Rainbow

Rainbow, also called celestial arch, covenant arch, rain arch, or old-time arch, is an optical and meteorological phenomenon that separates sunlight from its nearly continuous spectrum when the sun shines over raindrops. It is a multicolored bow with red on the outside and violet on the inside; The complete order is red, orange, yellow, green, blue, indigo (or indigo) and violet.

The rainbow effect can be observed whenever water droplets are in the air and sunlight is shining above the viewer at a low altitude. The most spectacular rainbow appears when half of the sky is still dark with rain clouds and the viewer is in a clear sky location. Another place to enjoy the rainbow is near waterfalls.

The rainbow does not really exist as a place in the sky, but it is an optical illusion whose apparent position depends on the position of the observer. All raindrops refract and reflect sunlight in the same way, but only the light of some of them reaches the eye of the beholder.

These drops are perceived as the rainbow to that observer. Its position is always in the opposite direction of the sun with respect to the observer. A curious thing is that inside the rainbow the clouds and the sky have a brighter illumination than outside it.

From an airplane it is possible to have the opportunity to see the full circle of the rainbow, with the shadow of the airplane in the center.

It is difficult to photograph the full arc, which requires a viewing angle of 84 °. For a 35mm camera, a focus lens of 19mm or less is required, however most cameras have lenses over 28mm (more details on the angle of view you see here).

We can see rainbows of different sizes because, to estimate their width, our brain only has the size of the viewing angle that corresponds to it. If near his image there are distant objects such as mountains, the rainbow will appear larger. If the rainbow is near less distant objects, it will appear smaller. It is fundamentally the same illusion that makes the moon, sun or constellations appear larger when they are near the horizon.

The first accurate theoretical explanation of the rainbow was made by Descartes in 1637. Knowing that the size of the raindrops did not appear to affect the observed rainbow, he experimented with focusing light rays through a large glass sphere filled with Water. By measuring the angles as the rays emerged, he concluded that the first arc was caused by a single internal reflection within the raindrop and that the second arc could be caused by two internal reflections.

He was able to get his results from the refractive law and correctly calculated the angles of both arcs. However, he was unable to explain the colors.

Isaac Newton was the first to demonstrate that white light was composed of light of all the colors of the rainbow. With a glass prism, he decomposed white light into the full spectrum of colors and, with another, recombined the light beam into white light. It also demonstrated that red light is refracted less than blue light, which led to a complete explanation of the optical effect of the rainbow.

Sometimes a second weaker rainbow is seen outside the main rainbow, and it is due to a double reflection of sunlight on the raindrops. Due to the extra reflection, the colors of the arc are reversed when compared to the main rainbow, with blue on the outside and red on the inside. This other arc is called the secondary rainbow.

Some people have claimed to have seen up to three rainbows in the sky at one time. But the scientific reports of this phenomenon were so rare - only five were recorded in 250 years - that until now many scientists believed they were as real as a pot of gold at the end of the rainbow.

These legendary optical rarities, caused by three reflections of each ray of light within a raindrop, have finally been confirmed, thanks to photographic perseverance and a new weather model that provides the scientific underpinnings to find them.

Although incredibly rare, tertiary and quaternary rainbows are natural products of the combination of scattering, refraction and reflection within raindrops. These are the same processes that create all rainbows, but they are pushed to their extremes to produce these variants.

Refraction is when sunlight “doubles” by changing medium, passing, for example, from air to water and vice versa. Water droplets refract each of the sunlight colors at a slightly different angle. This is called scattering, which separates colors to create a rainbow.

Most of these multicolored lights go through the raindrop, but some are reflected. The spherical curves of a drop concentrate the reflections at 138 degrees from the sun. This concentrated light is bright enough to create a visible primary rainbow.

A double rainbow occurs because not all light comes out of the raindrop. Part is reflected back to the raindrop and goes through the whole process again. Although this light is weaker, it is sometimes also bright enough to produce a secondary rainbow outside the first one.

The third series of reflections creates a tertiary rainbow. It is even fainter than the secondary rainbow and much harder to find because instead of forming away from the sun, a tertiary rainbow appears around it. To see him, observers have to look at all the brightness emanating from him.

This may be why only five scientifically informed observers have described the tertiary rainbow during the last 250 years.

A researcher named Raymond Lee went after the descriptions of the five observers and found common elements. All the rainbow described as tertiary appeared for a few seconds against a background of dark clouds about 40 degrees from an intensely bright sun.

The researcher used a mathematical model to predict what can produce conditions for seeing the tertiary. First, they needed dark clouds as well as heavy rain or almost uniformly sized drops. Under these conditions, if the sun breaks through the clouds, it can project a tertiary rainbow.

When Lee presented his findings, it sparked heated debate. Some scientists insisted that the descriptions of the past were wrong and that the tertiaries are too weak to be seen in the sunshine.

Since then, researchers Michael Grossman and Michael Theusner have been taking pictures of tertiary and quaternary rainbows. The day Grossman photographed the tertiary rainbow, he first recalls seeing a secondary rainbow. When the rain intensified, he knew he had to turn to the sun.

With the naked eye it is very difficult to say that it is visible. But the treasure hunt with a camera is worth it.

In addition to the rainbows described above, there are also twin or twin rainbows and supernumerary rainbows.

Unlike a double rainbow, which consists of two separate arcs from the rainbow, the very rare twinned rainbow appears as two rainbows that split from a single base. The colors in the second rainbow, instead of reversing as in a double rainbow, appear in the same order as the primary rainbow. It is sometimes observed in combination with a double rainbow.

The cause of a twinned rainbow is the combination of different sizes of water droplets falling from the sky. Due to the air resistance, raindrops flatten as they fall, and flattening is more prominent in large drops of water. When two raindrops of raindrops of different sizes combine, each produces slightly different rainbows, which can combine to form a twinned rainbow.

A supernumerary rainbow, also known as a stacked rainbow, is an infrequent phenomenon consisting of several weaker repeated rainbow colors within the main arc, and very rarely outside the rainbow as well. secondary. Supernumerary rainbows are slightly highlighted and have streaks of color that do not fit the usual pattern.

It is not possible to explain its existence using classical optics geometrics. Supernumerary rainbows are caused by interference between light rays that follow slightly different paths, with slightly different lengths, within the raindrops.

Given the different refraction angles of rays of different colors, the interference patterns are slightly different, so each bright band is differentiated in color, creating a miniature rainbow.

Supernumerary rainbows are most evident when raindrops are small and of similar size.