Does light travel in a straight line? Let’s find out…
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This simple activity has surprising results! We’re going to bend light using plain water. Light bends when it travels from one medium to another, like going from air to a window, or from a window to water. Each time it travels to a new medium, it bends, or refracts. When light refracts, it changes speed and wavelength, which means it also changes direction.
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Diffraction happens when light goes around obstacles in its path. Sound waves diffract bend around obstacles, so if you’re stuck behind a pillar at a concert, you can still hear just fine.
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Ever play with a prism? When sunlight strikes the prism, it gets split into a rainbow of colors. Prisms un-mix the light into its different wavelengths (which you see as different colors). Diffraction gratings are tiny prisms stacked together.
When light passes through a diffraction grating, it splits (diffracts) the light into several beams traveling at different directions. If you’ve ever seen the ‘iridescence’ of a soap bubble, an insect shell, or on a pearl, you’ve seen nature’s diffraction gratings.
Scientist use these things to split incoming light so they can figure out what fuels a distant star is burning. When hydrogen burns, it gives off light, but not in all the colors of the rainbow, only very specific colors in red and blue. It’s like hydrogen’s own personal fingerprint, or light signature.
Astronomers can split incoming light from a star using a spectrometer (you can build your own here) to figure out what the star is burning by matching up the different light signatures.
Materials:
- feather
- old CD or DVD
Here’s what you do: Take a feather and put it over an eye. Stare at a light bulb or a lit candle. You should see two or three flames and a rainbow X. Shine a flashlight on a CD and watch for rainbows. (Hint – the tiny “hairs” on the feather are acting like tiny prisms… take your homemade microscope to look at more of the feather in greater detail and see the tiny prisms for yourself!
What happens when you aim a laser through a diffraction grating? Here’s what you do:
Materials:
- laser pointer
- diffraction grating (you can use an old CD also)
Download Student Worksheet & Excercises
Exercises
- Which light source gave the most interesting results?
- What happens when you aim a laser beam through the diffraction grating?
- How is a CD different and the same as a diffraction grating?
- Why does the feather work?
Click here to go to next lesson on Useful Diffraction
Do you have thick or thin hair? Let’s find out using a laser to measure the width of your hair and a little knowledge about diffraction properties of light. (Since were using lasers, make sure you’re not pointing a laser at anyone, any animal, or at a reflective surface.)
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Have you ever wondered why you just can’t just shine a flashlight through a lens and call it a laser? It’s because of the way a laser generates light in the first place.
The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
That’s a mouthful. Let’s break it down.
Let’s do an experiment that shows you how a laser is different from light from a flashlight by looking at the wavelengths that make up the light.
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Click here to go to next lesson on Light Interference
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.
Laser light is also coherent, which means that the light is all in synch with each other, like soldiers marching in step together. Since laser light is coherent, which means that all the light waves peaks and valleys line up. The dark areas are destructive interference, where the waves cancel each other out. The areas of brightness are constructive interference, where the light adds, or amplifies together. LED light is not coherent because the light waves are not in phase.
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Polarization has to do with the direction of the light. Think of a white picket fence – the kind that has space between each board. The light can pass through the gaps int the fence but are blocked by the boards. That’s exactly what a polarizer does.
When you have two polarizers, you can rotate one of the ‘fences’ a quarter turn so that virtually no light can get through – only little bits here and there where the gaps line up. Most of the way is blocked, though, which is what happens when you rotate the two pairs of sunglasses. Your sunglasses are polarizing filters, meaning that they only let light of a certain direction in. The view through the sunglasses is a bit dimmer, as less photons reach your eyeball.
Polarizing sunglasses also reduce darken the sky, which gives you more contrast between light and dark, sharpening the images. Photographers use polarizing filters to cut out glaring reflections.
Materials:
- two pairs of polarized sunglasses
- tape (the 3/4″ glossy clear kind works best – watch second video below)
- window
Click here to go to next lesson on Shadows
The electromagnetic spectrum shows the different energies of light and how the energy relates to different frequencies. The wavelength (λ) equals the speed of light (c) divided by the frequency (ν), or λ = c / ν. The speed of light is: c = 3 x 108 m/s (300,000,000 meters per second).
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Do you see where the “visible light” rainbow section is in the electromagnetic spectrum image below? This small area shows the light that you can actually see with your eyeballs. Note that the “rainbow of colors” that make up our entire visible world only make up a small part of all the light, from 400-700 nm (nanometers, or 10-9.
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You can demonstrate the primary colors of light using glow sticks! When red, green, and blue cold light are mixed, you get white light. Simply activate the light stick (bend it until you hear a *crack* – that’s the little glass capsule inside breaking) and while wearing gloves, carefully slice off one end of the tube with strong cutters, being careful not to splash (do this over a sink).
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Is white or black a color? No and no. White is the mixture of all the colors (red, orange, yellow, green, blue indigo, and violet), so technically white isn’t a color of light but rather the combination of colors. Black is also not technically a color. In outer space, it’s pitch-black dark because there’s no light. In a room with the lights off, it’s also black. Black is the absence of color.
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When you change the wavelength, you change the color of the light. If you pass a beam white light through a glass filled with water that’s been dyed red, you’ve now got red light coming out the other side. The glass of red water is your filter. But what happens when you try to mix the different colors together?
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Your eyes have two different light receptors located on the back of the eyeball. These are the rods, which see black, white and grays can detect different intensities. The cones can detect color when the light strikes the cells that have a color-sensing chemical reaction that gets activated and sends a pulse to the brain. There are three cones: red, which can detect red wavelengths and some orange and yellow, green cones (which can also detect blue and yellow) are the most sensitive to light, and blue cones.
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Gummy bears are a great way to bust one of the common misconceptions about light reflection. The misconception is this: most students think that color is a property of matter, for example if I place shiny red apple of a sheet of paper in the sun, you’ll see a red glow on the paper around the apple.
Where did the red light come from? Did the apple add color to the otherwise clear sunlight? No. That’s the problem. Well, actually that’s the idea that leads to big problems later on down the road. So let’s get this idea straightened out.
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Click here to go to next lesson on Martian Sunsets
Have you ever wondered why the sky is blue? Or why the sunset is red? Or what color our sunset would be if we had a blue giant instead of a white star? This lab will answer those questions by showing how light is scattered by the atmosphere.
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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.
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Click here to go to next lesson on Path Difference
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”.
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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.
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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…
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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:
- 1 tongue-depressor size popsicle stick
- Three 3″ x 1/4″ rubber bands
- 2 index cards
- 3 feet of string (or yarn)
- scissors
- tape or hot glue
Click here to go to next lesson on Pressure Waves.
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.
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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!).
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Click here to go to next lesson on Speed of Sound.
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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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.
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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!
Click here to go to next lesson on Big Ears.
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.
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Click here to go to next lesson on Elasticity and Inertia.
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.
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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.
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.
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.