Reflection occurs when waves bounce back from a barrier they cannot pass through. Refraction occurs when waves change direction as they enter a new medium at an angle. Diffraction occurs when waves spread out around an obstacle or after passing through an opening in an obstacle. Wave interference occurs when waves interact with other waves. Constructive interference increases wave amplitude. Destructive interference decreases wave amplitude.
- AAAS.6-8.12.D.4, 9
- Describe wave reflection, refraction, and diffraction.
- Explain how wave interference affects the amplitude of waves.
diffraction: bending of a wave around an obstacle or through an opening in an obstacle
reflection: bouncing back of waves from a barrier they cannot pass through
refraction: bending of waves as they enter a new medium at an angle and change speed
standing wave: wave appearing to stand still that forms when a wave and its reflected wave interfere
wave interference: interaction of waves with other waves
Introducing the Lesson
Introduce wave interference with a simple demonstration. While students observe, drop a pebble into a large shallow pan of water. As the waves start to spread out from the disturbance in concentric circles, drop another pebble into the water at the other side of the pan. Point out how the waves caused by the two pebbles meet and interfere with each other. Call on students to describe how the waves change when this happens. Conclude the demonstration by telling students they will learn about wave interference in this lesson.
Students can simulate wave interference with the activity at the following URL. The simulation allows students to move step by step through wave interference so they can clearly see how interference affects wave amplitude.
Pair less proficient readers and English language learners with other students, and have partners work together to make a table comparing and contrasting the three types of wave interactions described in the lesson: reflection, refraction, and diffraction. Tell them to include definitions, examples, and sketches of each type of wave interaction.
Ask a few students to set up and present a demonstration of the effect the size of the opening in an obstacle on the diffraction of waves. Suggest that they generate water waves in a shallow pan with an obstacle placed across the middle of the pan. The obstacle should have an adjustable opening. One possibility for the obstacle is a row of children’s play blocks. Students could remove one block at a time to show how increasing the opening size changes the amount of diffraction of the waves. Schedule time for the students to present their demonstration to the rest of the class. Have the other students make simple sketches of what they observe each time the size of the opening changes.
Give students a hands-on opportunity to explore wave interactions and wave interference using the activities collectively titled “Four Important Properties of Waves” at the URL below. In each activity, students perform a guided inquiry activity and then answer a series of questions to form a deeper understanding of the wave phenomenon in question.
Students often incorrectly think that when waves interact with a solid surface, the waves are destroyed. Remind students that waves transfer energy and that energy cannot be destroyed. Discuss wave interactions as energy transfers at boundaries between two different media. You can learn more about boundary behaviors of waves at this URL: http://www.physicsclassroom.com/Class/sound/U11L3c.html.
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.
Lesson Review Questions
Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.
Check students’ mastery of the lesson with Lesson 19.3 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.
Points to Consider
You were introduced to sound waves in this chapter, and you will learn more about them in the next chapter.
- How do you think we hear sound waves?
- What properties of sound waves might determine how loud a sound is?