Making “skyentific” observations with a cell phone Observation It’s quite possible to make scientific observations of different natural phenomena in everyday life: all you need for it is to document the observation and make certain measurements. Here we will focus on making observations with a device that most pupils carry around with them: a cell phone. One example of a natural “skyentific” phenomenon that can be documented with a cell phone is the rainbow. In an ancient Greek the universe was divided into two regions: * A spherical Earth, central and motionless (the sublunary sphere). * A spherical heavenly realm centered on the Earth, which may contain multiple rotating spheres made of aether All sky phenomena belonged to the astronomy. So rainbow is a sublunary astronomical phenomenon. Exploration Impressive photos of a rainbow can be made even on a cell phone camera. It’s advisable to encourage pupils to take photos of rainbows during the summer holidays. In the Fall, while teaching optics on physics lessons, one could even hold a photography contest. This is the situation especially in the northern countries like Finland. It is also in many cases easier to study optical phenomena from a good photo than in a nature. And as it was said, especially then, when the phenomenon is rarely seen. So here the photos and videos of a rainbow a the starting point of study. You can not solve the nature of rainbow jus watching photos, but you can make good observations, which can lead to the good questions. The answers are in experiments, that you can make or you can read what other scientists have done earlier. That is how science goes. The rainbow is a light phenomenon that covers practically the entire field of optics from colours to interference. It provides innumerable questions and topics for research on lessons of physics, history and even religion. Here are some questions that photos of rainbows might evoke: 1. What part does the rainbow play in the Bible? 2. How do the appearances of the rainbow in the Bible relate to physics? 3. In what angle is the first rainbow visible and why is this? In what angle is the second rainbow visible? 4. Why couldn’t a pot of gold be found at the end of the rainbow? 5. What is the darker area between the first and second rainbows called and what is the reason for it? 6. One can see colourful thin bows below the rainbow. What are they and how do they come about? 7. Is the mirror image of the rainbow in the water reflected from the same raindrops as the real rainbow? 8. Why can’t a rainbow be the result of a total internal reflection (as erroneously stated even in many textbooks)? Concept introduction The rainbow is a complex natural phenomenon, and it took well over two thousand years for scientists to solve its mysteries. The first explanations of any scientific value were made by Greek philosophers about 2500 years ago. And the last word on the matter has not been said yet, as there are still many unexplained questions about the rainbow. Here, however, are answers to at least some of the questions, as well as references to sources of knowledge. Some of the questions can be quite demanding even for high school students, while others are suitable for pupils of elementary school. 1. According to the Old Testament, the rainbow is a sign of the covenant: God produced the rainbow on the sky after the deluge as a promise that it wouldn’t happen again. 2. In order for the rainbow to appear on the sky after the deluge, God would have had to change the optic qualities of water from what they were during it – in other words, reading the Bible as a textbook of natural sciences leads to rather absurd explanations. This might be a good moment to explain to pupils that science and religion deal with two different aspects of human life and their premises are fundamentally different from each other. 3. The appearance of a rainbow is a complex optic phenomenon. It was first solved in a somewhat sensible way by the famous 17th century French scientist and philosopher René Descartes. There is a lot of material to be found about the rainbow in both books and on the Internet. Here’s one link to start out with: http://en.wikipedia.org/wiki/Rainbow There is at least one book in Finnish about the rainbow that is intended for the general public. The book has been sold out ages ago, but it’s worth keeping an eye on in second-hand bookshops: 4. Rainbow is not an immutable object, but a series of reflections of waterdrops seen from a certain angle. As the viewer moves, the rainbow moves as well, which makes it impossible to find ”the end of the rainbow”. 5. The dark area between two rainbows is called Alexander’s dark band. It is in the direction where light beams are not reflected from raindrops. A good explanation about the matter can be found also here http://en.wikipedia.org/wiki/Rainbow. 6. The thin bows are caused by interference. Their origin was discovered by scientist only in the 20th century. More on this phenomenon can be found here. http://en.wikipedia.org/wiki/Alexander's_band 7. The rainbow reflected in the water is a reflection from different raindrops and is thus a ”different rainbow”. A good explanation about this and many other phenomena can be found in M. G. J. Minnaert’s classical work Light and Color in the Outdoors: 8. The reasons for this being so are numerous. a) Light is always reflected in the same angle inside a raindrop. If it’s reflected once by total internal reflection this recurs always in the same way and the light is ”captured” in the raindrop. b) The rainbow would be much brighter if the light would be fully reflected in the raindrops. c) The rainbow would be seen at a different angle if the light would be fully reflected in the raindrops. Generalization of the concept It’s usually impossible to capture an entire rainbow on a cell phone camera. For doing that, the video camera on a cell phone may come in handy. Rainbow video