7 Standing Waves

Light & Optics Unit Becky McCoy

Lesson Title: Standing Waves Timing: 50 Minutes Target Audience: High School Conceptual Physics Course (9th – 12th grade)

Objectives: Students Will Be Able To: • Define a standing wave. • Identify properties of a standing wave. • Explain differences between light and sound waves. The Teacher Will Be Able To: • Identify and correct lingering misconceptions of light waves. • Prepare students for Ray v. Wave Battle. • Give students opportunities to understand standing waves conceptually.

Standards Assessed: New York State, The Physical Setting 4.3 iii. identify nodes and antinodes in standing waves 4.3 vi. predict the superposition of two waves interfering constructively and destructively (indicating nodes, antinodes, and standing waves)

Misconception(s) Addressed: •

Standing waves have no energy or motion.

Prior Knowledge: Previous class discussions on wave superposition. Aim: Explore properties of standing waves in light and sound. Concept Map Vocabulary: • • •

Standing Wave Node Superposition

Necessary Preparation: COPIES • Battle! Assignment • Battle! Rubric (saved with Lesson 8) MATERIALS • Something to watch YouTube videos • Two jump ropes • 12 to 15 feet of 1/8" nylon cord

• • •

Interference Destructive Interference Beat

Light & Optics Unit Becky McCoy • 1 foot of 1/8" nylon cord • Electric drill and chuck key • 1 20-penny bent nail • #2 barrel swivels (found in the fishing section of sporting goods) SET UP • Assemble standing wave demonstration • Have computer/projection set up for the start of class

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Light & Optics Unit Becky McCoy

Lesson Plan

Aim: Explore properties of standing waves in light and sound. Physics Push-Up: Standing Wave Videos (7 minutes) Have students watch clips of the Double Dutch, Surfing, Standing Wave on a string, and Laboratory Standing Wave Videos and write what each video tells them about standing waves – to be collected. • Double Dutch: http://www.youtube.com/watch?v=A6Tub5_NXOA# • Surfing: http://www.youtube.com/watch?v=uFwEGrxNp5Q&feature=PlayList&p=998B26BD06D6A520 &playnext=1&playnext_from=PL&index=24 • Standing Wave on a String: http://www.youtube.com/watch?v=MT7EpS4OX3k • Laboratory Standing Wave: http://www.youtube.com/watch?v=lNg_woUaPgM

Activity: Standing Wave Demonstrations (25 minutes) Materials: • Two jump ropes • 12 to 15 feet of 1/8" nylon cord • 1 foot of 1/8" nylon cord • Electric drill and chuck key • 1 20-penny bent nail • #2 barrel swivels (found in the fishing section of sporting goods) Procedure: 1. Begin by having several students do double dutch or jump rope and discuss how this is one ½ wavelength and the next demonstration will show multiple ½ wavelengths. 2. Before presenting the demonstration with the drill, provide some background information on the electromagnetic spectrum. This activity will lead to a discussion about wavelengths in the electromagnetic spectrum (as in harmful UV rays). 3. Prior to the demonstration, you will need to bend a 20-penny nail to a right angle. 4. Attach a swivel to each end of the nylon cord. 5. Tie the 1-foot piece of cord to one of the swivel holders. This is the piece of cord that a student will hold during the demonstration. 6. Slide the bent nail through the eye of the other swivel. 7. The nail end should be put into the drill bit fitting and tightened securely with the chuck key. 8. To ensure the safety of your students, it is imperative that the cord does not break during the demonstration. Be sure to test it before you present it to your students. 9. Ask a student to hold one end of the cord. 10. Plug in the drill and the demonstration begins. The less tension you apply, the more waves will appear. You can also vary the speed and reverse the direction of the drill to get different wave effects. Experiment and have fun!

Light & Optics Unit Becky McCoy Questions to Ask: • The length of the wave is measured as the distance from wave crest to wave crest. What happens to the length of the wave when the drill speeds up, i.e., when more energy is added? (The wavelength shortens.) • What occurs to the wavelength when the drill is slowed? (The wavelength increases.) • UV radiation is a relatively short wavelength. It is shorter than visible light. What is the energy of UV radiation relative to visible light? (It has higher energy.) • Why do you think UV radiation is of such great concern? (Because it has so much energy, ultraviolet light in large doses can be damaging.) • We hear so much about ultraviolet radiation these days. What about even shorter wavelengths, such as x-rays and gamma rays that also come from the sun? Do they cause damage too? (Yes.)

Activity Summary: Light v. Sound Waves (13 minutes) Standing waves are a phenomena we usually only deal with when teaching sound, but it is worth mentioning during this unit in order to help students understand more about destructive interference of waves. Use the video Standing vs. Traveling Waves (http://www.youtube.com/watch?v=03cUrkjqdE0) to show the difference between standing and traveling transverse and longitudinal waves. This is also a time to explain the differences between sound and light waves, specifically, light does not need a medium to travel through, but sound does. Information like this might help teams build their arguments for the Ray v. Wave Debate next class. As an early preview for the next unit on sound, show this video of a Chladni Plate (http://www.youtube.com/watch?v=h3N_VeQ6pLs) which makes the nodes and beats of sound waves very clear!

Homework: Ray v. Wave Battle Research (5 minutes) Students should prepare arguments for the Ray v. Wave Battle next class by researching (via the internet, textbook, and class notes) for their team.

Exit Strategy: Team Assignments Teams for the Battle will be assigned during the time allocated for explaining the homework.

Extension Activity: Discuss examples of standing waves in sound (stringed instruments specifically). Find a couple videos to show or have an instrument in class.

Assessment: • •

Physics Push-Up responses Teacher evaluation of student discussion throughout class

Resources: Standing Wave Demonstration: http://www.ucar.edu/learn/1_5_2_22t.htm

Light & Optics Unit Becky McCoy Double Dutch: http://www.youtube.com/watch?v=A6Tub5_NXOA# Surfing: http://www.youtube.com/watch?v=uFwEGrxNp5Q&feature=PlayList&p=998B26BD06D6A520&playnext=1&p laynext_from=PL&index=24 Standing Wave on a String: http://www.youtube.com/watch?v=MT7EpS4OX3k Laboratory Standing Wave: http://www.youtube.com/watch?v=lNg_woUaPgM

Notes & Adaptations:

Light & Optics Unit Becky McCoy

Battle! Ray v. Wave Theories of Light Challenge: You must use class notes, textbooks, and internet resources to build the case for your team’s argument. Task: Your team must be prepared with the following… • • • • •

2 physical phenomena that provide evidence for your model. 1 drawing that demonstrates your strongest argument. 1 example of another law in Physics that supports your argument. 2 questions for the other team. 1 theory that explains the flaw in your model.

Tips: It will help if you’re familiar with the other team’s argument so you can strengthen your own. The success of the battle will be determined by the rubrics everyone will fill out at the end of class.