This is one of a set of almost 40 diagrams exploring Rainbows.
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About the Diagram
- The areas of sky around a rainbow may appear blue or grey depending on weather conditions and the amount of cloud in the sky. But these areas outside, inside and between primary and secondary rainbows tend to appear tonally different from one another:
- Alexander’s band can be explained by the fact that fewer photons are directed from this area of the sky toward an observer.
- The raindrops that form a primary rainbow all direct exiting light downwards towards an observer so away from Alexander’s band.
- The raindrops that form a secondary bow all direct light upwards, so away from Alexander’s band, before a second internal reflection directs light downwards towards an observer.
- Alexander’s band is named after Alexander of Aphrodisias, an ancient Greek philosopher who commented on the effect in his writing.
The most common atmospheric rainbow is a primary bow.
- Primary rainbows appear when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, is refracted again as it escapes back into the air, and then travels towards an observer.
- The colours in a primary rainbow are always arranged with red on the outside of the bow and violet on the inside.
- The outside (red) edge of a primary rainbow forms an angle of approx. 42.40 from its centre, as seen from the point of view of the observer. The inside (violet) edge forms at an angle of approx. 40.70.
- To get a sense of where the centre of a rainbow might be, imagine extending the curve of a rainbow to form a circle.
- If your shadow is visible as you look at a rainbow its centre is aligned with your head.
- A primary rainbow is only visible when the altitude of the sun is less than 42.4°.
- Primary bows appear much brighter than secondary bows and so are easier to see.
- The curtain of rain on which sunlight falls is not always large enough or in the right place to produce both primary and secondary bows.
A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then travels towards an observer.
- A secondary rainbow always appears alongside a primary rainbow and forms a larger arc with the colours reversed.
- A secondary rainbow has violet on the outside and red on the inside of the bow.
- When both primary and secondary bows are visible they are often referred to as a double rainbow.
- A secondary rainbow forms at an angle of between approx. 50.40 to 53.40 to its centre as seen from the point of view of the observer.
- A secondary bow is never as bright as a primary bow because:
Some Key Terms
A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.
- Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
- Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
- A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
- It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
- When the sun is behind an observer then the rainbow will appear in front of them.
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