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)

Reflection

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

• For light waves, it cancause different colors (e.g. bubbles, oil spills)

• For sound, it causes the vibrations to happen at same time (beats)

Interference

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 manywavelengths of each are there to the left?