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# homeschoolsciencegeek

### Middleschool

I decided to spend another day on mirrors and had the students look at images in flat mirrors and do some ray tracing with real mirrors.  I’ve already described this lab in detail a few years ago so I’m just going to link to that post, Physics 026 – Ray Optics.  I also had students look at concave mirrors and use them to project a real image on a sheet of to find the focal length of the mirror – also described in post mentioned above.

At the end of class we watched  Physics Girl explain why mirror flip images horizontally:

And we watched Crash Course Astronomy on telescopes.

We started class by watching Doc Schuster’s video on Geometric Optics Intuition.

I have a set of 6 mirrors (convex and concave) that students used in Science Fusion lab Spoon Images (in the online resources).  As the title suggests you can use spoons for this lab since the spoon can be used as concave mirror (looking into the ‘bowl’) or a convex mirror (looking at the ‘bottom’ of the spoon).   I also stepped the students through ray tracing for a concave mirror with the image at various locations and we discussed whether the images were virtual or real, upside down or rightside up and whether or not the image size was different than the object size.    If you can project the image onto a piece of paper then the image is real, but if you can’t project it, like your image in a flat mirror, then its a virtual image – it just appears to be behind the mirrors but there is no light going behind the mirror.

We finished early and there were only 4 students today so I let them play Jishaku which uses very strong magnets and Laser Battle, which involves strategically placing mirrors to reflect light on to your enemie’s tower.  Laser Battle is very similar to one of the Science Fusion labs where students make a maze in a cardboard box and then have to use mirrors to bounce light (bright flashlight or laser pointer) through the maze.

This is one of my favorite topics to teach.  I’ve already written about the physics of why the sky is blue a few years ago so I’m just going to link to that old post.  We spent some time at the beginning of class talking about refraction and headless polar bears.  This is the same effect that causes your straw to look bent in a glass of water – the light bends different amounts when its going through air/glass/air (head of the bear)  or water/glass/air (body of the bear).  Its just more dramatic in the polar bear photo because the glass holding back the water (and the bear) is very very thick.

I like using this example below with a shopping cart to explain why light bends when it goes from one material to another. The cart is traveling on smooth pavement and going from bottom right to top left.  But when the cart encounters the grass it slows down.

The top right wheel hits the grass first so it slows down, while the other wheels are still on pavement and going at the faster speed, this causes the cart to change direction.  By the time all four wheels are on the grass, going the slower speed the direction of the cart has changed.   This is the same thing that happens to light waves as they cross from one medium to another.  In going from air to glass, the light slows down and changes direction.  How much it bends depends on the difference in the speed of the light waves in each material, hence light going from the polar bear’s head in air, through glass and back out in air, is bent a different amount than the light coming from its body which starts in water instead of air.

Science Fusion Module J has a lab, Refraction with Water that we did in class.  Students fill a beaker (a smooth clear glass would work fine) half way with water and observe a pencil and spoon when placed inside the beaker. Students notice right away that the pencil and spoon look bigger below the water line and they change the apparent size of the spoon by moving it inside the glass.  We talked about how the curvature of the beaker was acting like a lens and they did not see the magnification if they placed the spoon in the rectangular water tank since the sides are flat.

The other Science Fusion lab students did was Comparing Colors of Objects in Different Colors of Light.  Last week, we had discussed why a blue ball looks blue – because it reflects blue light and absorbs the other colors (wavelengths).  So today I made a viewing box out of  shoe box – a whole to peek through and a hole in the top to shine a flashlight.

Students had to predict what four different colored objects, (white, black, red and yellow) would like under different colored light.  Then they put the objects in the box, one at a time, and placed a different filter (red, blue, green) over the hole in the box so the item was illuminated with red, blue or green light. The photos below show the view in the box for the yellow object. With no filter it appears yellow as predicted, but red light makes it look a bit orange, blue light makes it a teal color (according to my students) and the green light makes it a yellowish-green.  You don’t need fancy filters to do this experiment, you can use colored cellophane.  The point of this lab is to show that the apparent color of an object depends not only on the object but also on the wavelength of light that is shining on it.

We had a bit of time at the end of class and watched the following videos:

We started the unit on Light (Science Fusion Module J) today with a slide show on the electromagnetic spectrum.  I actually found this slideshow (below), which I think may have been a student’s assignment and showed parts of it to my class as we talked about the spectrum.  I really stressed how these visible light, X-ray, radio waves, etc are all the same thing, and just have different wavelengths and frequencies.  I spent quite a bit of time discussing how the longer wavelengths means a lower frequency and shorter wavenlengths have a higher frequency but they are all traveling at the same speed, the speed of light.

We also watched this video, What is Light? by Kurzgesagt – In a Nutshell.

I have two simple spectrometers that came with a spectroscope analysis kit (one came with the kit and I bought an extra one) and had students look at red and blue LED lights (Light Blox light sources) and read off the wavelength of the light.  I also made the different colored flames with the different salts (potassium chloride, cupric chloride, lithium chloride, etc most of which come in the spectroscope analysis kit).

When using the spectroscope to look at white lights or sun light reflecting off a surface (do NOT point it directly at the sun) you get a very nice rainbow of colors, showing that white light consists of all the colors (wavelengths) of visible light.

You can see the spectroscope analysis kit in action with the colored flames in this older post from my chemistry class.  I’ve definitely gotten my money’s worth out of this kit and it came with enough chemicals that I’l be able to keep using it for many more years since you only use a few crystals each time.  There are instructions on line for making simple spectroscopes at home but none of the ones I’ve made work as nice as this, and the fact that this at least a rough scale for measuring wavelength is a big plus.

At the end of class, I had the kids fill out this electromagnetic spectrum worksheet that is available for free from Cloey Holzman on the Teachers Pay Teachers website.

We started class with the following videos on sonic booms, SONAR and echolocation.

We played around with the free app, SignalScope X by Faber Acoustical.    The app uses the microphone in your phone or iPad and displays the sound waves on the screen like an oscilloscope. Below is a waveform that I made by humming a note.  You could use this app to measure the period (T) of the wave, time between crests or troughs and then calculate the frequency (1/T).  For the wave shown the period is roughly 5ms (0.005s) and frequency = 1/0.005 = 200 Hz. We used it to look at sound waves produced by the tuning forks.

While searching for labs  I found a video of the speed of sound lab that I did with the high school class, but they were using wider tubes and the resonance was much easier to hear, so I repeated that experiment with the middle school class using the boom whacker tubes and it worked much better. Since the tubes were so much wider we had to use big plastic containers to hold the water instead of graduated cylinders.  When you move the tube up and down with a tuning fork over the opening, you will hear the sound get louder when the length of the tube, L, is equal to 1/4 of the wavelength of the sound. Students found resonance (the length where the sound got louder) for four different frequencies (tuning forks) and calculated the speed of sound for each one ( speed = wavelength x frequency).  They all found values close to 330 m/s.

Here’s the video of tuning forks demonstration, the speed of sound demo is around 3:20.

Learn from Yesterday, live for today, hope for tomorrow. The important thing is not stop questioning ~Albert Einstein

graph paper diaries

because some of us need a few more lines to keep everything straight

Evan's Space

Wonders of Physics

Gas station without pumps

musings on life as a university professor

George Lakoff

George Lakoff has retired as Distinguished Professor of Cognitive Science and Linguistics at the University of California at Berkeley. He is now Director of the Center for the Neural Mind & Society (cnms.berkeley.edu).