1. Explain 3 ways the fundamental frequency of a string can be altered. Do now: Paraire, 15 Kohi-tātea 2016 1)Changing the length Shorter string = shorter λ = higher f 2)Changing the tension force in the string Tighter string = faster wave speed = higher f 3)Changing the mass per unit length of the string Heavier string = slower wave speed = lower f

2. This Week’s Learning: Reminders: 1)Standing Wave H/W is due – solutions up 2)Intereference this week – test next week Interference of sound and E/M waves -multi-slit interference, diffraction gratings -key concept: path difference -explain rather than describe and calculate -Textbook Chapter 4 (Activity 4B, 4C)

3. Play with wave interference and sound (2 taps dripping, then light)

4. 4 2 Source Interference  Having 2 sources of concentric waves will produce a pattern like this  There appear to be lines radiating out from between the sources

5. 5 2 Source Interference  Anti-nodal lines are lines of constructive interference. ie the water is choppy  Nodal lines are lines of destructive interference. ie the water is flat

6. 6 Revision Exercise:  The n value is called the “order number”  It tells you how many wavelengths further one wave has travelled compared to the other  n = integer: antinode (constructive interference)  n = half-integer: node (destructive interference) n=0 n=1

7. Constructive antinodal lines: Destructive nodal lines: pd = n λ pd = (n – ½) λ

8. 8 2 Source Interference  If the waves were sound, what would a person walking from A to B hear?  A series of loud and soft noises as they moved across the antinodal and nodal lines A B