Monday, August 31, 2009
Saturday, August 29, 2009
A Physics Digest—Part 6: Light
Last time I described how the electromagnetic spectrum spans a huge frequency range—from radio waves to gamma rays, a factor of a trillion or so. Only the tiniest part (one billionth) of that range, in the very middle, is the light visible to our eyes. Everything we see is confined to that incredibly narrow strip—it’s our constricted view of life.
One property of visible light that’s crucial to our sense of sight is reflection. If ordinary objects didn’t reflect the sun’s light, we’d not see them (sort of like the Klingon cloaking device on Star Trek). Ordinary things don’t emit light on their own, so we need reflections from the sun or other light source to know they’re there.
While light waves reflect from opaque objects, they penetrate materials like glass and water—wherein they move more slowly. When a light wave is forced to slow down upon entering a transparent medium, it will bend or refract. If you look at a spoon sitting in a glass of water, it appears bent. Refraction is also used in eyeglasses and telescopes, to focus rays of light.
In air or in a vacuum all the colors (all frequencies) of light move at the same speed, but in glass the low frequencies (reds) move faster than high frequencies (blues). Thus light can get separated into its constituent colors upon entering a glass prism and becoming refracted. The same process creates a rainbow; each raindrop acts like a tiny prism.
Sunlight appears white because it contains all colors mixed together. (Actually, it’s a little yellowish, since the sun emits a little more light energy at yellow wavelengths.) So how does a red balloon appear red, when only “white” sunlight strikes it? The balloon absorbs all light frequencies other than red; only the red wavelengths are reflected. If all the colors get absorbed, it’s a black balloon.
Why is the sky blue? White sunlight enters our atmosphere and some of it gets scattered by particles in the air. Blue (high) frequencies get scattered more than any other color, so the sky gets filled with “blue rays” of light. Why is the sunset red? When overhead, the sun is whitish-yellow, but as it sets (or rises) its rays must then travel a longer path through the atmosphere to reach our eyes. The lowest frequencies (reds) are the least scattered color. So red rays penetrate the thick atmosphere best and are the ones we see.
Our eyes’ retinas contain light sensing rods and cones. Cones (at the center) respond to color and also give us acute central vision. Rods (surrounding the center) do not sense color and give us less detail in peripheral vision. They can “see” at light levels a hundredth of what cones need, however, so in dim light we can still see, but with fewer details and no color.
Two or more overlapping electromagnetic light waves can either reinforce or interfere with each other. Scientists have learned much about light—its speed, its color composition, and the nature of the media it’s traversing—by studying the reinforcement and interference patterns that the waves make.
Although we see most objects by the sunlight they reflect, a few things emit light themselves. Fireflies do it beautifully. An incandescent light does it by getting heated up so much that it glows. When some types of gas are zapped by electrical energy, the electrons in their atoms are bumped up to a higher energy state. These excited electrons quickly drop back to their usual lower energy level, and emit photons of light. This is how neon and fluorescent lamps glow.
The electrons in some elements don’t immediately jump back down to a lower energy state right away, but hold on for a while. This is phosphorescence. And when the emitted light waves are all in step with each other (reinforcing each other) we get the intense beam of a laser.
Every element emits its own unique signature of photons (i.e., color of light) when excited electrically. Astronomers use these signatures to determine what elements are present in various stars, so we don’t have to travel there to sample them directly.
One property of visible light that’s crucial to our sense of sight is reflection. If ordinary objects didn’t reflect the sun’s light, we’d not see them (sort of like the Klingon cloaking device on Star Trek). Ordinary things don’t emit light on their own, so we need reflections from the sun or other light source to know they’re there.
While light waves reflect from opaque objects, they penetrate materials like glass and water—wherein they move more slowly. When a light wave is forced to slow down upon entering a transparent medium, it will bend or refract. If you look at a spoon sitting in a glass of water, it appears bent. Refraction is also used in eyeglasses and telescopes, to focus rays of light.
In air or in a vacuum all the colors (all frequencies) of light move at the same speed, but in glass the low frequencies (reds) move faster than high frequencies (blues). Thus light can get separated into its constituent colors upon entering a glass prism and becoming refracted. The same process creates a rainbow; each raindrop acts like a tiny prism.
Sunlight appears white because it contains all colors mixed together. (Actually, it’s a little yellowish, since the sun emits a little more light energy at yellow wavelengths.) So how does a red balloon appear red, when only “white” sunlight strikes it? The balloon absorbs all light frequencies other than red; only the red wavelengths are reflected. If all the colors get absorbed, it’s a black balloon.
Why is the sky blue? White sunlight enters our atmosphere and some of it gets scattered by particles in the air. Blue (high) frequencies get scattered more than any other color, so the sky gets filled with “blue rays” of light. Why is the sunset red? When overhead, the sun is whitish-yellow, but as it sets (or rises) its rays must then travel a longer path through the atmosphere to reach our eyes. The lowest frequencies (reds) are the least scattered color. So red rays penetrate the thick atmosphere best and are the ones we see.
Our eyes’ retinas contain light sensing rods and cones. Cones (at the center) respond to color and also give us acute central vision. Rods (surrounding the center) do not sense color and give us less detail in peripheral vision. They can “see” at light levels a hundredth of what cones need, however, so in dim light we can still see, but with fewer details and no color.
Two or more overlapping electromagnetic light waves can either reinforce or interfere with each other. Scientists have learned much about light—its speed, its color composition, and the nature of the media it’s traversing—by studying the reinforcement and interference patterns that the waves make.
Although we see most objects by the sunlight they reflect, a few things emit light themselves. Fireflies do it beautifully. An incandescent light does it by getting heated up so much that it glows. When some types of gas are zapped by electrical energy, the electrons in their atoms are bumped up to a higher energy state. These excited electrons quickly drop back to their usual lower energy level, and emit photons of light. This is how neon and fluorescent lamps glow.
The electrons in some elements don’t immediately jump back down to a lower energy state right away, but hold on for a while. This is phosphorescence. And when the emitted light waves are all in step with each other (reinforcing each other) we get the intense beam of a laser.
Every element emits its own unique signature of photons (i.e., color of light) when excited electrically. Astronomers use these signatures to determine what elements are present in various stars, so we don’t have to travel there to sample them directly.
Friday, August 28, 2009
Wednesday, August 26, 2009
A Physics Digest—Part 5: Electricity and Magnetism
Our world is filled with electrical devices—in fact, all matter is electrical in nature. Every atom contains negatively-charged electrons orbiting a positive nucleus of protons. Positives and negatives attract each other. This is what holds an atom together. So what keeps all those mutually repulsive protons squeezed into an atom’s nucleus? The strong nuclear force—which we’ll get to in a later entry.
Some materials can be charged with static electricity by rubbing them. Shuffle across a rug in winter (when the humidity is low) and touch a door handle—watch the spark. Rub a balloon on your shirt and stick it to a wall: electrical attraction.
Some materials are good conductors of electricity; e.g., metals, which also conduct heat well. Other materials are poor conductors; e.g., rubber, so we coat electrical wires with rubber, in order to be able to handle them without a shock.
When electrical charges move along a wire, we have an electrical current; which can be thought of as analogous to water flowing through a hose. Voltage corresponds to pressure in the hose, and a battery is like the pump that pushes the water.
We can have direct current (DC), in which all the electric charge flows in one direction, or alternating current (AC), in which the charge reverses direction (60 times a second for the electric power coming into our houses). Voltage can do work as it pushes current, such as when it turns an electric motor. We measure the work that is done by the power it produces—in watts.
The discovery of magnetism predated our grasp of electricity by a few millennia. The Greeks played with magnetic stones they found. Material gets magnetized when clusters of atoms line up, military style. That lineup creates directional properties, for which we assign a north and a south pole to the magnet. Where did we get that convention? The Earth’s iron core creates a magnetic field that points towards the north and south poles. A magnetized compass needle aligns itself with this field. Some birds have a built-in magnetic compass to navigate themselves via Earth’s magnetic field.
A wonderful property of matter is that electricity and magnetism are coupled. When electricity flows in a wire, it sets up a magnetic field around it. Conversely, if you move a wire through a magnetic field, it causes an electric current to flow in the wire. Better yet, if you move a wire carrying current through a magnetic field a force on the wire can be felt. This is the essence of electric motors. That force can do work rotating a motor, maybe even moving an electric car.
One more fascinating property of electricity and magnetism is that their interaction creates electromagnetic radiation. If we vibrate a wire that’s carrying an electric current back and forth, we cause the wire’s surrounding magnetic field to similarly vibrate. This interaction sends out an electromagnetic wave, which happens to travel at the speed of light. In fact, these waves are light! They can move through the vacuum of space, like the heat radiation we saw earlier.
How fast we wave the wire back and forth determines the frequency of the electromagnetic wave. At low frequencies we get radio and microwaves (the latter can cook food). At mid frequencies we get infrared, visible light, and ultraviolet radiation. At high frequencies we get X-rays and gamma rays. These are all the same kind of waves, all moving at the speed of light.
On to more light next time.
Some materials can be charged with static electricity by rubbing them. Shuffle across a rug in winter (when the humidity is low) and touch a door handle—watch the spark. Rub a balloon on your shirt and stick it to a wall: electrical attraction.
Some materials are good conductors of electricity; e.g., metals, which also conduct heat well. Other materials are poor conductors; e.g., rubber, so we coat electrical wires with rubber, in order to be able to handle them without a shock.
When electrical charges move along a wire, we have an electrical current; which can be thought of as analogous to water flowing through a hose. Voltage corresponds to pressure in the hose, and a battery is like the pump that pushes the water.
We can have direct current (DC), in which all the electric charge flows in one direction, or alternating current (AC), in which the charge reverses direction (60 times a second for the electric power coming into our houses). Voltage can do work as it pushes current, such as when it turns an electric motor. We measure the work that is done by the power it produces—in watts.
The discovery of magnetism predated our grasp of electricity by a few millennia. The Greeks played with magnetic stones they found. Material gets magnetized when clusters of atoms line up, military style. That lineup creates directional properties, for which we assign a north and a south pole to the magnet. Where did we get that convention? The Earth’s iron core creates a magnetic field that points towards the north and south poles. A magnetized compass needle aligns itself with this field. Some birds have a built-in magnetic compass to navigate themselves via Earth’s magnetic field.
A wonderful property of matter is that electricity and magnetism are coupled. When electricity flows in a wire, it sets up a magnetic field around it. Conversely, if you move a wire through a magnetic field, it causes an electric current to flow in the wire. Better yet, if you move a wire carrying current through a magnetic field a force on the wire can be felt. This is the essence of electric motors. That force can do work rotating a motor, maybe even moving an electric car.
One more fascinating property of electricity and magnetism is that their interaction creates electromagnetic radiation. If we vibrate a wire that’s carrying an electric current back and forth, we cause the wire’s surrounding magnetic field to similarly vibrate. This interaction sends out an electromagnetic wave, which happens to travel at the speed of light. In fact, these waves are light! They can move through the vacuum of space, like the heat radiation we saw earlier.
How fast we wave the wire back and forth determines the frequency of the electromagnetic wave. At low frequencies we get radio and microwaves (the latter can cook food). At mid frequencies we get infrared, visible light, and ultraviolet radiation. At high frequencies we get X-rays and gamma rays. These are all the same kind of waves, all moving at the speed of light.
On to more light next time.
Tuesday, August 25, 2009
Sunday, August 23, 2009
A Physics Digest—Part 4: Sound
Sound begins when something vibrates—sending a sound wave through the air. The ear then gets stimulated and electrical impulses reach the brain. So does a tree make a sound when it falls? By this definition, only if an ear and a brain are present to sense the air’s vibrations.
While light and heat waves can travel through a vacuum, sound waves need a medium—a solid, liquid, or gas. (So a tree falling in outer space would be silent.) Sound waves travel at different speeds in different materials (they move faster in solids than air). A sound wave is transmitted within a medium when something makes a part of that medium vibrate. That part jostles neighboring parts, and the disturbance moves along—just like heat. Throw a stone into a pond and watch the circular vibration waves move outward. While the disturbance (energy) flows along, the material itself doesn’t. Watch a cork on the surface of the water bob up and down and notice that the cork doesn’t move outward with the wave.
The rate of vibration of sound is defined as its frequency. Our ears can register frequencies as low as 20 vibrations per second (Hertz), up to as high as 20,000 Hertz (although my aging ears quit at about 12,000). Sound waves transmit energy, which is often measured as loudness. Our remarkable ear can sense loudness levels of greater than a range of a factor of a trillion, so we squeeze that down into a workable logarithmic range of about 120 decibels.
Sound can become particularly attractive when we talk about music—although it can be very subjective. One person’s music is another’s noise. As a less subjective evaluation, we’d pretty much all of us define a jet engine as making a lot of noise.
Qualities of musical sounds that describe their properties are pitch (the same as frequency) and timbre. When a musical instrument makes a sound, it is composed of a fundamental tone (the basic pitch) and various overtones, which are whole-number multiples of the fundamental. The particular mix of these multiple tones defines the timbre of the instrument—its unique sound quality.
Musical instruments make sounds in three ways: by vibrating strings, vibrating air columns (horns and woodwinds) and vibrating surfaces (cymbals and drums). All of these sounds are simulated in a stereo system when an electrical signal causes a speaker surface to vibrate.
Our ear loves regular mathematical relationships between musical pitches. When the pitch of one sound is twice that of another, we hear an octave. When the ratios of pitches are whole number fractions—like 5/4 and 4/3—we hear musical intervals of a third and a fourth, respectively. That’s harmony!
While light and heat waves can travel through a vacuum, sound waves need a medium—a solid, liquid, or gas. (So a tree falling in outer space would be silent.) Sound waves travel at different speeds in different materials (they move faster in solids than air). A sound wave is transmitted within a medium when something makes a part of that medium vibrate. That part jostles neighboring parts, and the disturbance moves along—just like heat. Throw a stone into a pond and watch the circular vibration waves move outward. While the disturbance (energy) flows along, the material itself doesn’t. Watch a cork on the surface of the water bob up and down and notice that the cork doesn’t move outward with the wave.
The rate of vibration of sound is defined as its frequency. Our ears can register frequencies as low as 20 vibrations per second (Hertz), up to as high as 20,000 Hertz (although my aging ears quit at about 12,000). Sound waves transmit energy, which is often measured as loudness. Our remarkable ear can sense loudness levels of greater than a range of a factor of a trillion, so we squeeze that down into a workable logarithmic range of about 120 decibels.
Sound can become particularly attractive when we talk about music—although it can be very subjective. One person’s music is another’s noise. As a less subjective evaluation, we’d pretty much all of us define a jet engine as making a lot of noise.
Qualities of musical sounds that describe their properties are pitch (the same as frequency) and timbre. When a musical instrument makes a sound, it is composed of a fundamental tone (the basic pitch) and various overtones, which are whole-number multiples of the fundamental. The particular mix of these multiple tones defines the timbre of the instrument—its unique sound quality.
Musical instruments make sounds in three ways: by vibrating strings, vibrating air columns (horns and woodwinds) and vibrating surfaces (cymbals and drums). All of these sounds are simulated in a stereo system when an electrical signal causes a speaker surface to vibrate.
Our ear loves regular mathematical relationships between musical pitches. When the pitch of one sound is twice that of another, we hear an octave. When the ratios of pitches are whole number fractions—like 5/4 and 4/3—we hear musical intervals of a third and a fourth, respectively. That’s harmony!
Saturday, August 22, 2009
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