Wave - A disturbance that transfers energy from one place to another.
Ocean waves are caused by the wind. The wind is the disturbance and it causes energy in the water to be transferred from one place to another in the form of an ocean wave.
If there is a boat on the ocean water, the wave will cause anything floating on the water to be disturbed so the energy carried by the wave lifts the boat as it passes.
Most waves require something to travel through: a medium. Any form of matter can be a medium. Waves that require a medium to travel are known as mechanical waves.
Electromagnetic waves, such as light from the sun, can travel through empty space. They do not require a medium to travel through.
Waves are created when a source of energy causes a medium to vibrate. The vibrating up and down or back and forth is the source of the wave. A boat's propellers disturb the calm surface of the water. They transfer energy to the water and cause its particles (the medium) to vibrate. This creates a wave. The movement of a boat through water also creates a transfer of energy and a wave.
Types of waves
There are three types of waves: Transverse, longitudinal, and combination.
Transverse waves - If you make a wave with a piece of rope (as we did in class), the wave moves from one end of the rope to the other while the rope itself moves up and down. In transverse waves, the waves move at right angles to the direction of the medium. The highest parts of these waves are called crests and the lowest parts are troughs. The distance between the crests of two waves is the wavelength. The distance between the trough and the crest is the amplitude.
Longitudinal waves - If you stretch out a Slinky, you can produce a longitudinal wave. These waves move the particles of the medium parallel to the direction the waves are traveling. They either both move back and forth or up and down. In the spring, the coils that are close together are called compressions while those that are far apart are rarefactions.
Combination waves - These waves have characteristics of both longitudinal and transverse waves. They occur as surface waves between two media (such as air and water). The water, and anything on it, moves up and down like a transverse wave but it also moves back and forth like a longitudinal wave.
Light
Light travels from one place to another in waves. Another name for light is electromagnetic radiation. Scientists created the electromagnetic spectrum to show the wavelengths of different types of waves as well as those of the colors of visible light.
See Through Lab - We took a break at this point to complete a lab that showed the differences between transparent, translucent, and opaque materials. The students looked through a clear plastic Zip-Loc bag, a piece of wax paper, and a piece of poster board. They discussed how things looked through each.
Transparent, translucent, opaque
Light normally travels in a straight line. When light strikes an object it can be reflected, absorbed, or transmitted. Most objects reflect or absorb light. An object that reflects or absorbs all the light that hits it is opaque. Opaque objects are things that you can't see through like the poster board.
Transparent objects transmit light. When light strikes something transparent it passes right through. The transparent object from our demonstration was the Zip-Loc bag.
Translucent objects allow some light to pass through them. They scatter light as it goes through so you can usually tell that something is behind a translucent object but you can't see the details. Wax paper is translucent.
Mirrors
Mirrors reflect light. Light passes through the glass and hits the coating on the back of the mirror. It is then reflected, allowing you to see an image. There are three types of mirrors: Plane mirrors, concave mirrors, and convex mirrors.
Plane mirrors are flat mirrors and are the ones the students probably use most. Plane mirrors produce images that are right side up and are the same size as the object being reflected.
Concave mirrors have a surface that curves inward like a bowl. These mirrors reflect rays of light that meet at a focal point. Concave mirrors can produce an image like that produced in a plane mirror. They can also produce images that are upside down and/or larger or smaller than the object being reflected. Since these mirrors can be used to produce magnified images, they are used as make-up or shaving mirrors.
Convex mirrors have surfaces that curve outward. The rays reflected by these mirrors start at a focal point and spread out. Passenger-side rearview mirrors on cars are an example of convex mirrors. They spread out rays of light allowing for a larger reflection area but, because you can see more, images appear smaller and further away. This is the reason for that "Objects in mirror are closer than they appear" message on rearview mirrors.
Refraction and prisms
When light enters a new medium at an angle, the change in speed that occurs causes the rays to bend or change direction. This is refraction.
Refraction of light can cause us to see something that is not really there. This explains rainbows and mirages (see below).
Prisms allow us to see refraction of light. When white light hits the angles on a prism, it refracts and separates into its component colors (all those colors of the electromagnetic spectrum). Longer wavelengths mean the light waves will be bent less by the prism so colors like orange and red will be bent or refracted less than blue or purple.
Prism Lab - We went outside with a prism and saw it refract the white light from the sun into the colors of the spectrum.
Water Prism Lab - We didn't have much luck with this one but that may have been because we didn't cut big enough slits in the poster board. Try this again at home and experiment with different sized slits.
You will need a poster board circle cut to fit over the lens of a flashlight. Cut a very thin slit across the circle, stopping about 1 cm from the edges. Tape the circle to the front of the flashlight. Place a glass of water on the edge of a flat chair and have a partner hold a piece of white paper near the floor at the edge of the chair. Darken the room and hold the flashlight at an angle to the surface of the water. Try changing the angle of the flashlight and moving the paper until you see the spectrum of colors.
The water acts as a prism to refract or bend the light from the flashlight.
Rainbows
Raindrops can act as tiny prisms. When light from the sun hits raindrops (or water from a garden sprinkler or hose), the light is bent by the water.
Mirages
Air higher in the atmosphere is cooler than that near the road. Light travels faster when it reaches the warmer air so the rays bend as they travel downward. This refracted light appears to come up from the ground making the rays look like they were reflected off a smooth surface such as water.
Color
Colors depend on how objects reflect and absorb light. Each object absorbs some wavelengths of light and reflects others. Our eyes see objects as the color of the light they reflect. So, a red object appears red because the red wavelengths of light are reflected by that object. All other wavelengths are absorbed. Blue objects reflect the blue wavelengths. Green things reflect green wavelengths.
Black and white are a bit different. Black objects appear black because they absorb all wavelengths of light. White objects, on the other hand, reflect all the wavelengths.
Blender Lab - This lab showed how light waves blend to produce white light.
Each student was given a 10-cm. diameter poster board circle. The students divided their circles into six equal sections then used markers (red, orange, yellow, green, blue, and violet) to color each section. It's important to go in that order so the colors match their order on the electromagnetic spectrum. They then pushed a pencil through the center of the circle to create a spinner. As the circle spins, the colors do seem to blur together. We couldn't make our circles blur enough to go grey but I did see some dull blues. To make white, we would need to include the 7th color of the spectrum, indigo.
Chromatography Lab - Chromatography is used to separate the components of colored mixtures. The original lab called for using candy such as Skittles but I tried that at home and didn't have great results. Instead, we used markers. The link to the original lab is below if you'd like to try it with Skittles or M&Ms.
Cut a 3"x3" square from a coffee filter. Add a dot of each color marker about 1 cm from the edge of the coffee filter.
Make a salt solution by mixing 1/8 tsp. of table salt into 3 cups of water. Pour a little of the solution into a clear glass or cup. You want the level to be below the dots but the salt solution should touch the very bottom of the coffee filter. Use a binder clip to clip the coffee filter on the side of the cup.
Capillary action will draw the water up through the coffee filter. It will pass through the dots of color and start to separate them. This happens because some dyes are more likely to stick to the paper while others want to mix with the salt water.
References:
The labs "See Through," "Water Prism," and "Blender" were all from Physics for Every Kid.
VanCleave, J. (1991). Physics for Every Kid: 101 Easy Experiments in Motion, Heat, Light, Machines, and Sound. San Francisco: Jossey-Bass.
The Chromatography lab was found on this website: http://chemistry.about.com/od/chemistryexperiments/ht/candychroma.htm
No comments:
Post a Comment