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14.3 W ave Interactions. Reflection Waves bounce back when they meet a surface or boundary (movie: reflection) At a boundary free to move, the wave reflects in same shape. At a boundary that doesn’t move (fixed), the wave is inverted.
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14.3 Wave Interactions • Reflection • Waves bounce back when they meet a surface or boundary • (movie: reflection) • At a boundary free to move, the wave reflects in same shape. • At a boundary that doesn’t move (fixed), the wave is inverted. • This is what happens for mirrors (light) and echoes (sound)
Diffraction • When waves pass the edge of a boundary, new waves are made. • This makes it look like the waves bend around the boundary. • why we can hear things we can’t see
Refraction • Waves also bend when they go from one medium to another • (movie: refraction) • That’s how eye glasses and magnifying glasses work (light)
Interference • Waves combine to make a new wave • Draw them on top of each other to see new wave • Constructive interference • Crest/troughs or compressions/rarefactions line up • Amplitude increases • Destructive interference • Crest/troughs or compressions/rarefactions don’t line up • Amplitude decreases, or can completely cancel out. • (movie: comparing const/destr interference)
Interference • Constructive interference • Destructive interference
Interference • For light waves, it can cause different colors (e.g. bubbles, oil spills) • For sound, it causes the vibrations to happen at same time (beats)
Standing Waves • The wave vibrates so that it looks like it’s standing still or flip- flopping back and forth. • Really waves are reflecting back and traveling in both directions at once. • (movie: standing waves) • Places where there are no vibrations (movement) are called nodes. Here there is perfect destructive interference, the waves have cancelled each other out. • Places of maximum vibration are called antinodes. Here there is perfect constructive interference and the waves have added together.
Standing Waves • This can only happen at certain wavelengths, related to the length of the medium. • More about this in • the next unit on sound • How many wavelengths of each are there to the left?