1. Diffraction of Water WavesPg

2. Properties of Waves
In order to study the properties of waves, a ripple tank is used
A shallow, glass-bottomed tank on left
~2cm of water is added
Light from a source above the tank passes through the water and illuminates a screen on the table below
The light is made to converge by waves crests and diverge by wave trough, creating bright and dark areas on the sheet of paper located below the tank

3. Diffraction
If you observe straight wave fronts in a ripple tank, you can see that they travel in a straight lines if the water depth is constant and no obstacles are in the way
If the waves pass by an edge of an obstacle or through a small opening, the waves spread out

4. Diffraction
Diffraction is the bending of a wave as the wave passes through an opening or by an obstacle
The amount of diffraction depends on the wavelength of the waves and the size of the opening

5. Practice
1. what do you think will happen if you change the width of the slit but keep the wavelength fixed?
As the slit width decreases, the amount of diffraction increases
2. what do you think will happen if you change the wavelength of the wave but keep the width of the slit fixed?
As the wavelength decreases (or the frequency increases), the amount of diffraction deceases
3. When does diffraction become more noticeable?
If waves are to undergo more noticeable diffraction, the wavelength must be comparable or greater than the slit width ---- for small wavelengths (like visible light), you need to have extremely narrow slits to observe diffraction

6. Diffraction Occurs for sound and light waves as well
However, we study the phenomenon first for water waves because their long wavelength allows for easier observation of the effects of diffraction

7. Diffraction (note)
The bending of a wave as the wave passes through an opening or by an obstacle
Occurs for water, sound, light…..
Depends on the size of the opening and the wavelength of the wave
As the wavelength increases, the amount of diffraction increases
As the size of the slit increases, the amount of diffraction decreases
becomes noticeable when the wavelength > slit (or wavelength/slit >1)

8. Practice
4. Explain the difference between the diffractions observed.

9. Diffraction & Sound
The relationship between wavelength, width of the slit, and extend of diffraction is similar so sound waves
You can hear the sound through an open door, even if you cannot see what is making the sound
The primary reason sound waves diffract around the corner of the door is that the sound waves have long wavelengths compared to the width of the doorway

10. Practice
5. Why do lower frequency sounds diffract more than higher frequency sounds?
6. If a marching band is approaching on a cross street, which instrument will you hear first, a high pitched piccolo or a low pitched bass drum?

11. Practice
7. Bats use echolocation to detect and locate their prey – insects. Why do they use ultrasonic vibrations for echolocation rather than audible sound?
Textbook page 461, #1 , 2

12. Interference of Water WavesPg

13. Interference
When two waves cross paths and become superimposed, they interact in different ways
The interaction between waves in the same medium is called interference

14. Interference
If the crest of one wave coincides with the crest of the other, then the waves are in phase and combine to create a resultant wave with an amplitude that is greater than the amplitude of either individual wave
Called constructive interference
The point where the constructive interference occurs is referred to as an anti-node

15. Interference
For two waves that differ in phase by ½ wavelength (or 180 degrees), the crest of one wave coincides with the trough of the other wave
The two waves combine and create a resultant wave with an amplitude that is smaller than either of the individual waves
Called destructive interference
The point where the destructive interference occurs is referred to as the node

16. Interference
In order for interference to occurs, the waves must have the same frequency and a fixed phase difference
Waves that meet this condition are said to be coherent

17. Interference (note)
Phenomenon that occurs when two waves in the same medium interact
Waves must be coherent (same frequency and a fixed phase difference)
Two types:
Constructive – waves have displacements in the same direction
Destructive – waves displacements in opposite directions

18. Two-Point Interference
In the diagram below, the two point sources are coherent (i.e. same frequency and phase) and have equal amplitudes
Successive wave fronts travel out from the two sources and interfere with each other, producing a symmetrical line pattern of constructive and destructive interference

19. Two-Point Interference
The pattern includes lines of maximum displacement (anti-nodal lines), caused by constructive interference, separated by lines of zero displacement (nodal lines), causes by destructive interference

20. Practice
1. What happens to the pattern when the frequency increases?

21. 2. What happens when the distance between the sources increases?

22. Two-Point Interference – the math
You can measure wavelength using the interference pattern produced by two point source and develop some mathematical relationships for studying the interference of other waves Textbook: pg. 468, #1-7 Worksheet – two point interference
|PnS1 – PnS2| = ( n – ½)