Lasers light is different from light from a flashlight in a couple of different ways. Laser light is monochromatic, meaning that it’s only one color.
Imagine you have a coherent light in a shoebox, and you cut two narrow slits out the side and shine the light on the far wall. The distance from one slit to the wall isn’t going to be exactly the same as the other, so there’s a “path difference”.
We’re going to use a laser pointer and a protractor to measure the microscopic spacing of the data tracks on a DVD and a CD. The really cool part is that you’re going to use an interference pattern to measure the spacing of the tracks, something that you can’t normally see with your eyes.
This experiment is also known as Young’s Experiment, and it demonstrates how the photon (little packet of light) is both a particle and a wave, and you really can’t separate the two properties from each other. If the idea of a ‘photon’ is new to you, don’t worry – we’ll be covering light in an upcoming unit soon. Just think of it as tiny little packets or particles of light. I know the movie is a little goofy, but the physics is dead-on. Everything that “Captain Quantum” describes is really what occurred during the experiment. Here’s what happened…
To show how light acts like a wave, you can pass light through a glass of water and watch the rainbow reflections on the wall. Why does this happen? We’ve already covered this in a previous lesson, but basically when the light passes through the glass and the water, it bends to give different frequencies of light and therefore different colors.
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This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!
Sound is a form of energy, and is caused by something vibrating. So what is moving to make sound energy?
Molecules. Molecules are vibrating back and forth at fairly high rates of speed, creating waves. Energy moves from place to place by waves. Sound energy moves by longitudinal waves (the waves that are like a slinky). The molecules vibrate back and forth, crashing into the molecules next to them, causing them to vibrate, and so on and so forth. All sounds come from vibrations.
Materials:
A sound wave is different from a light wave in that a sound is a mechanical wave, which requires particle interaction in order to exist. Light waves can travel in the vacuum of space, and we’ll talk more about this in our next section when we get to light.
Think of your ears as ‘sound antennas’. There’s a reason you have TWO of these – and that’s what this experiment is all about. You can use any noise maker (an electronic timer with a high pitched beep works very well), a partner, a blindfold (not necessary but more fun if you have one handy), and earplugs (or use your fingers to close the little flap over your ear – don’t stick your fingers IN your ears!).
Sound is a type of energy, and energy moves by waves. So sound moves from one place to another by waves; longitudinal waves to be more specific. So, how fast do sound waves travel? Well, that’s a bit of a tricky question. The speed of the wave depends on what kind of stuff the wave is moving through. The more dense (thicker) the material, the faster sound can travel through it.
When something vibrates, it pushes particles. These pushed particles create a longitudinal wave. If the longitudinal wave has the right frequency and enough energy, your ear drum antennas will pick it up and your brain will turn the energy into what we call sound. The higher the amplitude, the more energy the wave has. Intensity of a measure of a wave’s power per unit area, and is measured in Watts per square meter.
It may seem like walking across a balance beam and listening to your favorite song are very different activities, but they both depend on your ears. Ears are the sense organs that control hearing, which is the ability to detect sound. Ears also sense the position of the body and help maintain balance when you walk a balance beam or ride a bike.
Imagine a pebble being dropped into a lake. Waves of water go off in all directions. A similar thing happens when a car driving down the street honks its own. Waves go off from the car in all direction. The difference is that these are not waves of water, but instead are sound waves, which travel through the air. If you are nearby, some of those sound waves make it to your ear.
Here’s a video that shows you how everything works together so you can hear:
The pinna, or outer ear, which is the part of your ear that you can see, gathers up some of the sound waves, sends them down the ear canal, and eventually they strike the eardrum. The eardrum is a thin membrane that vibrates like a drum when the waves hit it. The vibrations pass three tiny bones, called the hammer, anvil, and stirrup, as well as a membrane called the oval window, causing them all to vibrate.
From the oval window, the vibrations go to the cochlea, liquid-filled space lined with hairs. The vibrations make waves in the cochlea’s liquid, just like waves in a pond, causing the hairs to move. The movement of the hairs sends a nerve impulse through the auditory nerve to the brain. The brain interprets the message and “tells” you what you have heard.
This video is an old instructional film shown to pre-med students in the early 50s you might enjoy watching:
Along with hearing, the ears play a major role in balance. Inside the ears are semicircular canals which are lined with hairs and full of liquid. When the body moves in one direction, the liquid in the semicircular canals move, causing the hairs to move. This sends a message to your brain, which gives instructions for the body’s muscles to contract or relax. This keeps you balanced.
There’s a cool video of a camera going inside the ear… watch out for the wax!
How do you think animals know we’re around long before they see us? Sure, most have a powerful sense of smell, but they can also hear us first. In this activity, we are going to simulate enhanced tympanic membranes (or ear drums) by attaching styrofoam cups to your ears. This will increase the number of sound waves your ears are able to capture.
Have you ever been in a thunders storm? Here’s how you can use wave speed to figure out how far away that lightning strike really was.
Although the speed of a wave is found by multiplying the frequency by the wavelength, it’s important to note that the wave speed doesn’t change if you change the wavelength or frequency. What actually happens is that if you change the frequency, you also change the wavelength so the wave speed will remain constant. The speed of the wave depends only on the properties of the medium it’s traveling through, so the only way to change the wave speed is to change the properties of the medium itself.
Let’s review interference and nodes for a minute…
How waves interact with each other depends on whether the waves are in phase or not. If they are in-step (in phase) with each other, then it’s easy to add up to double the displacement (constructive interference). If they are completely out of step, then they cancel each other out (destructive interference). We’ve seen this for transverse waves, but what about compression waves, like for sound waves?
Destructive interference happens when a compression and rarefaction section meet and the next effect is that there’s no push or pull on the particles. The waves don’t destroy each other (as the name implies), but rather they cancel out the effect of each other when they interact with each other, which results in no sound at all. Which is kind of odd to think about: two sound waves interacting to make no sound at all. This happens at the nodes, where there’s no particle displacement.
Beats refers to the pattern that two similar waves make when they interfere with each other. A beat pattern is one that varies in volume (amplitude) as the waves constructively and destructively interfere with each other. The beat frequency is the wave that forms when you hear the volume go from high to low volume.
The Doppler Effect describes how moving sound waves can shift frequencies for either the observer, the source, or both, depending on the motion of each. You’ll find Doppler shifts with waver, sound, and light waves. For sound waves, if the source is moving toward the observer, the observer will hear higher pitch sounds. And if the source moves away from the observer, then they will hear lower pitch sounds. The frequency at the source didn’t change, only what the observer perceives is difference because there’s motion between them. It’s a shift in the apparent frequency.
Sound can change according to the speed at which it travels. Another word for sound speed is pitch. When the sound speed slows, the pitch lowers. With clarinet reeds, it’s high. Guitar strings can do both, as they are adjustable. If you look carefully, you can actually see the low pitch strings vibrate back and forth, but the high pitch strings move so quickly it’s hard to see. But you can detect the effects of both with your ears.
When waves go from air to water, they must pass through a boundary between the two, and depending on the properties of two mediums, the wave will do one (or more) of four possible behaviors: reflect, diffract, transmit transmit through, and/or refract.
Everything is vibrating. Absolutely everything is wiggling and jiggling, and most of those things are doing it really fast! Now, I can hear you saying “Hey…maybe you need to check your eyesight or lay off the coffee because in my house, I’m not seeing everything jiggling.”
You can easily make a humming (or screaming!) balloon by inserting a small hexnut into a balloon and inflating. You can also try pennies, washers, and anything else you have that is small and semi-round. We have scads of these things at birthday time, hiding small change in some and nuts in the others so the kids pop them to get their treasures. Some kids will figure out a way to test which balloons are which without popping… which is what we’re going to do right now.
This is one of my absolute favorites, because it’s so unexpected and unusual… the setup looks quite harmless, but it makes a sound worse than scratching your nails on a chalkboard. If you can’t find the weird ingredient, just use water and you’ll get nearly the same result (it just takes more practice to get it right). Ready?
NOTE: DO NOT place these anywhere near your ear… keep them straight out in front of you.
Sound is everywhere. It can travel through solids, liquids, and gases, but it does so at different speeds. It can rustle through trees at 770 MPH (miles per hour), echo through the ocean at 3,270 MPH, and resonate through solid rock at 8,600 MPH.
Sound is made by things vibrating back and forth, whether it’s a guitar string, drum head, or clarinet. The back and forth motion of an object (like the drum head) creates a sound wave in the air that looks a lot like a ripple in a pond after you throw a rock in. It radiates outward, vibrating it’s neighboring air molecules until they are moving around, too. This chain reaction keeps happening until it reaches your ears, where your “sound detectors” pick up the vibration and works with your brain to turn it into sound.
You can illustrate this principle using a guitar string – when you pluck the string, your ears pick up a sound. If you have extra rubber bands, wrap them around an open shoebox to make a shoebox guitar. You can also cut a hole in the lid (image left) and use wooden pencils to lift the rubber band off the surface of the shoebox.
When a guitarist plucks a string to start the vibration, it not only vibrates the string, but it also vibrates the entire box of the guitar. This is called a forced vibration, which means that the motion of the original source vibration is also causing another object to vibrate (the box of the guitar). Since the box is larger than the string, it amplifies the vibration and makes it louder.
An electrical signal (like music) zings through the coil (which is also allowed to move and attached to your speaker cone), which is attracted or repulsed by the permanent magnet. The coil vibrates, taking the cone with it. The cone vibrates the air around it and sends sounds waves to reach your ear. Here’s how speakers work and also how to make your own out of cardboard (it really works!):
We’ve already looked at standing wave patterns that are created when a reflected wave interferes with an incoming wave. It looks like the wave is fluctuating in place, when really it’s just an optical illusion of two waves interfering with each other. The point is, this effect are created at specific frequencies called harmonics, and now it’s time to learn about vibrational modes using a really cool experiment by Ernst Chladni.
A vibrational mode is the standing wave pattern that give the highest amplitude vibrations with the least amount of energy input. If you vibrate an object at it’s natural frequency, you’ll get the highest amplitude during the vibration. Sometimes the amplitude (which is related to the energy of the vibration) that the object vibrates at is so high that the object will actually will tear itself apart. Here’s a video where the wind was blowing the bridge, which started a natural vibration in the bridge which tore itself apart.
Ella Fitzgerald was famous for breaking the wineglass with her voice at the end of the Memorex commercial:
Let’s see this in slow motion using lab equipment (video courtesy of MIT):
There are four general categories of musical instruments: guitars and pianos are examples of vibrational strings, trombones and flutes are examples of the open-and air column instruments, organ pipes are examples of the closed-end air instruments, and drums are examples of vibrational mechanical instruments.
When you blow into the mouthpiece of an instrument, the vibrations create frequencies, and the ones that resonate with the air in the tube inside the instrument are the ones you hear as a loud sound. When an instrument is open at both ends, it’s called an open-end air column. Here’s how to figure out the frequencies of these types of instruments:
Have you ever blown across a glass bottle? If so, you’ve played one of these instruments! Pipe organs are also closed-end air instruments because one end is sealed. The difference in sealing one end affects the types of frequencies that the instrument can create because the standing wave pattern that is created is from the incident (incoming) waves interfering with the reflected waves bouncing back when they hit the sealed end of the instrument.
Have you ever put a cardboard tube up to your ear? What you hear depends on whether the tube is right up against your ear or offset so there’s a space between your head and the tube, because it goes from being a closed end to an open end air column, which changes the standing wave pattern inside. Here’s how to figure it out:
Once upon a time, people used record players to hear music. Records were these big black discs that played on a machine. Spinning between 33 and 45 times per minute on a turntable, people used to listened to music just like this for nearly a century.
Edison, who had trouble hearing, used to bite down hard on the side of his wooden record player (called a phonograph) and “hear” the music as it vibrated his jaw.
One common misconception is the idea that noes and antinodes are the same as the crest and trough of a wave. They’re not. A node is a place on the wave that is permanently at rest. An antinode is where the wave is at its maximum (it will travel through a large up and a large down displacement).
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When a wave travels from one medium to another, like sound waves traveling in the air and then through a glass window pane, it crosses a boundary. Whether the wave continues to the new medium (and even how it goes through), or whether it bounces and reflects back, or a bit of both depends on the boundary.
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Straight waves are what happens when something moves back and forth in a medium like water. These are interesting when they hit a diagonal plane barrier, because when the incident wave reaches the barrier, the waves always reflect at the same angle that they approached the barrier with (called the Law of Reflection).
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Waves bend when they go from one medium to another when the speed changes. It’s a really important topic in light (not so much with sound), because it’s how lenses, eyes, cameras, and telescopes work. The bending of sound waves happens naturally in the air above the earth when it’s warmer than the surface of the earth. The sound waves that travel through the warmer air are faster and the ones that travel through cooler air are slower. When the sound waves go from warmer to cooler air (less dense to more dense air), they become bent back down toward the surface.
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When waves pass around small (we’re talking small compared to the wavelength of the wave) objects, they diffract. People in the audience of a concert can hear really well if they are sitting right behind a pillar because the sound waves are large enough to bend around it (which is actually because of both diffraction and reflection effects). Diffraction helps sound bend around obstacles. You can sometimes hear conversations around corners because of diffraction.
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Often two (or more) waves travel through the same spot. If you’ve ever listened to an orchestra, you’re hearing the sounds from many different instruments all playing at the same time. If more than two boats are on the lake, their wakes churn up the water together. Here’s how we handle this in physics…
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What if two waves of the same wavelength and amplitude travel in the same direction along a stretched string? What will the string look like? We know about the idea of superposition adding the waves together, but what does the string actually look like?
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In previous lessons we’ve learned that energy is the ability to do work, and that work is moving something a distance against a force. The concept of energy is fairly easy to see as far as lifting things or pushing things go. We are exerting energy to lift a box against the force of gravity. We are exerting energy to pedal our bike up a hill. But how does this energy stuff relate to light, electricity, or sound? What’s moving against a force there?
Damping is when a spring, swing, or other vibrating object loses its energy over time. It means that without adding energy into the system, like pumping on a swing or hitting a drum head, the object will eventually come to its non-vibrating (equilibrium) position.
Imagine the kid on the swing again. Why does the kid move past the equilibrium point without stopping?
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The concept of frequency is very important to understanding energy. When it comes to electromagnetic waves it is frequency that determines whether the wave is radio, light, heat, microwave or more. It’s all the same type of energy, it’s the frequency that determines what that energy actually does. With sound energy the frequency determines the pitch of the sound.
Imagine a police car on the side of the road with lights and sirens on full blast. You’re also parked and you hear the same frequency (say 1,000 Hertz) of the siren. However, if you’re driving at 75 mph toward the police car. you’re going to hear a higher frequency (1096 Hz), and if you’re driving away at 75 mph, you’re going to hear a lower frequency of 904 Hz. Why is that?
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The restoring force slows down the object as it moves from its resting but speeds it up when it heads back to the resting position, and that’s what creates the vibration. We’re going to take a look at the forces in a pendulum from the point of view of Newton’s Laws.
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You’ll need a pendulum for this experiment. A pendulum is really nothing more than a weight at the end of something that can swing back and forth. The easiest way to make one is to get a string and tape it to the edge of a table. (The string should be long enough so that it swings fairly close to the ground.) Tie a weight to the bottom of your string and you’ve got a pendulum.
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Since we can’t see soundwaves as they move through the air, we’re going to simulate one with rope and a friend. This will let you see how a vibration can create a wave. You’ll need at least 10 feet of rope (if you have 25 or 50 feet it’s more fun), a piece of tape (colored if you have it), a slinky, and a partner. Are you ready?
This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!
Sound is a form of energy, and is caused by something vibrating. So what is moving to make sound energy?
Molecules. Molecules are vibrating back and forth at fairly high rates of speed, creating waves. Energy moves from place to place by waves. Sound energy moves by longitudinal waves (the waves that are like a slinky). The molecules vibrate back and forth, crashing into the molecules next to them, causing them to vibrate, and so on and so forth. All sounds come from vibrations.
Materials:
Some waves need a medium to travel through while others do not. Mechanical waves need a medium for the wave to travel through to transport energy. Ocean waves, jump ropes, pendulums, sound, and waves in a stadium are all examples of mechanical waves.
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The words particle and wave are two words you’ll see in nearly every area of physics, but they are actually very different from each other. A particle is a tiny concentration of something that can transmit energy, and a wave is a broad distribution of energy that fills the space it passes through. We’re going to look at particles in more depth later, and instead focus on understanding waves.
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Sound is a type of energy, and energy moves by waves. So sound moves from one place to another by waves; longitudinal waves to be more specific. So, how fast do sound waves travel? Well, that’s a bit of a tricky question. The speed of the wave depends on what kind of stuff the wave is moving through. The more dense (thicker) the material, the faster sound can travel through it.
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Sound moves faster in solid objects than it does in air because the molecules are very close together in a solid and very far apart in a gas. For example, sound travels at about 760 mph in air, 3300 mph in water, 11,400 mph in aluminum, and 27,000 mph in diamond!
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