# Questions of the Day: The water wave shown has a period of 2.0 s. a/ What is its speed? b/ How long does it take to travel a distance equal to one wavelength?

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Questions of the Day: The water wave shown has a period of 2.0 s. a/ What is its speed? b/ How long does it take to travel a distance equal to one wavelength? 9.0 m 1.5 m Given: A = 1.5 m = 6.0 m T = 2.0 s Unknown: v=? v= f or v= d/t v= 6.0/2.0 v= (1/2)  v= 3.0 m/s  T of a wave is the time to travel 1.   is the distance a wave travels in 1T. 6.0 m

Thought questions: Why can bass be heard more around corners than treble? Why are sound waves able to “get around” corners, but light waves are not? Why does AM radio reach to areas behind hills better than FM? And just where do photos of stars get those cool spikes???

Diffraction – the _____________________into the region behind a___________________________________________. Diffraction is a property of ____________ The waves can be: 1. _________________ 2. _________________ 3. _________________ 4. _______________________________________ Diffraction is only significant when _______________________________________________ _______________________________________________ > d or  ~ d spreading of waves barrier, obstacle or an opening waves. sound light water electrons or other particles the wave's is large compared to the size d of the obstacle or opening.

Ex. Plane waves incident on an opening: plane waves This is the area “behind” the opening This is the area “behind” the opening ___ d, so the wave ________________. d opening  w is different behind the opening? v diffracts It isn't. > d >

Ex. Waves incident on an opening: plane waves d _____ d, so _____________diffraction occurs. This is the area “behind” the opening This is the area “behind” the opening  w is different behind the opening? little It isn't. << << d

obstacle area behind obstacle around a corner diffracted waves Ex. Plane waves diffracting ________________________

Which example shows diffraction? A B ____ d __ d  ______________ d d no bending into shadow bending into shadow Shadow of obstacle << diffraction ~ Shadow of obstacle

Which case below will result in more diffraction? A B > d < d more diffraction less diffraction

Diffraction is explained by thinking of _______________ within the opening as a __________________ that creates new ___________________ The diffracted wavefront is the ____________________ of all of these wavelets from all the ________________ within the opening. This idea was first explained by _________________________ each point point source "wavelets." "envelope" Christian Huygens. point sources Huygens plane waves diffracted waves envelope

diffraction through an opening…

AM vs FM: which diffracts more? Is that good or bad for reception?  typical AM frequency: _____________________  typical FM frequency: _____________________ Because ________ diffracts more, it can _________________ obstacles, so there are less ___________________________. AM  ________ f  ___________  _________ diffraction 1000 kHz = 1 x 10 6 Hz 100 MHz = 1 x 10 8 Hz lower longer more AM bend around "shadow" zones AM: FM:

So why does sound diffract ____________ than light? And why do bass sounds diffract _________ than treble? typical sound :____________________ typical visible light : __________________ Longer ’s diffract______________. typical bass : ____________ typical treble : __________ Longer ’s diffract ___________. Light can be made to diffract by making ________________________________________. more 10 cm – 1 m 10 -6 m – 10 -7 m more the opening or obstacle very small more 1 m 10 cm more

support vane mirror business end of telescope Rigel diffraction spikes Light diffracts around the sharp edges of the support vanes and gets magnified in the eyepiece. Diffraction is most noticeable when the source is a single point.

The Hubble Ultra Deep Field: The same piece of sky that you would view if you looked through an eight- foot-long soda straw. There are 4 stars in the photo, the rest are galaxies (10.000 of them). The stars are all in our galaxy. How can you tell the stars from the galaxies? The diffraction spikes

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