Rainbows are only visible at certain angles because they result from the refraction and reflection of sunlight through water droplets suspended in the air. To observe a rainbow, sunlight must hit the water droplets at a specific angle between 40 and 42 degrees relative to your line of sight.
When sunlight meets a drop of water, it penetrates inside and refracts, meaning it slightly changes direction upon entering. Inside the drop, this light continues its path and bounces off the inner wall like a mirror: this is called internal reflection. After this bounce, it exits the drop and refracts again as it returns to the air, resulting in a second deviation. In the process, white light is separated into different colors because each color is deviated at a slightly distinct angle. It is because these colors leave the drops along distinct paths that an observer perceives a multicolored arc.
To see a rainbow, sunlight must enter the water droplets, change direction, exit, and then reach your eye at a precise angle of about 42 degrees. If you are positioned just right, the refracted and reflected light forms a colored circle around a point exactly opposite the sun (called the antisolar point). Basically, each color exits the water droplet at a slightly different angle: red exits at about 42 degrees, while violet exits at around 40 degrees. This difference explains why the rainbow always displays the same colors and why they always appear in the same order: red on the outside, violet on the inside. At different angles, the light scattered by the droplets simply will not be visible to you, and that's exactly why the rainbow seems to appear only at certain precise angles.
To see a rainbow, you absolutely must position yourself with the Sun at your back. It's really a matter of angle: light enters the raindrops, reflects, and comes out towards you at a precise angle. If you change your observation position, it’s completely different water droplets that capture and send the light to your eye, forming another image of a rainbow. Therefore, everyone sees a unique rainbow, specially tailored to their own position relative to the rain and the Sun. That’s also why the rainbow seems to move away when you try to get closer: it's impossible to catch!
The intensity and clarity of a rainbow greatly depend on the atmospheric conditions at the moment. When there is a lot of fog or dust, visibility quickly decreases as light struggles to pass clearly through these particles. An overly covered sky often hides the rainbow since the sunlight cannot directly reach the water droplets necessary for its formation. In contrast, following a good quick shower with the sun reappearing immediately after, the air is often clear, and rainbows become particularly visible and sharp. Additionally, a cleaner atmosphere after rain allows light to circulate easily, intensifying the colors we see.
A double rainbow is caused by a double internal reflection of light within water droplets. The second arc appears less bright, and its colors are inverted compared to the first one.
Unlike terrestrial rainbows seen from the ground, rainbows observed from an airplane can form complete circles, highlighting their true circular nature.
It is impossible to approach a rainbow: no matter how much you walk or drive towards it, it retreats at the same pace, as its angle of appearance always depends on your position relative to the sunlight.
The presence of mist or fog sometimes allows for the appearance of a 'white rainbow' or 'fogbow', which follows the same optical principle as a typical rainbow, but without the bright colors!
The apparent size of a rainbow depends on the height of the sun in the sky. The lower the sun is on the horizon, the larger the rainbow appears. Conversely, a very high sun produces a very low rainbow, which may even be invisible from the ground.
Light is strongly concentrated at the precise angle necessary for the formation of a rainbow, creating a bright band. Within this band (toward the center of the rainbow), a greater amount of light is reflected, while outside, the brightness is more diffuse. This is why the inside appears brighter.
Yes, we then talk about a lunar arc or lunar rainbow. This rare phenomenon is caused by the refraction of light from the moon (and not from the sun) in water droplets. Due to the low light intensity, it often appears whitish to the human eye.
The rainbow is not a physical object, but rather an optical effect related to the relative position of the observer, raindrops, and the sun. Thus, as one moves forward, one encounters new water droplets that refract light in ever different ways, creating the illusion that the rainbow is constantly moving away.
A double rainbow forms when light undergoes two reflections inside water droplets. The second arc is fainter, and its colors are reversed because the light exits the droplet at a different angle after two bounces, resulting in the inversion of colors and a lower light intensity.
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