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SF Physics 23 – Optics

For this class I went over how the human eye works, including color vision and then I had the students use the Light Blox to see what different lens shapes do to incoming light.  They traced the lens on their paper and then placed the Light Blox on one of the lines on their paper and traced it, with the help of a ruler, on both sides of the lens.  You can use a laser pointer for this but its not as easy to see the beam as the Light Blox. They did this for both converging and diverging lenses and a couple of them also did it for a right angle prism.IMG_5426

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Students also made an image of a bright window with a small concave lens and moved a piece paper around until they found the focal length of the lens – where the image was in focus (sorry my photo is NOT in focus). We talked about how this was a real image since you could project it on to a piece of paper.

One of the students was trying to see the image from a diverging lens (skinny in the middle) and another student noticed it made his eye look smaller and closer together (photo below).  IMG_5425

Lastly they took prisms (these are part of the lens/prism kit linked to above) outside to see the spectrum of sun light, i.e. make rainbows.

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R2 Physics 26 – Seeing Color

Before class, students read 13.2 Discharge Lamps in How Things Work the Physics of Everyday Life and watched:

There are two different types of light ‘detectors’ on our retina, rods and cones.  Rods are very sensitive to low light but not to color, and cones are sensitive to different wavelengths (colors) of light.  Humans have three types of cones, one for predominately blue light, one for green and one more sensitive to red light (photo below).  Cones need fairly bright light so in low light conditions we are mainly detecting light with the rods, which is why we don’t see colors very well in dimly lit rooms.

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Image from wikipedia

But not everyone sees colors in the same way.  Color blindness, or to be more accurate, color vision deficiency happens when one set of cones is not as sensitive as it should be. This is an inherited trait and affects men more often than women because the responsible gene lies on the X chromosome, which men only have one of and women have two.  Its unlikely for women to have the recessive gene on both of their X-chromosomes.  My husband is red-green color blind which means his vision isn’t as sensitive to red light, so a pink shirt might look white to him.  There are some great apps, including ColorDeBlind, that let you see what the world looks like with different color deficiencies.  The photo below shows how the fruit appears when you have ‘normal’ color vision and the photo on the right is what it looks like if your red-green color blind.  You can see the blue is pretty much the same but reds, oranges and greens all look like different shades of yellow and brown.

colorblind

Animals can have different numbers of cones then us and they can cover different wavelengths or colors.  Some birds, fish and insects have 4 different cones, giving them tetrachromatic (4-color) vision.   Here’s a link to an article on color vision in animals in Cosmos magazine.

Unknown-1I happen to have a book of pseudoischromatic plates for testing color blindness where if you have ‘normal’ color vision you will see a number among the dots, but if you are color blind then you will not see any numbers, or you might see another number.  Enchroma, a company producing glasses that enhance the color vision of people with color vision deficiencies, has an online color vision test you can take for free here.

For the lab portion of the class, I did the colored flame demonstration, putting different chemicals (strontium chloride, lithium chloride, etc) over a butane burner and showed how they produced different colored flames.  Students looked at the flames with the handheld spectrometers and saw the light had different wavelengths.  The different chemicals produce different colored light because their energy levels are different and you can use a flame test to help identify chemicals. You can find a more detailed lab in this post from my chemistry class last year.

IMG_5312Students also did the color viewing box that I did with the middle school class a few weeks ago.  They made predictions for how different colored objects would appear under different colored light and then did the experiment to see if they were correct. We used different colored filters and a flash light to produce different colors of light.

Since this chapter also discussed fluorescence, I brought out our collection of fluorescent rocks and a shortwave Ultraviolet (UV) light.  The rocks look very boring in normal sunlight (photo on the left), but under UV light the rocks give off visible light.   The UV light excites the atoms in rock and when they relax to the ground state they give off visible light.  Just like the flames, the color depends on the elements involved.  The light I used was an old Raytech which doesn’t seem to be available anymore, but this one by UVP looks similar and is about the same price ($60) I paid for mine.  Not all rocks fluoresce but you can buy fluorescent rock collections online or buy them at local rock and gem shows.  You can read more about fluorescent rocks in this geology.com article.  This article also has some safety tips for using these UV lamps – they can damage your eyes and skin, so you need to make sure its only pointed at the rocks.

 

In class we watched a video on blackbody radiation by Physics Girl

and Why is Blue so Rare in Nature? by It’s Okay to Be Smart, which is a very cool video on how animals appear to be blue without using blue pigment.

 

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SF Physics 22 – More mirrors

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.

R2 Physics 25 – Light

Students read Ch 13.1 Sunlight in How Things Work, the Physics of Everyday Life and watched the following videos before class:

At the beginning of class I lectured a bit on light, refraction and polarization, using my water tank of science (below) to demonstrate scattering and polarization.  The tank is filled with water and a bit of powdered milk to help scatter more light.  The flashlight is scattered by the powdered milk and scatters more blue light so when you look at the water near the flashlight it appears blueish.

But if you look at the flashlight through the the entire tank of milky water the light looks yellow… kind of like the sun, because the blue light has been scattered away leaving mostly yellow/red light to reach our eyes.  This is exactly why the sky looks blue during the day, the atmosphere is made of molecules that easily absorb and re-emit blue light but not the other wavelengths of light. During a sunset or sunrise the sunlight travels through more atmosphere since its coming in at an angle and the blue light is all scattered away by the time the light reaches you, making it look  red-orange and yellow.

Students measured the index of refraction of an acrylic cube (or other shape) by tracing the cube on a piece of paper and using one of the lightblox, trace the incoming and outgoing light beam.  The easiest way to do this is put two dots on the path and use a ruler to connect the dots.IMG_5239

Then connect the two light rays through the cube so you can see the path of the light.

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Draw a dashed line perpendicular (normal) to the surface of the cube and measure the incident angle and the angle of the light inside the cube.

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Once you have the two angles you can use Snell’s law to find the index of refraction of the cube.  The index of refraction of air, n1, is 1.00.  The cube is most likely acrylic so the index of refraction should be close to 1.49.

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SF Physics 21 – Mirrors

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.

IMG_5230We 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.

 

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