Why is the sea blue? The sea is a fascinating place. It is vast, blue and surging, which has been praised by literati in past dynasties. Do you know why blue water is blue? If you pour seawater into a plastic bottle, you will find that it is no different from ordinary water, colorless and transparent. However, the endless sea in our eyes is really blue and amazing. It turns out that there is optical knowledge in physics hidden here. Sunlight is a kind of composite light, which consists of seven colors: red, orange, yellow, green, cyan, blue and purple. Seven colors have different wavelengths, and the longer the wavelength, the longer it is. It is most easily absorbed by seawater. Blue light and purple light with short wavelength have weak penetration ability and are easy to scatter and reflect when encountering seawater. What we see is the color of blue light and purple light left after being scattered by seawater. Because human eyes are not as sensitive to purple as blue, the ocean we see appears blue. In the shallow sea near the beach, because there are more suspended solids and larger particles in seawater, the absorption of green light is weak and the scattering ability is strong. Therefore, the sea water near the coast is mostly light blue or green. As for the seawater collected by plastic bottles, because the capacity is very small, the scattering degree of its seven colors of light is very close, which can not be accurately distinguished by human eyes, so we can see that the seawater in plastic bottles is as colorless and transparent as ordinary water. Knowing that seawater absorbs, scatters and reflects different colors of light, you must be able to explain why the sky is blue!