1. Apply superposition principle

2. Differentiate between constructive and destructive

inference.

3. Predict when a reflect wave will be inverted.

4. Predict whether specific traveling waves will produce a

standing wave.

5. Identify nodes and antinodes of a standing wave.

12.4 Wave interactions

Wave Interference• A phenomenon which occurs when two WAVES MEET while

traveling along the same medium. • The interference of waves causes the medium to take on a

shape which results from the SUPERPOSITION of the two individual waves.

The two waves meet, produce a

net resulting shape of the

medium, and then CONTINUE on

doing what they were doing

before the interference.

Constructive interference• Occurs where the two interfering waves have a displacement in

the same direction. The result is a LARGER AMPLITUDE.

2 units1 unit

-2 units-1 unit

MAXIMUM constructive interference occurs when the waves are in PHASE (phase difference is 0o or 360o) and crest superposes on crest or trough on trough.

The point of maximum displacement of a medium when two waves are interacting is called an ANTI-NODE.

Destructive interference• Occurs where the two interfering waves have a displacement in

the opposite direction. Destructive interferences result a SMALLER amplitude.

• Maximum destructive interference occurs when two waves of equal frequency and amplitude whose phase difference is 180o or ½ λ meet at a point. Maximum destructive interference results in the formation of NODES. Which are regions of ZERO displacement of the medium

Constructive Destructive

principle of superposition• When two waves interfere, the resulting displacement of the

medium at any location is the ALGEBRAIC SUM of the displacements of the individual waves at that same location.

Displacement of Pulse 1

Displacement of Pulse 2 = Resulting

Displacement

+1 +1 =

-1 -1 =

+1 -1 =

+1 -2 =

+2

-2

0

-1

Example #1Determine type of interference of each section as constructive or destructive.

I

II

III

Example #2Apply superposition principle to determine result of interference by sketch the resultant wave.

CLASS WORK – today’s date1. Two waves having the same amplitude and the same

frequency pass simultaneously through a uniform medium.  Maximum destructive interference occurs when the phase difference between the two waves is

a.   0° c.  90° b.   180° d. 360°

2. The diagram shows two pulses, each of length, traveling toward each other at equal speed in a rope. Which diagram below best represents the shape of the rope when both pulses are in region AB?

a.

b.

c.

d.

3. Maximum constructive interference between two waves of the same frequency could occur when their phase difference is

a. 1λ b. ¼ λ c. ½ λ d. 1 ½ λ

4. The diagram below represents shallow water waves of wavelength λ passing through two small openings, A and B, in a barrier. How much longer is the length of path AP than the length of path BP?

a. 1λ b. 2λ c. 3λ d. 4λ

5. The diagram below represents shallow water waves of constant wavelength passing through two small openings, A and B, in a barrier. Compared to the length of path BP, the length of path AP is how many wavelength longer?

6. Which statement best describes the interference at point P?a. It is constructive, and causes a longer wavelength. b. It is constructive, and causes an increase in amplitude. c. It is destructive, and causes a shorter wavelength. d. It is destructive, and causes a decrease in amplitude.

7. The diagram shows two sources, A and B, vibrating in phase in the same uniform medium and producing circular wave fronts.  Which phenomenon occurs at point P?

a. destructive interference b. constructive interference c. reflection d. refraction

8. Determine the interference pattern

Reflection of a PulseFixed Point Floating Point

after

before

after

before

The reflected pulse is INVERTED. This inversion can be explained by Newton's third law of action-reaction.

The reflected pulse have the SAME DIRECTION as the incident pulse.

Changing MediumsFast Slow

after

before

Slow Fast

after

before

less dense denser denser less dense

• The wave speed is always GREATEST IN THE LEAST dense rope.

• The wavelength is always GREATEST IN THE LEAST DENSE rope.

• The frequency of a wave is NOT ALTERED by crossing a boundary.

• The reflected pulse becomes INVERTED when a wave in a less dense rope is heading towards a boundary with a denser rope.

• The amplitude of the incident pulse is always greater than the amplitude of the reflected pulse.

Characteristics of transmitted pulse and reflected pulse

• A pulse moves from a very thick rope into a thin string. Circle the term that makes the statement true.

a. The transmitted pulse will lose / gain amplitude. b. The transmitted pulse will lose / gain speed. c. The transmitted pulse will lose / gain energy. d. The reflected pulse will / will not come back on the opposite

side.

Example

Two sources in phase in the same medium

Constructive: Point A, B are anti-nodesDestructive: Point C, D, E, F are nodes

creststroughs

Standing Waves• A WAVE PATTERN what results when two waves of the SAME

frequency, wavelength, and amplitude travel in OPPOSITE DIRECTIONS and interfere

• A standing wave pattern is formed as the result of the perfectly timed interference of two waves passing through the same medium. A standing wave is NOT actually A WAVE; rather it is the PATTERN.

Nodes and anti-nodes in a standing wave

Nodes: the points of ZERO displacement of the resultant wave

Antinotes: the points of MAXIMUM displacement of a medium

The distance between two successive nodes is ½ λ

standingWaveDiagrams1/StandingWaveDiagrams1.html

• Standing wave patterns are only created within the medium at SPECIFIC FREQUENCIES OF VIBRATION. These frequencies are known as HARMONICS.

• ..\..\RealPlayer Downloads\Standing Wave on a String.flv

• Standing waves can be created for both transverse and longitudinal waves.

• pipe-waves.html

1st harmonic

2nd harmonic

3rd harmonic

Harmonic # of Nodes # of Antinodes Pattern λ

1st 2 1

                                                      

2L

2nd

                                                      

L

3rd

                                                      

2/3 L

4th

                                                      

½ L

5th

                                                      

2/5 L

6th

                                                      

1/3 L

nth n + 1 n --

Standing waves in water• Standing waves in water are produced most

often by periodic water waves REFLECTING FROM A BARRIER.

Example #1

• What is the number of nodes and antinodes in the standing wave shown in the diagram?

8 nodes

7 antinodes

Example #2The diagram represents a wave moving toward the right.                    

Which wave shown below could produce a standing wave with the original wave?

1 2 3 4

Example #3• Two waves traveling in the same medium and

having the same wavelength (λ) interfere to create a standing wave. What is the distance between two consecutive nodes on this standing wave?

1. λ 2. ½ λ3. ¼ λ4. ¾ λ

Class work – today’s date

1. Sketch a pulse that shows the superposition of the pulse pairs below.

2. A pulse with a height of +0.5 meter encounters a second pulse with a height of +2.3 meters.

a. The two pulses interfere: ___________________________

b. The resulting height of the medium when the pulses interfere will be: ______________ m

3. A pulse with an amplitude of 0.4 meter moves from a thick, heavy cord where its speed is 2.0 meters per second into a much thinner string. The speed and height of the pulse after it is transmitted could be:

a. Amplitude = 0.5 m; Speed = 1.0 m/s b. Amplitude = 0.3 m; Speed = 1.0 m/s c. Amplitude = 0.5 m; Speed = 3.0 m/s d. Amplitude = 0.3 m; Speed = 3.0 m/s

4. Which of the following is transmitted by a pulse? (1) energy and mass (2) mass only (3) energy only

5. The energy contained in a pulse is related to its: (1) amplitude and speed (3) width and speed (2) amplitude only (4) speed only

6. As pulses travel they lose: (1) amplitude and speed (3) width and speed (2) amplitude only (4) speed only

7. A pulse moves from a very thick rope into a thin string. Circle the term that makes the statement true.

(a) The transmitted pulse will lose / gain amplitude. (b) The transmitted pulse will lose / gain speed. (c) The transmitted pulse will lose / gain energy. (d) The reflected pulse will / will not come back on the opposite side.

8. Sketch the superposition of the following sets of pulses.

9. The grid below represents a 10.0 meter long string.

a. Sketch the standing wave that this string would produce if it were to have SIX nodes.

b. Draw a circle around each ANTINODE on the string. c. Determine the wavelength of this standing wave.

_________________m d. Assuming that this wave moves at 2.0 meters per second,

calculate its frequency and period.

10. Two point sources produce a pattern of overlapping circular waves. The solid lines in the diagram represent wave crests while the dotted lines represent wave troughs. Mark a “C” in the boxes that indicate constructive interference and a “D” in the boxes that indicate destructive interference.

11. Sketch a wave that will completely destructively interfere with the wave shown below. What is the phase difference between these two waves?

12. At the point when the two waves shown below completely overlap, what will the superposition of the two waves look like? Draw a sketch of the wave produced during this interaction.

13. A standing wave is produced as a result of a combination of _______________ and _______________________. The main features of standing waves are: ______________________ at which minimum motion of the medium occurs. _______________________ at which maximum motion of the medium occurs.

14. Determine the wavelength of the standing wave shown below. Identify one node and one anti-node.

15. A (NODE/ANTINODE) is the result of an alternating phase difference of 0 and 180 degrees between two waves passing through each other. (Circle one)

16.To produce a standing wave, two waves must: o be moving in _______________________________ o have the same: _____________________________ _____________________________ _____________________________

17. The grid below represents a 10 meter long string.

– Sketch the standing wave that this string would produce if it were to have SIX nodes.

– Draw a circle around each ANTINODE on the string.– Determine the wavelength of this standing wave.

_________________m– Assuming that this wave moves at 2.0 meters per second,

calculate its frequency and period.