Where is the rainbow?
Liu Yike
Today(2020/8/26), there was a rainbow appearing in the sky at 7:29 p.m. in Binzhou, Shandong. I went out to see the rainbow quickly with photos that my friends upload. Unfortunately, I didn't see it even thought I spent about 10 minutes looking for it in the west of the sky, which is where the sun was.
it is public knowledge that, Issac Newton did such an experiment. In a dark room, a ray of sunshine went through a small hole and shone on a triangular prism. And we can see a "rainbow" on the white wall behind the prism. This phenomenon is named Dispersion(or Material Dispersion).
Dispersion is main law about rainbow. It occurs because light with different wavelength has different refractive index. Purple light is deflect a lot, but red light isn't, so sunshine is divided into a colorful band.
If we remove the wall from the experiment, we will see the "rainbow" by facing the sun. But why was the true rainbow across from the sun? After Searching for information on the Internet, I got the answer. Dispersion do be one of the principles of rainbow. So do reflex. Just like triangular prism, water droplet refract light twice. The difference between water droplet and triangular prism is that when the light first wants to leave the droplet, it was totally reflected.
TR(Total Internal Reflection) is another law about rainbow. When a ray of light go through the surface between two different things, it will be reflected and refracted.When the light go from optically denser medium to optically thinner medium, incident angle is smaller than refraction angle. If incident angle is so big that refraction angle is 90°, we call it critical angle. If incident angle is bigger than critical angle, there will be only reflection and no refraction, we call it TR.
It was because of Dispersion and Total Internal Reflection that the rainbow appeared in the east at dusk.
Here is the method to find rainbow. After the rain, walk outdoors. Define solar elevation angle as θ. Turn your back to the sun. Then observe the position that has a 42-θ degree angle to you. That's where the rainbow is.
Observing closely, the rainbow is everywhere in our daily life, the foam under the sun, the oil film on water, one side of the CDs, the wings of butterflies, etc. But the "rainbow" color do not actually exist on the object. These color comes from their microscopic structure. When light passes them, because of reflection, refraction, interference, diffraction and so on, rainbow appears.
I think this might be why nature is so fascinating.