Shadows, Diffraction, and Observation

My supervisor observed my Honors class yesterday.

My students had asked for a lesson on one of our advanced standards:

• 9.4 Advanced WPM I can draw ray and wave diagrams to explain shadows, diffraction, and interference.

Their reasoning for asking for this particular standard was that it seemed "hard," as well as interesting, and they wanted a more formal lesson on it, while some of the other advanced standards they felt they could learn more independently. (This quarter, they can choose up to five advanced standards to assess, rather than me choosing which advanced standards everyone will work on. For the honors students, this seems to motivate them to be less passive about their learning.)

I used a variation on this presentation (transformed into PowerPoint just in case the wireless network went down) as the framework for the lesson. Their Do Now/Opener was to write down what a shadow is. After soliciting students' definitions, we got into the distinctions between umbra and penumbra, related the new ideas to ideas they'd encountered in earlier years (though there was some resistance to thinking farther back than the first time I mentioned the words in a vocabulary list) as well as to eclipses and the shadows around openings like doors. (I wish I had the time to spend a few weeks on this, so that the model they built was truly theirs, but class end on June 12th...)

Then we talked about the shadows around and the light passing through really small gaps. A student passed out World's Cheapest Diffraction Slit (~1in razor slit in a piece of tissue box) to everyone in the room, and we looked at a really bright light bulb. Students were engaged and interested. I heard comments of general enthusiasm as well as good observations about the colors and the orientation of the diffraction pattern when the slit was rotated. Students recorded qualitative observations and then went to computers to play with PhET's Waves & Interference simulation. Most of the students found several different patterns in the light on the screen and the intensity graphs, although there were significant problems with some of the computers which cut into the time-on-task for a couple of the groups. Their homework was to finish finding three different patterns if they had not already done so.

Today, we will whiteboard and discuss their findings and go back to our brief discussion of what happens when waves overlap (from the slinky activity we did when we talked about wave anatomy) in search of an explanation that fits what we observed.

Most of the students easily picked up the vocabulary introduced in the beginning of the lesson, using 'umbra' and 'penumbra' accurately to describe the shadows in the second situation introduced. Most students also noticed the spreading of the light wave after it passed through the single slit. The whiteboards they create today will be their second formative assessment for this standard. They will take a quiz on (among other topics) drawing ray and wave diagrams for a single slit tomorrow.