A pink rainbow was observed from parts of Western England on August 7, 2017 which is a rare occurrence. Pink rainbows occur as optical effects depending on how we view phenomena and is caused by the refraction of sunlight from a reddish evening sun by droplets of water in the Earth’s atmosphere.
In the evening the angle of sunlight reaching an area on Earth makes it pass through a greater depth of atmosphere than in the daytime. In such a circumstance, colours occupying shorter wavelengths of light such as violet, indigo, blue, and green are optically invisible while the colour with the longest wavelength – red is most visible such that as the evening progresses we see mostly reddish sunlight, causing a pink rainbow after refraction by airborne water droplets (BBC, 2017). To understand this phenomenon, we must investigate the science of atmospheric optics further.
Atmospheric optics is a broad science, and pink rainbows are only one part. Another interesting effect due to atmospheric optics besides pink rainbows is rainbow coloured clouds. Clouds can sometimes be seen as having the colours of the rainbow.
The scientific nomenclature for such clouds is iridescent clouds. The cause is similar to pink rainbows in that these effects are caused due to the diffraction of sunlight by water droplets or by tiny crystals in the air. The optical phenomenon responsible for this effect is called diffraction, which is the slight bending of light as it passes along the edge of an object.
This diffraction is caused by the water droplets and ice crystals that cause sunlight diffracted to spread through the object that is the cloud, creating the optical effect of rainbow coloured clouds. The phenomenon is scientifically called irrisation or iridescence. The best method for viewing this phenomenon is to place a foreground object such as a building or a mountain in front of the sun in one’s line of sight and then observe the clouds. The full spectrum of the rainbow is not usually visible, although they can sometimes be vividly visible (D. Byrd, 2016).
Fig: The rare iridescent cloud
Atmospheric Optics and Rainbows
The science of atmospheric optics mostly consists of the scattering of light. The Sun is normally the main source of illumination. Sunlight is always scattered, but usually the scattered light is moving forwards, and seems as if it is not scattered to the eye.
The atmosphere consists of molecules and particles that act to accentuate the scattering of light depending on their refractive index. Even the colour of the sky is a property of the selective scattering of sunlight by bound electrons of molecules in the atmosphere based on the depth of atmosphere encountered by sunlight.
Although molecules of the same substance may be similar, the particles in the atmosphere exhibit an incredible heterogeneity. In substances in the atmosphere such as clouds and water droplets, the molecules are packed together, forming a coherent array. In an incoherent array, such as in the total atmosphere, the scattered light is dependent on the sum of their refractive indexes.
The wave property of light is much more pronounced in a coherent array than an in an incoherent array, such that greater interference is observed in the wavelengths of light as it encounters a coherent array like that provided by water droplets.
Particles forming a coherent array in the atmosphere having a certain wavelength interfere and interfere with the wavelengths of sunlight, sometimes resulting in visible refractive effects. Diffraction by iridescent clouds is similar to the effects of a prism, whereby the forward motion of sunlight is bent according to angular separation of colours in light comprising of different wavelengths by an object.
The sight of a rainbow is dependent on the refractive index of water and the angle sunlight has at the location of incidence relative to the angle of the observer’s line of sight. The latter two, called the solar point and the anti-solar point, must produce a scattering angle of not more than 42o.
In a vacuum a rainbow is hypothesized to be spherical. However in the atmosphere, the shape of the rainbow is attributed to how the plane is defined in that the intersection of the incident rays and the scattered rays produce a semicircle. The angular separation of refracted sunlight by water droplets is scattered in an elliptical plane, forming the bow of the rainbow (C.F. Bohren, undated). In rainbows the incident sunlight is usually white light.
The pink rainbow however, can be attributed to refraction of a more reddish sunlight due to its passage through a greater depth of the Earth’s atmosphere. In such a situation, the parts of the Sun’s colour spectrum with shorter wavelengths were rendered optically invisible to the observer. The phenomenon responsible for pink rainbows is the selective scattering of sunlight by the Earth’s atmosphere in the evening combined with precise scattering of parts of sunlight having greater wavelengths by water droplets.