Rainbow: a photometeor of hope

Typology: content exploration with experimental protocol 

Description: The rainbow is one of the most beautiful natural phenomena. It became a symbol of hope and unity, during the days of world emergency due to COVID-19. During the confinement period, in an initiative that started in Italy and spread quickly across Europe, several children and their family created their own rainbow with the message "everything will be fine!".

And what is the rainbow? How is it formed? Nature is a source of uncertainty and strength (which we often neglect), which has always fascinated man. This fact, coupled with curiosity, often triggers scientific research.

The rainbow is a luminous phenomenon (photometeor). Sunlight is composed of all the colors of the luminous spectrum, which we can only observe when a ray of white light passes through a prism. This ray undergoes refraction as it passes through the prism, and is divided into wavelengths ranging from infrared to ultraviolet. The extremes are not visible, so we can only observe the wavelengths from red to violet (which are part of the visible light radiation). Isaac Newton was the first scientist that  demonstrated that white light is made up of all the colors of the rainbow, through a glass prism. Likewise, when sunlight passes through a drop of water it works just like the prism and, therefore, it is possible to separate the light into its colors. 

Thus, according to IPMA - Instituto Português do Mar e da Atmosfera (national institution that carries out weather forecasts), the rainbow can be observed when the sun (or the moon) is on the observer's back (at a low altitude or angle) and there is rain, drizzle or fog, in the observed direction. The rays of sunlight (or lunar) pass through the drops of water and are refracted in the drops of rain. After the refraction, a part of the rays of light undergoes one or two reflections (on the internal face of the drops) and then it occurs a second refraction, exactly in the direction of the observer. In this way, groups of concentric arches appear, with colors ranging from purple to red.

The most spectacular rainbow appears when half the sky is still dark, with rain clouds, and the viewer is in a clear sky. Sometimes, it is possible to observe two rainbows at the same time: a main one (primary rainbow) and, on top, another, slightly larger and less bright (secondary rainbow). The last one is the result of the drops of water reflecting the sun's rays twice. The primary rainbow has purple on the inside (40º radius) and red on the outside (42º radius). In its turn, the secondary rainbow has red inside (50º radius) and purple outside (54º radius). When observing a rainbow on an airplane, it shows a full circle and not a simple arc. The rainbow can also be white in color, with a white band that appears in fog days: the water droplets are so small that they reflect weaker colors (usually with the outer rim red and bluish inside). The rainbow, of enormous beauty, aroused the curiosity of science, but it is also a source of inspiration for literature, the arts or even myths. Popular belief says that at the ends of the rainbow there will be a pot of gold. Has anyone found him yet? You already know the conditions for this photometeor, so the weather forecasts from IPMA - Instituto Português do Mar e da Atmosfera and the data obtained, namely, by the Arouca Meteorological Radar, are important to assess if we have a chance to observe this photometeor! Until we see it in nature, we suggest that you create your rainbow with colored rice. Download the attached experimental protocol!
Suggestions for exploring the theme:

• Experimental protocol attached in pdf file;
• IPMA FAQ: https://bit.ly/2WgOC6c; Images of the Mosaic of National Weather Radars (includes Arouca), through which you can monitor the hydrometeors. An important tool for meteorologists, namely to detect severe weather conditions (rain and strong winds, hail or even tornadoes) and, consequently, greater security for populations in the face of natural disasters, available at http://www.ipma.pt/ pt / otempo / obs.radar /;
• Educational video about the rainbow available at https://bit.ly/2z4vGzm; We appreciate the collaboration of IPMA in creating this educational resource.