaving each time? — GEW

Whenever you turn on a fluorescent lamp, a small amount of metal is sputtered away from the electrodes at each end of the tube. These electrodes are what provide electric power to the gas discharge inside the lamp and sputtering is a process in which fast moving ions (electrically charged atoms) crash into a surface and knock atoms out of that surface. Because sputtering is most severe during start up, a typical fluorescent tube can only start a few thousand times before its electrodes begin to fail. To avoid the expense and hassle of having to replace the tube frequently, you shouldn’t cycle the lamp more than once every ten minutes. If you will only be away for a minute or two, leave the lamp on. But if you will be away for more than about ten minutes, turn it off. Incidentally, the claim that a fluorescent lamp uses a fantastic amount of electric power during start-up is nonsense. It’s just a myth.

is a photon a specific unit of measurement of light? Has it been decided if ligh…

is a photon a specific unit of measurement of light? Has it been decided if light is a particle or a wave? Why? — J, Australia

There is no doubt about it: light is both a particle and a wave. While it is traveling, light behaves as a wave—for example, it has a wavelength. But when it is being emitted or absorbed, light behaves as a particle—for example, it may transfer momentum, angular momentum, and energy to whatever it hits. A photon is a quantum of light, the smallest packet of light that can exist. You can’t have half a photon of light—it’s all or nothing. The amount of energy in a particular photon of light depends on the frequency (or wavelength) of that light.

Is light a particle or a ray?

Is light a particle or a ray? — CG

Light is both a particle and a ray (a wave). Its wave character was known and understood for many years before its particle character was discovered. That a film of clear soap exhibits colors is one of many demonstrations that light travels as waves, and such demonstrations were well understood in the 19th century. But it wasn’t until the early 20th century that people discovered the particle character of light. They found that light is absorbed in discrete packets of energy or quanta, and these quanta of light energy were called photons. As a simple rule of thumb, you can think of light as exhibiting wave-like properties while it’s traveling, but particle-like properties when it’s being emitted or absorbed. This dual nature of light is complicated but unavoidable; it’s a consequence of the quantum mechanical nature of our universe.