How did the "double rainbow" of refreshing the screen come into being?
On September 3, the beautiful scenery of double rainbow in Beijing was swiped in the circle of friends.
Although a rainbow is a very common natural sight, even so, whenever a beautiful rainbow rises in the sky after the rain, it can still attract everyone's attention, and the extremely rare double rainbow is more than enough to "attract the eye."
So, how did the double rainbow form?
Why do we only see a rainbow in many cases?
A reporter from Science and Technology Daily interviewed Huang Zhimin, an agricultural meteorological expert in Hubei Province, to answer these questions one by one.
Double rainbow is actually neon and rainbow
Huang Zhimin said that when the air humidity is high, the water vapor in the air can be regarded as countless small water droplets.
When sunlight enters the small water droplets, two refraction and one reflection occur, which is the phenomenon of "refraction-reflection-refraction".
Sunlight is composed of 7 colors of red, orange, yellow, green, blue, indigo, and purple. When it is irradiated on water droplets, it will be refracted for the first time. Due to the different wavelengths of these 7 colors, their deflection angles It is also different. At this time, the sunlight will be decomposed into 7 kinds of monochromatic light; after the refracted monochromatic light reaches the back of the water drop, it will be reflected according to the reflection law; the reflected monochromatic light will return to the incident water drop. On the one hand, a second refraction will occur at this time, and water droplets will still be emitted in the form of monochromatic light.
The red light with the shortest wavelength has the smallest refractive index in the water droplets, so that the emitted light and the incident light form the largest angle, while the shortest wavelength purple light has the largest refractive index in the water droplets, making it refracted light and incident light. With the smallest included angle, other lights are distributed in sequence, and what people see with the naked eye is the "rainbow" with red outside and purple inside.
When we see the "rainbow" in the sky, it means that the sunlight is refracted and reflected in the water droplet, which means the existence of "neon".
The formation of neon is similar to that of rainbow, except that sunlight has undergone two refraction and two reflections in the water droplets, namely "refraction-reflection-reflection-refraction". Compared with the formation of rainbow, there is one more reflection process, so its refractive index is the smallest. The direction of the red light emitted by the red light forms the smallest angle with the incident direction, and the emergent direction of the purple light with the largest refractive index forms the largest angle with the incident direction. In the end, the color order we see will be exactly the opposite, showing the form of red inside and purple outside. .
However, the two reflections of sunlight after passing through water droplets lose more light energy than the one reflection. Therefore, the brightness of neon is much darker than that of rainbow, and it is generally not easy to be observed by people.
Therefore, we can only see neon when the energy of the light is large enough or the energy loss is small enough.
If it is visible, the corner radius of the rainbow is about 42 degrees, and the corner radius of the neon is about 52 degrees, so the neon is above and parallel to the rainbow.
When the rainbow and neon are seen by us together, it presents a beautiful double rainbow scene.
In theory, sunlight can be reflected multiple times in small water droplets.
But every reflection will cause light absorption and light attenuation.
So usually we can only see two rainbows at most.
Water vapor is enough to drop the water to make the rainbow double
The basic conditions for the formation of a rainbow are water vapor and sunlight. The adequacy of water vapor is related to the season and region.
Seasonally speaking, the temperature in winter is generally low, small water droplets are not easy to exist in the air, and there is less chance of rain, so there is generally no rainbow in winter.
There are more hot convective weather in summer, with frequent thunderstorms and showers, and it often clears up after raining in a hurry, so it is easier to see rainbows.
Geographically speaking, the appearance of rainbows needs to meet certain water vapor conditions. Therefore, in southern my country, places with plenty of rain and humid air are easier to see, and the chance of double rainbow beauty is also greater.
For example, the air humidity in Guangxi is relatively high, and sometimes even if it does not rain, there will be a beautiful rainbow.
Once you have the conditions for forming a rainbow, if you want to see a double rainbow, the size of the water droplets in the air is the key.
The size of the water droplets in the air determines the vividness of the rainbow.
When the water droplets in the air are large, the rainbow is bright; when the water droplets are small, the rainbow color is dim.
Therefore, if you want to see a double rainbow, the raindrops in the air need to be larger to ensure that the color of the neon is bright enough to be visible to the naked eye.
In addition, we see the rainbow from the ground as an arch, but in fact it is a complete circle.
The surface of the earth is a curved surface, and the water vapor in the air will be distributed according to the curved surface of the earth after the rain. Therefore, the rainbow seen outside the earth should be in the shape of a complete ring.
This is why the rainbow seen from the plane is a complete circle.
Once standing on the earth, the originally complete rainbow is no longer complete. This is because the other half of the rainbow is blocked by the ground.