Rainbow: How Are Rainbows Formed?

What happens to the sky that occasionally gives rise to a rainbow stretching across the horizon? Basic optics, some might say, believing they've answered something. But since some people attribute mystical powers to rainbows and others feel they understand absolutely nothing about physics while gazing at them in awe, in this article we'll unravel the mystery and tell you how rainbows work.

Introduction

We are here and are part of what we call life through an almost impossible succession of events. Fortuitous and accidental, limited and mortal, human beings enjoy a virtue that has redefined our passage through life, which otherwise would be merely eating and reproducing, subject to the irrefutable whims of evolution. That characteristic is curiosity, a motor that drives us to the adventure of understanding our surroundings, to the question about Being, about existence, and about how all things around us work.

Embedded in a world of increasing complexity due to technology that modifies our lives, the most elementary questions mix with the ones that are de rigueur, and these with the ones that present themselves as new every day. So, on this occasion, we'll try to give an explanation suitable for all audiences about how rainbows form.

Rainbows

For mythology and fantasy, rainbows have been all kinds of symbols and elements, both sources of power and space-time vortices. Artists have been inspired by them thousands of times, and not without reason, because the beauty that characterizes them is almost as great as the silent curiosity they cause in people. Many times we've had the explanation reduced to: The rainbow is an optical phenomenon that forms when rays of light pass through raindrops on a rainy day. But is that all? What does it mean? Science has an explanation for rainbows, and according to a school of thought I adhere to, life is the search for the elimination of ignorance through the recognition of it, we will describe how this rare and beautiful phenomenon occurs.

Refraction

One of the fundamental processes for the appearance of a rainbow is the effect known as refraction. What does this mean? Refraction is basically the way light 'curves,' the way it changes direction when traveling from one medium to another. In turn, the direction light takes is subject to the qualities of the medium through which it 'moves.' But to understand it better, let's think of a practical example before moving on to the classic prism diagram.

Imagine you're skating in a plaza. In this case, you would be the light and the plaza floor would be the medium. Keeping that in mind, you know that if you apply more or less force and the right movement, you'll skate across the plaza floor (the medium) at different speeds. Now imagine the plaza floor has different sections, where the floor changes in composition, texture, sliding qualities, hardness, etc. So while you go from an asphalt section to one with grass, for example, the medium has changed and with it the transport qualities. As you know, on grass the wheels of your skate will have a lot of difficulty sliding, which will imply a change in your speed.

If we add that the floor has irregular relief between each section, we can understand that your direction will be slightly modified as you advance, because the front wheels hit a certain floor before the rear ones. The amount of direction change will depend on the angle and speed at which you entered a different floor and the resistance it exerts, such as from grass to granulated tiles. That is, the speed and angle with which you passed from one medium to another will determine the subsequent direction.

The prism and dispersion

Having understood this, we can now focus on a light ray entering a glass prism. We said that one part of the light wave slows down before the other part, so the ray curves at the boundary between air and glass (actually part of the light reflects off the prism's surface, but most passes through). The light bends again when it leaves the prism, since one side of it accelerates before the other.

Refraction tells us about this example in these terms: "In addition to the bending of light as a whole, a prism separates white light into its component colors (that is, sunlight into all its colors). Different colors of light have different frequencies, allowing them to travel at different speeds when moving through matter or media."

A color that travels slower in glass will bend more sharply when passing from air to glass, because the speed difference is more pronounced. A color that moves faster in glass won't slow down as much, so it will bend less intensely. In this way, the colors that make up white light are separated according to their corresponding frequency when passing through the glass. If the glass bends light twice, as in a prism, the separated colors can be seen more easily. This is called dispersion.

Ingredients: Water and Sun

This is where water comes into play, and more specifically the effect of rain. Raindrops can refract and disperse sunlight in the same way a prism does. In fact, each raindrop acts as a prism as it falls. Under the right conditions, refraction produces a rainbow, which, being a phenomenon composed of each of the falling drops, cannot be classified as static. An individual raindrop has a different shape and consistency than a glass prism, but light affects it in a similar way.

So when white light hits a number of raindrops at a fairly low angle, it splits and you can see the colors red, orange, yellow, green, blue, indigo, and violet. In this way, each individual raindrop disperses white sunlight into its component colors. So why do we see wide bands of color, as if different areas of rain were dispersing into a single different color? Because we only see one color from each raindrop, falling ceaselessly from the clouds at a distance we cannot identify.

When a water drop A disperses light, only red light can come out at the correct angle to travel to the observer's eye. The other colored rays escape the drops at a smaller angle, so the observer cannot see them. Sunlight will reach all raindrops in the same way, so they will emit red light to the observer at an angle of 42 degrees relative to the Sun's radius.

On the other hand, drop B is at a lower height, so it will not emit red light to the observer. Instead, due to its angle, violet will be the color that travels to the observer's sight. The raindrops surrounding A and B will bounce different colors to the observer, always subject to the position of the drops and the angle at which they are observed.

Is there an end to the rainbow?

In reality, there is no 'end' because the effect is optical, and every time we move toward the rainbow, we see it further ahead of us. This is because each rainbow is different for each observer, since what generates the size, shape, and relative distance is the 42-degree angle from which we are situated in our cone of vision relative to sunlight. The rainbow's shape is an arc because we are in the middle of the visible spectrum, and the rest of the rainbow we cannot see because the horizon gets in the way. If we instead are on an airplane or a mountain, we might see a complete circle.

The mystery of the double rainbow

Sometimes a double rainbow is seen, one clearly marked and another weaker one above it. The weaker of the two rainbows is produced in the same way as the sharper one, but instead of the light reflecting once, it reflects twice and at a different angle. In fact, if you look closely, the colors of the second rainbow are inverted relative to the first, but for that you need a day with dark clouds.

And now you know how rainbows work, so you're ready to enjoy them with greater understanding and even show off explaining it to whoever shares it with you.