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Rainbow

　Many small spherical waterdrops drift through the air after it rains. these small waterdrops carry out much the same role as a prism by distributing light, and thus building a rainbow.


Therefore,viewing rain from the opposite direction of the sun is one of the conditions reauired to view a rainbow. But,unlike a triangular prism, the divection of the light through the inside of the waterdrops becomes somewhat complicated. When sun light hits water droplets floating in the air,the light comes into water droplets by first reflecting off the far surface of the water droplets and then being refracted at the surface again. Some of the light coming into the water droplets is reflected within the water drops and then refracted at the surface again before returning outside. The light reflected. at the surface of the water droplets is reflected in various directions from where the light hits the water droplets and it is not particularly strong in any direction. This is the sanne as light that comes into water droplets once and leaves again. However, light what eflected inside of the water droplets once and came out goes out at a specific angle by the difference in the refractive index by the wavelength of light. light at an angle of 42 degrees produces red color,and at about 40 degrees produces light of a blue (purple) color. Such refraction of light occurs in each of the many waterdrops floating in the air. In this way, a rainbow is made.


　Incidentally,another rainbow on top of this rainbow is called a "Secondary rainbow". When light reflects in the water droplets twice ,a secondary rainbow can be seen.


The secondary rainbow is different from primary rainbow in that the color order is reversed so the bottom color is red, while the top is purple. Furthermore, one must look more closely because the secondary rainbow is dimmer than the primary rainbow .A secondary rainbow can be seen in the picture below.


＜Click to enlarge（Displayed in a separate window）＞


◎【Video:Looking at a rainbow reproduced in the laboratory】
Fine beads are affixed onto the entire sarface of a sheet of black paper.(Refer the picture on the right.)In this experiment, beads play the role of water droplets in the air. When you shed light on the papcr and observed it from a fixed angle,you can see a rainbow.
＜Click to enlarge（Displayed in a separate window）＞
　How many colors are contained in the rainbow?
Aristotle in Greece said that a rainbow is red, green and blue,and sometimes appears
yellow.The first person who said that the number of colors in a rainbow was Isaac Newton
who became famous for his Law of universal gravitation.The color of a rainbow was said to be
three colors in the Edo era in Japan.Now,not all countries necessarily say that a
rainbow is seven colors.The number of colors in a rainbow is said to be six in America ,
five in Germany and Russia, and even two or three in some countries.The number of
colors changes by how neutral colors are identified.