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SF Physics 26 – Solar Bag

IMG_5934This was the last class for this year so we headed to the park with the All About Air Classroom Kit from Steve Spangler Science.  I had used the windbags in the high school class to demonstrate Bernouille’s principle – you can inflate a 2 meter long bag with one breath by taking advantage of this principle.  We started out with the windbags and I asked the students to put three big breaths in the windbags and then estimate how many breaths it would take to fill them.  Then I demonstrated how to do it with just one breath, hold the opening of the bag wide open about 10 inches from your mouth and blow a steady stream of air into the bag.  Increasing the speed of the air around the mouth of the bag lowers the pressure causing the air around the mouth of the bag to rush into the lower pressure area filling the bag!  IMG_5938

The solar bag is basically a black garbage bag that is 15 meters (50 ft) long!  We took it to the middle of the park and filled it with air, using the wind instead of blowing and then tied off both ends.   As the bag is warmed by the sun, the air inside will move at faster and faster speeds causing the bag to expand, giving the air a greater volume and therefore a smaller density than the surrounding air and the bag will float!DSCF9084.jpg

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This kit worked pretty well.  My only complaint is that the string provided in the kit to tether the solar bag (required!) is really cheap thin cotton string and it snapped on us a couple of times.  Luckily, we tethered both ends so we did not lose the bag.  If I had noticed the quality of the string before arriving at the park I would have used my own.  We did end up with a small hole in the solar bag at one point, but I always have duct tape in my backpack and quickly patched up the hole.  You have to do this in the morning when its still cool and it has to be a sunny day of course.  We had trouble getting the bag to go very high by 11 am because it was already getting too warm outside.  I also don’t think we put quite enough air inside the bag before closing it, but that kind of kept it more manageable.

 

 

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R2 Physics 29 – Lenses

We did another set of optics experiments this week, this time using lenses.  Many years ago I bought an Optics Discovery Kit from the Optical Society of America (OSA), which is a great little kit that comes in an plastic box very similiar to old VHS tape boxes.  Included in the kit are lenses, a bit of fiber optic, polarizers, a diffraction grating, a hologram and instructions for a number of experiments.  This kit is perfect for a homeschool family studying physics and I’m happy to say its still available – follow this link to purchase the Optics Discovery Kit.  This kit does NOT come with an optics bench but you don’t really need one.

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I have a few other sets of lenses so was able to put together three lab setups for class but basically followed the instructions that come with the kit.  Students used the lens to focus an image of a window (far away object) and measured the distance of the focused image from the lens to find the focal length of the lens.  They also measured the magnification produced by one of the lens by dividing the image size by the object’s actual size.

One group also proved the lens equation by measuring the image distance, i,  for a couple of different object distances, o.  They had already measured the focal length, f,  and could check to see if  1/f = 1/o + 1/i, which it did.

The optics bench in the photos above was the cheapest one I could find, and it shows when you try to use it.    Its nice for being able to slide the components along the bars and there is a ruler along the side (far side in the photo so you can’t see it) but the screws that hold the posts don’t tighten very well which can be annoying.  I’m really not that happy with it so I’m not going to provide a link.  I also have two meterstick optical bench kits which are really cheap, only $15 or so, if you already have your own meterstick.  These do NOT come with any optics so you have to purchase a few lenses. You can see the meterstick with stands in the photo below, but the plastic lenses in the photo are from the Optical Discovery Kit.  I did not take a picture of the third set up.  For our light source/object, one group used an iPhone and the other two groups used the LED Light Blox with wax paper taped over the end and in the case below, the number 4 draw on it.  This made it easier to focus the image. If you just tried to focus on the bright bulb it was difficult to tell when it was in focus, and you couldn’t really tell if the image was upside down or rightside up.  You could use any flashlight for this.

When students finished with the labs, they did some ray tracing worksheets for lenses.  Here’s a good webpage describing ray tracing for lenses by Hyperphysics at Georgia State University.

SF Physics 25 – Polarization

This video by Physics Girl is pretty much exactly what we did in class today.

I gave students 2 polarizers and they did a few different activities.  The first one was to look at the room lights through one of the polarizers while rotating it….. nothing really happens.  But when they looked at my iMac screen which gives off polarized light they noticed it went totally black when they had the polarizer in a particular orientation.  They also looked through 2 polarizers at the room lights and observed that there configurations of the two polarizers (crossed polarizers) where no light got through.

Then they stuck some packing tape (regular scotch tape does not work) on a transparency or a glass microscope slide and placed it between the polarizers.  I had a plastic ziplock bag on the table and on a whim stuck it between the polarizers. It was actually pretty interesting so I stuck some tape on it as well.  You can see in the photos below that the bag is orange/purple between the crossed polarizers and you can get a range of colors with the layers of tape.  The photo on the right is the bag with no polarizers… very  boring.  The video above explains how the orientation of the light rotates as it passes through some materials and the amount of rotation can change depending on the wavelength – hence the difference colors making it through different thickness of tape at the right orientation to pass through the second polarizer.

The other cool thing we put between crossed polarizers were the acrylic lens we used in previous labs.  When held on edge in between the crossed polarizers you can get a beautiful blue in the center (thickest) part of the lens and reds/yellows near the edge.  The colors fade as you turn the lens with respect to the polarizers, the most intense colors were with the lens at about 45 degrees to the axis of the polarizer.

You can do these experiments at home with out the polarizer films, as long as you have a pair of polarized sun glasses and a LCD computer screen or TV.  The light coming off the computer monitor/TV is polarized and that eliminates the need for one film and the polarized glasses work as the second film.  Put on your glasses and look at the computer, if you tilt your head and the screen goes dark or gets brighter than you are good to go.  Get some packing tape and put a few pieces overlapping on a ziplock bag and hold it up to your computer screen with the glasses on.  It helps to have a the computer set to show a primarily white screen – change your background or open a new document and leave it blank.  The photos of the lens above were taken using my iphone with my sunglasses and iMac.  Pretty much any transparent plastic items will do this – plastic forks, cups, etc.

IMG_5816One of my students got the Laser Maze game by ThinkFun and brought it to class.  Its a one player puzzle type game.  It comes with a deck of cards, each one giving you some initial layout for the board and tells you which pieces you may add to get the laser to the target pieces.  ThinkFun has a lot of single player puzzle games like this which are great for critical thinking and this particular one also lets kids gain some practice with mirrors and beam splitters.  It looks like ThinkFun also has a 2 player version called Laser Chess!

 

R2 Physics 28 – Optics

Students read Chapter 14.1 Cameras, in How Things Work the Physics of Everyday Life and watched the following video:

For the lab, there were three different ray tracing activities.  The first involved tracing an acrylic lens like the ones in the video above and using a light blox to trace the rays as they enter and exit different lens.

The second activity was to look at an image in a flat mirror and trace line of sight for the image and discover from measurements that the distance between the mirror and the object was about the same as the distance between the mirror and the image.  They also found that the angle of incidence and the angle of reflection were equal. The dashed lines show where the light appears to be coming from, but since it doesn’t actually pass behind the mirror, we draw them as dashed lines. I’ve described this lab before, Physics 026-Ray Optics and a detailed lab handout for this activity can be found

Lastly, while kids waited for a mirror or a light blox, they did some ray tracing worksheets for curved mirrors.  I printed out a list of ray tracing rules that go along with the worksheet. The worksheets can be found at AP PHYSICS – Mirror Lab.  This link will download the lab as a pdf.

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SF Physics 24 – Light Technology

Today we covered the last chapter in Science Fusion Module J, Light Technology.  We started class by watching the following videos:

I didn’t show this last video by Physics Girl in class, but its got a nice demo to show how fiber optics work just using a plastic bottle, water and a laser pointer.  I had some actual fiber optics so I used those in class but if we had had extra time I would have pulled out a bottle to do this demo.

Students used snap circuits to build some circuits using  LEDs and fiber optics.  I also had out our fluorescent rock collection and short ultraviolet (UV) light to show how the incident UV light causes the electrons in the minerals to become excited and when they relax they emit visible light.

I also had a HeNe laser and LED light blox out and talked about how the laser light was different – collimated beam, the light is polarized (all wiggling in the same direction) and its coherent (all the wiggles are lined up or in-phase with each other and make interference patterns).

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Lastly, my son has night vision goggles  and I let the students explore with them a bit.  The goggles we have use active illumination and emit infrared (IR) light which you can ‘see’ using your smartphone camera.  In the photo below left, the night vision goggles are off and this is what it looks like when they are on as well. But if you look at little bulbs on the side when the goggles are on using the camera on your phone you see they look violet (photo below right).  The camera sensor is more sensitive to IR than our eyes so its able to pick it up.

Since we finished the books and still have a few weeks left in the semester I think we’re going to do a bit of astronomy to finish up the year.

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