Diffraction of Water Waves. Pg

Transcription

1 Diffraction of Water Waves Pg

2 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

3 Practice 1. what do you think will happen if you change the width of the slit but keep the wavelength fixed? 2. what do you think will happen if you change the wavelength of the wave but keep the width of the slit fixed? 3. When does diffraction become more noticeable?

4 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)

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

6 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

7 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?

8 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

9 Interference of Water Waves Pg

10 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

11 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

12 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

13 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

14 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 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

15 Practice 1. What happens to the pattern when the frequency increases? 2. What happens when the distance between the sources increases?

16 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 P n S 1 P n S 2 = ( n ½) Textbook: pg. 468, #1-7 Worksheet two point interference