Why do rainbows appear when it rains?

Rainbow is caused by dispersion and reflection caused by sunlight shining 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.

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.

The size of water droplets in the air determines the brightness and width of the rainbow. When the water drops in the air are large, the rainbow is bright and narrow; On the contrary, if the water drops are small, the rainbow will be light and wide. We can't see the rainbow in front of the sun, but only in front of the sun, so the rainbow appears in the west in the morning and always in the east at night. 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 whether it was sunny or rainy from the position where the rainbow appeared in the sky. When there is a rainbow in the east, it is not easy to rain locally, but when there is a rainbow in the west, it is likely to rain locally.

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 is less chance of rain, so there is generally no rainbow in winter.

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 is round rather than arched, and the center of the circular rainbow is the direction of the airplane.

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.

Double rainbows often see two rainbows appear at the same time, and a concentric but dark secondary rainbow (also known as neon) appears outside the ordinary rainbow. The secondary rainbow is formed by two reflections of sunlight in water droplets. When sunlight passes through water droplets, it will be refracted, reflected and then refracted again. Our common rainbow (main rainbow) is formed by the reflection of light in water droplets. If light is reflected twice in a water drop, a second rainbow (neon) will be produced. The color arrangement of neon is opposite to that of the main rainbow. Because every reflection will lose some light energy, the brightness of neon lights is also very weak. 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 the low light intensity. Double rainbow over Scotland 1307, it was suggested in Europe that the rainbow was caused by the refraction and reflection of the sun light 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.