6 Wave Interference & Superposition

Light & Optics Unit Becky McCoy

Lesson Title: Wave Interference & Superposition Timing: 50 Minutes Target Audience: High School Conceptual Physics Course (9th-12th grade)

Objectives: Students Will Be Able To: • Discover and explain wave superposition and interference. The Teacher Will Be Able To: • Identify and correct misconceptions. • Allow students to learn about wave superposition and interference via inquiry.

Standards Assessed: New York State, The Physical Setting 4.3 i. compare the characteristics of two transverse waves such as amplitude, frequency, wavelength, speed, period, and phase 4.3 ii. draw wave forms with various characteristics 4.3 vi. predict the superposition of two waves interfering constructively and destructively (indicating nodes, antinodes, and standing waves) 4.3 vii. observe, sketch, and interpret the behavior of wave fronts as they reflect, refract, and diffract

Misconception(s) Addressed: • • • • •

Students view waves as a particle model. Rays and wave fronts are the same thing. There is no interaction between light and matter. Double slit interference shows light wave crest and troughs. Light exits in the crest of a wave and dark in the trough.

Prior Knowledge: Students will add upon their knowledge from the unit so far, as well as recall what they know about collisions from the mechanics unit earlier in the year.

Aim: Discover what happens when two waves are superimposed. Concept Map Vocabulary: • • •

Superposition Wave Interference Constructive

Necessary Preparation:

• •

Interference Destructive Interference Wave Front

• •

Wave Packet Node

Light & Optics Unit Becky McCoy COPIES • Wave Collisions Worksheet • Instructions to be placed at each station. • Homework MATERIALS • At least two Matchbox Cars • Playdough • Water • Food Coloring • Beakers • Computer • 3 Tennis Balls • Two Magnets • Coins • Hockey Puck (or similarly massive object) • Ripple Tank • CD player or other method of playing music SET UP • Each station will need to be arranged according to the activity listed below before the beginning of class.

Light & Optics Unit Becky McCoy

Lesson Plan

Aim: Discover what happens when two waves are superimposed. Physics Push-Up: Quick Write (5 Minutes) Students should make a prediction about what occurs when two waves collide. They can record their hypothesis in the first section of the Wave Collisions worksheet.

Activity: Collisions (20 Minutes) Materials: • At least two Matchbox Cars • Playdough • Water • Food Coloring • Beakers • Computer to show Youtube video: http://www.youtube.com/watch?v=yUc3wd4It8g • 3 Tennis Balls • Two Magnets • Coins • Hockey Puck (or similarly massive object) • Wave Collisions Worksheet Procedure: Objects will be set up in nine stations. Students should be grouped into nine groups and spend 2 minutes at each station (this leaves 2 minutes for instruction at the beginning). The idea of the stations is for students to recall what they learned in the mechanics section about collisions. Their prior knowledge, plus what they infer from these stations, will lead them to their hypothesis about what happens when waves “collide” (superposition). Students will change stations when the teacher pauses the music and any clean up should occur as each group finishes at each station. All observations should be recorded on the Wave Collisions Worksheet. Station 1 – Matchbox Cars: Students will crash the cars together and record observations. Station 2 – Playdough: Students will crash two Playdough balls together and record observations. Station 3 – Colored Water: Students will fill two beakers with water. They will add one or two drops of food coloring to one of the beakers and pour it into the uncolored beaker. They should record their observations of this “collision” of water. At the end, students should clean up after themselves. Station 4 – YouTube Video: Students should watch the video “Spectacular 1913 Train Collision” (http://www.youtube.com/watch?v=yUc3wd4It8g) and record their observations. Station 5 – One Tennis Ball: Students should bounce the tennis ball and record observations of the ball and the floor colliding.

Light & Optics Unit Becky McCoy Station 6 – Magnets: Students should cause a collision between two magnets and record their observations. Some students might record repulsion and others attraction. Either way, they are recording properties of magnets and, hopefully, will decide waves do not attract or repel each other. Station 7 – Coin and Hockey Puck: Students should cause a collision between the coin and hockey puck and record their observations. Station 8 – Two Tennis Balls: Students should roll the balls towards one another and record observations of the collision. Station 9 – Double Check: Students have the opportunity to revise their Physics Push-Up and make editions/deletions to their original ideas about wave collisions/superposition.

Activity Summary: Superposition & Ripple Tank (15 minutes) Review the activity with students. Allow a couple to share their experience and discovery. Be sure to find out if any predictions changed due to the activity stations. Begin lecture by defining a wave “collision” as superposition and defining interference. Draw pictures of constructive and destructive interference. Use the ripple tank to illustrate wave fronts. Ask students what will happen when wave fronts interfere. Create multiple wave fronts using a comb with only two prongs. Put a solid structure in the wave tank to create an area of wave “shadows” If the ripple tank is projected on a screen, students will be able to see where the interference and shadows are. Questions to Ask • What did you observe at each station? • Were you surprised by any of your observations? • Which of the stations seemed closest to wave superposition for you? • If single waves experience constructive and destructive interference, what might happen when wave fronts interfere? • When we use the ripple tank, what might interference of wave fronts look like? • What questions do you have?

Homework: (5 minutes) See Wave Interference Homework. Final project due first class of new unit.

Exit Strategy: 321 Exit Cards (5 minutes) 3 examples from the activity that are like wave superposition and interference. 2 examples from the activity that are not like wave superposition and interference. 1 example of how the ripple tank is a good analogy for light waves.

Light & Optics Unit Becky McCoy

Extension Activity: Draw more examples of constructive and destructive interference.

Assessment: • • •

321 Exit Cards Activity worksheets Physics Push-Up responses

Resources: “Spectacular 1913 Train Collision” http://www.youtube.com/watch?v=yUc3wd4It8g University of Dallas: http://phys.udallas.edu/C3P/Preconceptions.pdf

Notes & Adaptations:

Light & Optics Unit Becky McCoy Name _______________________________________________ Date ____________

Wave Collisions Worksheet Physics Push-Up: Create a hypothesis about what happens when two waves collide (we call this superposition) and record it in the space below.

MUSICAL STATIONS Record your observations from each station in the appropriate space below. You should change stations when the music stops. STATION 1: Matchbox Cars

STATION 2: Playdough

STATION 3: Colored Water

STATION 4: Spectacular 1913 Train Collision

Light & Optics Unit Becky McCoy STATION 5: One Tennis Ball

STATION 6: Magnets

STATION 7: Coin and Hockey Puck

STATION 8: Two Tennis Balls

STATION 9: Double Check

Light & Optics Unit Becky McCoy

Station ONE: MATCHBOX CARS

Crash the cars together and record what you observation on the Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station TWO: PLAYDOUGH Form two balls our of the Playdough. Crash the balls together and record your observations on the Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station THREE:

COLORED WATER Fill two beakers with water. Add one or two drops of food coloring to one of the beakers and pour it into the uncolored beaker. Record any observations of this “collision” of water on the Wave Collisions Worksheet. Don’t forget to clean up!

Light & Optics Unit Becky McCoy

Station FOUR: SPECTACULAR 1913 TRAIN COLLISION

Watch the video “Spectacular 1913 Train Collision” (http://www.youtube.com/watch?v=yUc3wd4It8g)

and record your observations on the Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station FIVE: ONE TENNIS BALL

Bounce the tennis ball and record observations of the ball and the floor colliding on the Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station SIX: MAGNETS Cause a collision between two magnets and record what occurs on your Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station SEVEN:

COIN & PUCK

Make the coin and hockey puck collide and record your observations on the Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station EIGHT: TWO TENNIS BALLS

Roll the balls towards one another and record observations of the collision on your Wave Collisions Worksheet.

Light & Optics Unit Becky McCoy

Station NINE: DOUBLE CHECK

Review your hypothesis from the Physics Push-Up and make any revisions. (If this is your first station, make your revisions as you go along!!)

Light & Optics Unit Becky McCoy Name ________________________________________ Date ____________

Wave Interference Homework Go to http://demonstrations.wolfram.com/WaveInterference/ and download the Wave Interference demonstration. Use this demonstration to answer the following questions. Write a paragraph explaining this demonstration.

What happens when you adjust the amplitude of both sources?

What changes when the frequency and phase are altered?

Move the location of the sources. What changes?

Light & Optics Unit Becky McCoy Create a hypothetical situation regarding two objects being dropped into standing water. Adjust the demonstration so that it reflects your story. Write your story below and print out the demonstration you design.