Another week of optics, which is fine by me.  I teach these topics a lot so it requires very little preparation.  I pulled out my aquarium tank, the powdered milk and my old slideshow on why the sky is blue and why sunsets are red.  Most things appear to be a certain color, like a blue ball, because when white light (composed of all the colors) hits the ball, all the colors but blue are absorbed and the blue light is reflected into our eyes.  So is the sky made of blue molecules?  No, because we can see through the air at night and see stars, it only appears blue during the day.  The sky is blue because the molecules that make up our atmosphere easily absorb blue light and then re-emit it in all directions, scattering blue light everywhere.   The easiest way to demonstrate this is by filling a clear container with water and then adding just a bit of powdered milk which acts as the scatterer.

If you aim a bright white light into the tank you will see the water looks very blue near the flashlight, but if your container is big enough, the blue light gets all scattered away and you’re left with red/orange light. Below is a photograph taken looking back at the flashlight.  You can see it looks very much like a sunset!  When the sun sets, the sunlight passes through more of our atmosphere since its coming in at an angle, so the blue light gets scattered away before it reaches our eyes, but the red and orange lights pass through the atmosphere and gives us a beautiful sunset.  This is also why sunsets are more spectacular after volcanic eruptions, more ash in the air means more scattering.

For the lab the students used the tank to look at the critical angle of refraction, which is the incident angle at which ALL the light is reflected at the interface and none passes through the interface.  You can see in the photo below that the laser is coming up through the water and reflects at the water/air interface as if its a mirror.  This will happen at any angle greater than the crictical angle, which is determined by the indices of refraction of the two materials. (You wouldn’t be able to see the laser if it was coming out of the water in this photo since there’s nothing there to scatter it, like the milk in the water, but we put an index card above to make sure no light was exiting the water.)

The other lab involved finding the index of refraction of an acrylic cube by measuring the angle of incidence and the angle of refraction inside the cube.

We traced the incident beam of laser light, the refracted beam exiting the cube, and the cube itself.  Then the students could connect the two beams by drawing the path of the light through the cube and measuring the angles.  Once they have the angles they plug them into Snell’s Law and find the index of refraction of the cube, which in the example below came out to 1.49.

We also talked about dispersion in class and how the index of refraction depends on the wavelength of light, which is why prisms separate out the colors and why we see rainbows in the sky.  Here’s a video by Physics Girl explaining rainbows.