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: