|MadSci Network: Physics|
You are likely recalling two separate experiments. The first one, involving a light source and a prism, demonstrates that what we perceive as white light is in fact a mixture of light of various colors, and demonstrates that the colors can be separated. The second, involving a light source and a liquid such as cloudy water demonstrates that light can be scattered.
The first experiment is easy to conduct, and students should notice that the red light is bent the least as it exits the prism, while violet light is bent the most.
For the second experiment, you need a small fishtank and a very small amount of milk. Try this out ahead of time so you know how much milk you need to add to the water in the tank (not likely more than a few tens of millilitres). Shine a flashlight through the water and observe two things: the color of the water through which the beam passes, and the color of the light source itself as you look at it from the opposite side of the fishtank. The flashlight itself should appear reddish-orange, and the beam it casts (as you look from the side) should have a bluish cast. The idea is that the milk particles scatter the light, just as a prism does, and just as dust and water vapor in our atmosphere do. Blue light is scattered the easiest, and so the water in the tank appears blue, and the sky appears blue. Red light is the hardest to scatter, and so it manages to arrive fairly directly, which is why the flashlight's light appears reddish, and why the sun has a very strong reddish color at sunrise and especially at sunset. Why doesn't the sun appear to be red during the day? Well, the answer is that it's just too bright to appear anything but white.
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