From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Feb 16 2005 - 09:43:40 PST
Message-Id: <l03110794be3933420588@[192.168.112.4]> Date: Wed, 16 Feb 2005 09:43:40 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Energy & waves
Hi Nathania
The summary for light interference is:
When the darks get darker the lights get lighter.
It is the total energy that is conserved not just the energy that hits a
wall in one spot.
So when you have light go through a single slit it spreads into a narrow
band across a distant wall. (The band is perpendicular to the slit.)
Open a second slit and darkness appears on the wall where waves from one
slit interfere with waves from the other. However, while it's very easy to
see the dark bands appear it is harder to see that the bright places became
brighter, because in those places there is constructive interference.
Average over the total interference pattern and you get the energy coming
through 2 slits.
Part 2
single slit interference
Luckily you didn't ask the truly hard question of why a beam propagates in
a straight line when there is no slit at all!
Take a single slit of width d.
Put a laser beam through the slit observe the diffraction pattern on a
distant wall.
Now make a new pair of slits of width d/10. place one of these slits where
the left edge of your single slit was, place the other one just right of
the center of where the single slit was.
Put the laser beam through the two slits and notice an interference pattern
on the wall with similar spacing between the maxima when compared with the
single slit pattern.
Notice that you can look at the single slit and notice that it has open
space where each of these two slits are. So the two slits are a sub part of
the big slit.
Now create two new slits d/10 wide. Put one of these into position just to
the right of the previous two slits. One just d/10 to the right of the left
wall one just d/10 to the right of center. Notice the pattern on the wall
is a double slit pattern shifted just d/10 to the right.
repeat this 5 more times. So the pattern you get from one slit is similar
to the patterns of pairs of double slits.
(Of course I ignored the sums from interference of pairs of slits which is
what gives the single slit pattern its broad maximum in the middle.)
Paul D
>I have been teaching my students about constructive and destructive
>interference, and the following question comes up again and again. What
>happens to the energy during destructive interference?
>
>We have talked about mechanical waves, including sound, as well as light
>waves (just did Young's double slit experiment) and I would like to be able
>to give an explanation that works across all these media/wave types.
>
>Also, I'm still personally unhappy at a gut level about the fact you can get
>interference from a single slit. I have read QED - The Strange Theory of
>Light and Matter, but I still don't like it! Any suggestions? Thanks!
>
>Nathania Chaney Aiello
>Mission San Jose HS
>Fremont, CA
>
>
>---------------------------------------------------------------------------
>
>To unsubscribe from pinhole, send an email to requests@exploratorium.edu
>with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
>of the email.
>
>To subscribe to the digest and only get 1 combined message a day, send an
>email to requests@exploratorium.edu with the words 'subscribe digest
>pinhole' (without the quotes) in the SUBJECT of the email.
>
>Check out what your colleagues have written on Pinhole in the Pinhole
>archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
>---------------------------------------------------------------------------
This archive was generated by hypermail 2.1.3 : Mon Aug 01 2005 - 16:06:47 PDT