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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Atmospheric optics | 4/5 | https://en.wikipedia.org/wiki/Atmospheric_optics | reference | science, encyclopedia | 2026-05-05T14:32:13.838616+00:00 | kb-cron |
A rainbow is a narrow, multicoloured semicircular arc due to dispersion of white light by a multitude of drops of water, usually in the form of rain, when they are illuminated by sunlight. Hence, when conditions are right, a rainbow always appears in the section of sky directly opposite the Sun. For an observer on the ground, the amount of the arc that is visible depends on the height of the sun above the horizon. It is a full semicircle with an angular radius of 42° when the sun is at the horizon. But as the sun rises in the sky, the arc grows smaller and ceases to be visible when the sun is more than 42° above the horizon. To see more than a semicircular bow, an observer would have to be able to look down on the drops, say from an airplane or a mountaintop. Rainbows are most common during afternoon rain showers in summer. A single reflection off the backs of an array of raindrops produces a rainbow with an angular size that ranges from 40° to 42° with red on the outside and blue/violet on the inside. This is known as the primary bow. A fainter secondary bow is often visible some 10° outside the primary bow. It is due to two internal reflections within a drop. The resulting secondary arc is some 3° wide and the colours are reversed, with blue/violet on the outside. Two internal reflections produce a bow with angular size of 50.5° to 54° with blue/violet on the outside. The region between a double rainbow is often noticeably darker that the sky within the primary bow and that beyond the secondary bow. It known an Alexander's Dark Band. The reason for this apparent reduction in sky brightness is that, while light from the sky enclosed within the primary bow comes from droplet reflection, and light beyond the secondary bow also comes from droplet reflection, there is no mechanism for the region between the bows to reflect light in the direction of the observer. Generally speaking, larger the droplets make for brighter bows. A rainbow spans a continuous spectrum of colors; the distinct bands (including the number of bands) are an artifact of human color vision, and no banding of any type is seen in a black-and-white photograph of a rainbow (only a smooth gradation of intensity to a maxima, then fading to a minima at the other side of the arc). For colors seen by a normal human eye, the most commonly cited and remembered sequence, in English, is Isaac Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv).
== Mirage ==
A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at". This is the same root as for "mirror" and "to admire". Also, it has its roots in the Arabic mirage. In contrast to a hallucination, a mirage is a real optical phenomenon which can be captured on camera, since light rays actually are refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water. Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and "Fata Morgana", one kind of superior mirage consisting of a series of unusually elaborate, vertically stacked images, which form one rapidly changing mirage. Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the Sun, or a green ray shoots up from the sunset point. Green flashes are actually a group of phenomena stemming from different causes, and some are more common than others. Green flashes can be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well. A green flash from the Moon and bright planets at the horizon, including Venus and Jupiter, can also be observed.
=== Fata Morgana ===