Mirages are optical phenomena in which light rays are bent due to thermal variations in the refraction index of air, producing displaced or heavily distorted images of distant objects. A mirage is also called a shimmer, optical haze which can work like a fog or mist, blurring objects seen at a distance. Optical haze occurs in a layer of air next to the ground where warmer air flows up and colder air descends, creating wind patterns known as convective currents. The difference in how the warm and cold air refract light causes objects seen though the layer to blur. A mirage is an optical phenomenon in which light is refracted through a layer of hot air close to the ground, giving the appearance of there being refuge in the distance, while a rainbow is a multicolored arch in the sky, produced by prismatic refraction of light within droplets of rain in the air. A rainbow shimmers with beauty, hope and promise.
The rainbow is a pattern with many colors or an arc of spectral colors, usually identified as red, orange, yellow, green, blue, indigo, and violet, that appears in the sky opposite the Sun as a result of the refractive dispersion of sunlight in drops of rain or mist. You probably know that rainbows are produced by sunlight passing through a raindrop. The light is bent or refracted because the denser water causes the light to travel more slowly. That light, now separated into its component wavelengths (colors), is reflected off the back of the raindrop and back out producing a colorful arc across the sky.
Rainbows are actually circles, centered on a point directly opposite the Sun and the Sun must be behind you to see a rainbow. We are only able to see the portion of that circle which is above the horizon. A rainbow is round because the process is based on angles. Light from the Sun hits the little water droplets after a rain. The water droplets act like little prisms. Different colors are sent out at different angles. If the Earth were not in the way, a rainbow would be a complete circle. This is the reason why you can never find the pot of gold at the end of the rainbow.
Around 350 BC, Aristotle (384–322 BC) Greek philosopher was the first to really try to describe the rainbow. He presumed that the rainbow was caused by reflection of sunlight in the clouds. The light was reflected at a certain angle. That means the rainbow consists of a cone of rays. Aristotle was the first to explain the rainbow’s circular shape and that the rainbow is not located at a definite place on the sky, but is seen in a certain direction.
Around 300 BC, Epicurus (341–270 BC) a Greek philosopher and a prolific author wrote a letter to his friend Pythocles in which he explained many celestial and meteorological phenomena including the rainbow. The rainbow may be produced by the reflection of the solar rays on the moist air; or it may arise from a particular property of light and air, in virtue of which these particular appearances of color arc formed, either because the shades which we perceive result directly from this property, or because, on the contrary, it only produces one single shade, which, reflecting itself on the nearest portions of the air, communicates to them the tints which we observe. As to the circular form of the rainbow, that depends either on the fact of’ the sight perceiving an equal distance in every direction, or the fact of the atoms taking this form when re-uniting in the air; or it may he caused by its detaching from the air which moves towards the moon, certain atoms which, being re-united in the clouds, give rise to this circular appearance.
Around 250, Alexander of Aphrodisias Greek alchemist and philosopher wrote about light reflecting off raindrops, which causes a noticeable brightening of the sky inside the primary bow. Similarly, a noticeable darkening of the sky between the primary and secondary bows is caused as light is reflected away from our eyes. This area is known as Alexander’s Dark Band, who first described the phenomenon. Alexander said the rainbow actually consists of two rainbows, the primary and the secondary.
About 703, the Venerable Bede an English monk composed On the Nature of Things where he talked about rainbows. The rainbow with its four colors is formed in the air from the directly opposed Sun and the clouds. This happens when the tip of a my of the Sun that was beamed into a shallow cloud is repulsed and the ray is reflected back toward the Sun, like wax giving back the image of a ring. The rainbow takes its fiery color from the sky, its purple color from the waters, its blue color from the air, and its color green as grass from the Earth.
In 817, Iyyubh or Job of Edessa Nestorian a Christian philosopher and a an East Syriac writer wrote his Book of Treasures, which was a brief encyclopedia of science. Job identified red and green in the rainbow and also identified them as fire and water, explicitly relating their presence to the way the rainbow was formed as sunlight mingling with clouds. He said the rainbow’s red was due to the fiery nature of the sun and its green was due to the watery nature of the clouds.
Around 1230, Robert Grosseteste (1168-1253) English statesman, philosopher, theologian and scientist pioneered rainbow theory at Oxford. Grosseteste wrote that changes in atmospheric conditions, and in quantity and quality of light, altered the shape of the light rays to create the many colors in any single rainbow. Grosseteste’s cosmology of light lent visibility to the harmony of the spheres, and gave a physical dimension to divine illumination. In the world of matter, he thought light was all-pervasive, for every natural body has in itself a celestial luminous nature and luminous fire. Grosseteste presents a quantitative law for the refraction of light, while in the third part of the essay he discusses the phenomenon of the rainbow. Grosseteste noted that visible objects are of a nature similar to the nature of the shining and sparkling sun, the radiation of which, combined with the radiation of the external surface of a body, completes the total perspective of vision.
In 1266, Roger Bacon (1214-1292) English Franciscan Friar Philosopher and scholar measured the angle of the rainbow as being 42°. In his Opus Magnus, Bacon gives an explanation of the rainbow saying it is a geometrically correct natural phenomena as he passed light through a glass bead to produce the rainbow.
In 1304, Theodoric of Freiberg (1250-1310) German monk and physicist discovered the actual scientific cause of the rainbow. Theodoric proposed the hypothesis that each raindrop in the cloud makes its own rainbow. He watched what happened to sunlight as it passed through a large globe filled with water and after observing the diffraction of sunlight in a circular bottle he verified his hypothesis. He documented the reflection and refraction processes that occurs when the sun shines through rain drops. He explained in detail the colors of the primary and secondary rainbows, the positions of the primary and secondary rainbows, the path of sunlight within a drop, how light beams are refracted when entering the atmospheric droplets, then reflected inside the droplets and finally refracted again when leaving them. He explained the role of the individual drops in creating the rainbow and the phenomenon of color reversal in the secondary rainbow.
In 1637, René Descartes described a rainbow by reducing it to a study of one water droplet and how it interacts with light falling upon it. Descartes was the first person to give a full explanation of how a rainbow is formed. He wrote that since rainbows appear not only in the sky, but also in the air near us, whenever there are drops of water illuminated by the Sun, as we can see in certain fountains, that rainbows would arise only from the way in which the rays of light act on these drops and pass from them to our eyes. He showed that if one traces the path through a spherical raindrop of parallel light-rays entering the drop at different points on its surface, each emerges in a different direction, but there is a concentration of emerging rays at an angle of 42 degrees from the reverse direction to the incident rays, in exact agreement with the observed angular size of rainbows.
Since some colors are refracted more than others in a raindrop, the rainbow angle is slightly different for each color, so a raindrop disperses the sun’s light into a set of nearly overlapping colored arcs. Descartes also showed that light-rays which are internally reflected twice inside a raindrop emerge concentrated at an angle of 50 degrees from the reverse direction to the incident rays. This rainbow is naturally less intense than the primary rainbow, since a light-ray always loses some of its intensity at every reflection or refraction. The 50 degrees represents the angle of maximum deviation of doubly reflected light from the reverse direction. Red is concentrated on the inside of the secondary rainbow and violet is concentrated on the outside.
In 1666, Isaac Newton (1642-1727) English scientist and mathematician explained the rainbow’s most spectacular aspect is its colors as he showed that white light being refracted in a prism is split up in colors. The color scattering results from the fact that the index of refraction is dependent on the color wavelength. Each color in the sunlight thus has its own rainbow. What we see is a collection of rainbows, each slightly displaced from to the rest. Newton worked out the angle of the red rainbow, 42° 2′ and for the violet rainbow, 40° 17′. This gives a rainbow of 1° 45′. This would have been the width of the rainbow if the sunlight were parallel, but it isn’t; the sun disk has a diameter of half a degree. Newton concluded that the width of the rainbow should be 2 degrees and 15 minutes.
In 1803, Thomas Young (1773-1829) English polymath came up with many theories of the physical nature of light and he made notable scientific contributions with his studies of the rainbow. Young showed that the waves from two wave sources creates alternating light and darkness. In other words, in some directions the light interferes constructively, in other directions it interferes destructively. Young pointed out that the supernumerary arcs could be caused by constructive and destructive interference of sunrays which have traveled different ways through the raindrop, so the distances traveled can turn out to be an odd number of half wavelengths (darkness) or an even number of wavelengths (light). In some rainbows, faint arcs just inside and near the top of the primary bow can be seen. These supernumerary arcs arise from the interference of light along certain rays within the drop.