The University of Cambridge was closed. The plague had seen to that. It would open when the plague was over. However, plague couldn't stop Isaac Newton from studying. He was at his family's home in England while the university was closed, and, while there, made his three major scientific discoveries. The one I am going to talk about today was his discovery concerning light. One day, he darkened his room and made a tiny hole in the shades. This produced a stretched version of the sun. He decided to experiment by throwing light on a screen using a prism. It produced a top blue edge and a lower red edge. Then he decided to try sending the light down a tube before it reached the prism. The result was amazing. Instead of mainly white light, he saw a band of many colors. All the colors of the rainbow, in fact. He conducted other experiments and decided that light is many colors, not just one. (One of the experiments he conducted involved sticking things in his eyes. He almost lost his eyesight. Don't repeat his experiment. Being a genius does not guarantee that you will make wise decisions. It also does not guarantee that you will escape unscathed, although many famous scientists do seem to.)
There are two types of rainbows, primary and secondary. In a primary rainbow, light is reflected once as it travels through a raindrop.The colors are dispersed as they enter and leave the drop. It is likely that the colors depend on the position of the drop in the sky. Red light is seen from raindrops at an angle of 42 degrees to the line of the horizon, while blue light is seen from those at 40 degrees. All other colors are seen from drops between those. A secondary rainbow will form outside a primary one. The light is reflected twice by each raindrop, and the order of the colors is reversed. Red light is seen from raindrops that are at an angle of 50 degrees to the horizon, and blue light from those at 54 degrees.
Well, that's all for now. Just remember not to repeat Isaac Newton's experiment!