|OBJECTIVES: Distinguishing between electromagnetic and physical waves. Comparing diffraction, reflection and refraction. VOCABULARY: diffraction interference reflection refraction wave MATERIALS: mirrortuning forkalien ballslinky laser flashlight|| |
Students determine types of waves generated by different items.
| BACKGROUND: |
Waves are a means by which energy travels. Many different particles move in waves. The waves on an ocean are physical waves caused mainly by wind. Light is an electromagnetic wave caused by excited electrons. The movement of a wave is complicated, but both electromagnetic and physical waves use similar ways to describe the motion.
Both electromagnetic and physical waves reflect, refract and diffract energy. These properties of wave motion are important because they explain how waves move. Waves (both physical and electromagnetic) if controlled, can produce products we use in our society. The phone, television, radio, microwaves, CD players, lasers, video players and many other items are products produced by understanding these waves. The details in these lessons emphasize the importance of understanding waves.
Diffraction is when a wave goes through a small hole and has a flared out geometric shadow of the slit. Diffraction is a characteristic of waves of all types. We can hear around a corner because of the diffraction of sound waves. For instance, if a wall is next to you when you yell, the sound will parallel the wall. The wall may stop, but the voice doesn"t; sound will almost turn the corner of the wall. This is diffraction.
Reflection is when waves, whether physical or electromagnetic, bounce from a surface back toward the source. A mirror reflects the image of the observer.
Refraction is when waves, whether physical or electromagnetic, are deflected when the waves go through a substance. The wave generally changes the angle of its general direction.
In this lab, students determine which situation illustrates diffraction, reflection, and refraction. Discuss the main definition of each term. Have students go to the different stations, perform the activity, and decide which motion the waves are producing. Some of materials shown in picture below.
The following will help in setting up stations.Fill a clear glass partially with water and put a pencil or other type of rod in it. Instruct students to look at the side. The students will see an off-centered pencil. The light is being refracted by the water, so our eyes see the pencil in two different mediums. Answer: electromagnetic (light); refraction Make 3 sets of slits with a razor in a sheet of aluminum foil (like diagram). Put the laser in a darkened corner with a box on the top (so students won"t stare at the light). Instruct students to put each of the slits in front of the light and record what they see. The light will get elongated which illustrates that the light is actually diffracted or "flaring outward." Where the slits are double, the light will have bright and dark areas. This is called interference. Do not expect students to know what actually causes this. Answer: electromagnetic (light); diffraction
Instruct students to hit the tines of a tuning fork (in kit) and place the tines on the surface of the water. Waves will move from the center of vibration. Notice that the pattern spreads from the center and causes little ripples. This shows not only diffraction but also interference similar to that in station 2. Answer: physical wave; diffraction Using a spoon, instruct students to hit the surface of the water in a pan of water. The ripples should be large enough to hit the end of the pie tin and reflect backwards. However, students might only see the spreading outward and call this diffraction. Answer: physical wave, reflection and/or diffraction (students should justify answer) Instruct two students extend the slinky in a straight line about 1 meter. One student should push back several of the coils and then release them. The waves will go down the slinky and reflect from the other end and bounce back. Answer: Physical wave, reflection. Instruct two students to extend the rope. One should snap their wrist to create an oscillating wave. The wave reflects backwards when it hits the other student. Answer: physical wave, reflection Instruct students to shine a small beam of light through a prism (in kit). If the light is angled correctly, they should get a rainbow effect. Also, put the prism on the laser light. (This shows the refraction better). Answer: electromagnetic (light); refraction Instruct students to shine a light on a mirror (in kit). (Students may also try the laser light). The light bounces off. Answer: electromagnetic (light); reflection If a beam of light is shone through a lens, the light will refract through the lens and create a different size image on the other side. Answer: electromagnetic (light); refraction The energy ball (in kit) will make a sound and cause light to flash when you use your fingers to complete the circuit. Answer: both physical (sound) and electromagnetic (light); diffraction of light and sound