When will the rainbow appear?

In summer, after the rain, the dark clouds fly away and the sun reappears. In the sky opposite the sun, there will be a semi-circular rainbow.

Rainbow is formed because sunlight is emitted in the air and refracted into water droplets.

We know that when sunlight passes through the prism, the direction of the foreground will be deflected and the original white light will change.

The line is broken down into seven color bands: red, orange, yellow, green, cyan, blue and purple.

After the rain, there are many tiny water droplets floating in the air. When the sun shines on them, they will refract and disperse.

Into seven colors of light. Many small water droplets refract sunlight at the same time, and then reflect into our eyes, and we will see one.

A semicircle rainbow. The ribbons of the rainbow are distinct, and the red ones are on the outermost side, followed by orange, yellow, green, cyan, blue and purple.

Color.

The size of water droplets in the air determines the bright color and width of the rainbow. When the water drops in the air are large, the rainbow is bright. also

Relatively narrow; On the contrary, if the water drops are small, the rainbow will be light and wide.

We can't see the rainbow when facing the sun, but only when facing the sun can we see the rainbow, so the rainbow in the morning appears.

In the west, the rainbow at dusk always appears in the east. But we can't see it. We can only see it from the sky by plane.

The appearance of rainbow is related to the weather change at that time. Generally, we can infer from the position of the rainbow in the sky that it will appear at that time.

It is sunny or rainy. When there is a rainbow in the east, it is not easy to rain locally, and when there is a rainbow in the west, it may rain locally.

Very sexual.

The visibility of rainbow depends on the size of water droplets in the air. The bigger the water drop, the brighter the rainbow, and the smaller the water drop, the less obvious the rainbow is.

Generally, in winter, the temperature is low, there are few water droplets in the air, and there are few opportunities for showers, so rainbows generally do not appear in winter.

Rainbow is an optical phenomenon in meteorology. When sunlight hits the raindrops in mid-air, the light is refracted and reflected, forming an arched color spectrum in the sky. The colorful colors of the rainbow are red, orange, yellow, green, cyan, blue and purple from outside to inside.

In fact, as long as there are water droplets in the air and the sun shines behind the observer at a low angle, an observable rainbow phenomenon may occur. Rainbow usually appears in the afternoon, when the rain clears. At this time, the air is less dusty and full of water droplets, and one side of the sky is dark because of rain clouds. However, observers can see the sunlight without being covered by clouds above or behind them, so rainbows are easier to see. Another place where rainbows are often seen is near waterfalls. When the weather is clear, you can spray water or mist into the air with your back to the sun, or you can make rainbows artificially.

Sunset is a rare phenomenon, which may appear in the night with strong moonlight. Because it is difficult for human vision to distinguish colors in the case of weak light at night, the night rainbow looks all white.

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principle

Rainbow is formed by the scattering and reflection of sunlight on small round water droplets in the air. When sunlight enters the water drop, it will be incident at different angles at the same time and reflected at different angles in the water drop. The reflection of 40 to 42 degrees is the strongest, which produces the rainbow we see. When this reflection occurs, sunlight enters the water drop, refracts once, then reflects on the back of the water drop, and finally refracts again when it leaves the water drop. Because water has a dispersive effect on light, the refractive index of light with different wavelengths is different, and the refractive angle of blue light is greater than that of red light. Because light is reflected in water droplets, the spectrum seen by the observer is reversed, with red light at the top and other colors at the bottom.

The optical principle that causes rainbows often sees two rainbows appear at the same time, and a concentric but dark secondary rainbow (also called neon) appears outside the ordinary rainbow. The secondary rainbow is formed by two reflections of sunlight in water droplets. The strongest reflection angle of the two reflections occurs at 50 to 53, so the position of the secondary rainbow is outside the main rainbow. Because there are two reflections, the color sequence of the auxiliary rainbow is opposite to that of the main rainbow, with the outer side being blue and the inner side being red. In fact, the auxiliary rainbow must follow the main rainbow, but sometimes it is invisible to the naked eye because of its low light intensity (see).

The rainbow does not actually appear in a specific position in mid-air. It is an optical phenomenon seen by the observer, and the position of the rainbow will change with the observer. When an observer sees a rainbow, its position must be in the opposite direction of the sun. The center inside the rainbow arch is actually an enlarged image of the sun reflected by water droplets. So the sky inside the rainbow is brighter than the sky outside the rainbow. The center of the rainbow arch is just the direction of the observer's head shadow, and the rainbow itself is 40 to 42 degrees above the line between the observer's head shadow and his eyes. Therefore, when the sun is higher than 42 degrees in the sky, the rainbow will be below the horizon and invisible. This is why rainbows rarely appear at noon.

The rainbow extends from one end to the other. With an ordinary 35mm camera, you need a wide-angle lens with a focal length below 19mm to capture the whole rainbow in a single frame. If you are on an airplane, you will see that the rainbow will be round rather than arched, and the center of the circular rainbow is the direction of the airplane.

Double rainbow, neon above, rainbow below.

Double rainbow over Scotland 1307, it was suggested in Europe that the rainbow was caused by the refraction and reflection of sunlight by water droplets. Descartes found in 1637 that the size of water droplets does not affect the refraction of light. He experimented by injecting water into the glass ball and got the refractive index of water to light. He mathematically proved that the main rainbow is caused by reflection at the water point, while the auxiliary rainbow is caused by two reflections. He calculated the angle of the rainbow accurately, but failed to explain its colorful colors.

Later, Newton discovered all the optical principles of rainbow formation after scattering sunlight into color with a glass rhombus.