Interference Objectives Investigate wave interference Describe constructive and

Interference

Objectives • Investigate wave interference. • Describe constructive and destructive interference.

Assessment 1. Which of the following is not an example of wave interference and the superposition principle? A. A car muffler produces sound to counteract noise from the engine. B. Ocean waves hit the shore and are absorbed by the beach. C. Noise-canceling headphones eliminate background noise by producing sound that is out of phase with the background noise. D. Light shown through two thin slits creates a diffraction pattern.

Assessment 2. Describe how two wave pulses can combine destructively. 3. In your own words, describe the difference in appearance between a wave with a single frequency and a wave composed of more than one frequency.

Physics terms • superposition principle • interference • constructive interference • destructive interference • phase

Multiple waves Examine this picture of the ocean. Notice that there are ripples on top of the waves. These ripples are actually smaller waves that are combining with larger waves.

Multiple waves Examine this picture of the ocean. Notice that there are ripples on top of the waves. These ripples are actually smaller waves that are combining with larger waves. Waves of different amplitudes, wavelengths, and frequencies are often present at the same time.

Rogue waves How are small ripples and the huge walls of water called “rogue waves” related? The answer lies in an understanding of interference.

Sine waves The simplest wave can be described by a single amplitude, wavelength, and frequency. These are referred to as sine waves.

Sine waves The simplest wave can be described by a single amplitude, wavelength, and frequency. These are referred to as sine waves. Graphs of amplitude vs. position and amplitude vs. time can be modeled using the sine function.

Sine waves The sine function repeats every cycle, or every 2π radians.

Sine waves The sine function repeats every cycle, or every 2π radians. What happens when two or more of these simple waves combine with each other?

Superposition principle The superposition principle says that the total amplitude at any point equals the sum of the amplitudes of all of the waves that occur at that same place and time.

Superposition principle When more than one wave is present, they can sum to make a larger or smaller amplitude wave.

Constructive interference When more than one wave is present, they can sum to make a larger or smaller amplitude wave. If the result is a larger amplitude wave, constructive interference has occurred.

Destructive interference Two waves can also add up to make a smaller wave.

Destructive interference Two waves can also add up to make a smaller wave. When two or more waves add up to make a smaller amplitude wave, destructive interference has occurred. If the amplitudes are exactly matched, there can be total destructive interference.

Investigation In Investigation 15 C you will experiment with wave pulse on a spring and observe how the pulses interact when they meet.

Investigation Constructive and destructive interference 1. Stretch the slinky out flat on a table or the floor – one group member holding each end. DO NOT overstretch the slinky!

Investigation 2. Send pulses down the slinky from each end and observe how they interact when they meet in the middle. Use your observations to answer the questions.

Investigation 3. Test the following scenarios:

Investigation The pulses travel fast, making it difficult to observe the exact moment they interact. Try using slow motion video to make observations, or use the video samples on the e-Book 15 C investigation page.

Investigation Questions a. When the equal pulses met in the middle, how was the amplitude of the combined wave different from the individual pulses? b. When the big and small pulses met in the middle, how was the amplitude of the combined wave different from the individual pulses? c. When the equal and opposite pulses met in the middle, how was the amplitude of the combined wave different from the individual pulses?

Investigation Questions d. Which scenarios experienced constructive interference? How do you know? e. Which scenarios experienced destructive interference? How do you know? f. For each scenario, describe what happened to the pulses after they met.

Interference and superposition In most real situations, many waves will be present. Some will interfere constructively; others will interfere destructively.

Interference and superposition In most real situations, many waves will be present. Some will interfere constructively; others will interfere destructively. The interference is often temporary, lasting only until the waves pass by each other.

Temporary interference Two opposite pulses start at opposite ends of this spring.

Temporary interference Two opposite pulses start at opposite ends of this spring. When they meet in the middle, they cancel.

Temporary interference Two opposite pulses start at opposite ends of this spring. When they meet in the middle, they cancel. They re-appear after passing through each other.

Assessment 1. Which of the following is not an example of wave interference and the superposition principle? A. A car muffler produces sound to counteract noise from the engine. B. Ocean waves hit the shore and are absorbed by the beach. C. Noise-canceling headphones eliminate background noise by producing sound that is out of phase with the background noise. D. Light shown through two thin slits creates a diffraction pattern.

Assessment 2. Describe how two wave pulses can combine destructively.

Assessment 2. Describe how two wave pulses can combine destructively. If an “up” pulse traveling to the left and a “down” pulse traveling the right reach other, they will interfere destructively. If they have equal amplitudes, they will cancel each other out. The cancellation is temporary. In the next instant they will reappear and continue on as before.

Assessment 3. In your own words, describe the difference in appearance between a wave with a single frequency and a wave composed of more than one frequency.

Assessment 3. In your own words, describe the difference in appearance between a wave with a single frequency and a wave composed of more than one frequency. Single frequency waves can be represented as simple sine waves. Multi-frequency waves are more complex in appearance; a multi-frequency wave typically has peaks within peaks, the repeating waveform a more serrated shape. giving