Apparent Position of a Rainbow
This is one of a set of almost 40 diagrams exploring Rainbows.
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Apparent Position of a Rainbow
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About the diagram
An overview of rainbows
An atmospheric rainbow is an arc or circle of spectral colours and appears in the sky when an observer is in the presence of strong sunshine and rain.
- Atmospheric rainbows:
- Atmospheric rainbows often appear as a shower of rain is approaching, or has just passed over. The falling raindrops form a curtain on which sunlight falls.
- To see an atmospheric rainbow, the rain must be in front of the observer and the Sun must be in the opposite direction, at their back.
- A rainbow can form a complete circle when seen from a plane, but from the ground, an observer usually sees the upper half of the circle with the sky as a backdrop.
- Rainbows are curved because light is reflected, refracted and dispersed symmetrically around their centre-point.
- The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
- An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer and the anti-solar point – the geometric centre of a rainbow.
- A section of a rainbow can easily disappear if anything gets in the way and forms a shadow. Hills, trees, buildings and even the shadow of an observer can cause a portion of a rainbow to vanish.
- Not all rainbows are ‘atmospheric’. They can be produced by waterfalls, lawn sprinklers and anything else that creates a fine spray of water droplets in the right conditions.
Overview of diagram
Visual processing refers to the way information begins to be processed the moment light enters the human eye. It then continues through a series of stages as signals travel towards and into the visual cortex where the stream is incorporated into the spread of neural activity that resolves into conscious visual experience.
As visual processing of the continuous stream of fluctuating wavelengths of light begins within the retina, it starts not only to compose information about the colours themselves, but also rudimentary information about the shape and movement of those colours. By the time this stage of visual processing is done it is ready to convey at least a dozen representations of a visual scene to higher brain regions.
Let’s look at trichromatic and opponent-processing first, two of the major forms of processing that take place within the eyeball as visual information is gleaned from the light entering our eyes and undergoes the initial stage of preparation that will ultimately be translated into our perceptions of the world.
Trichromacy, or in other words, the trichromatic theory of colour vision, seeks to explain how three types of cone receptors in the retina at the back of our eyes work in concert with bipolar cells to carry out their part of this initial stage of colour processing. Rod cells also contribute to the process but have a more important role in dealing with dim and dark conditions.
Opponent-processing, or in other words, the opponent-process theory of colour vision seeks to explain the second form of processing. Opponent-processing is associated with ganglion cells that work on the data they receive from trichromatic processing but also combine it with the output of other intercellular activities.
It is interesting to note that as both trichromatic and opponent-process theories developed over the last century, researchers and authors often pitted one theory against the other. But both processes are important in understanding how colour vision comes about. Trichromatic theory explains how visual information is encoded as light strikes the retina and opponent-processing explains a subsequent level of convergence of information, its assembly and coding before the data exits the retina along the optic nerve.
- Both trichromatic and opponent-processing take place within the retina of each eye without any comparison of the other.
- The information gathered by each eye is from a specific viewpoint (around 50 mm to the left or right on the nose).
- The two impressions will be compared and combined later to enable us to see a single three-dimensional, stereoscopic view of the world rather than a pair of flattened images.
We can think of each of the layers of retinal cells involved in trichromatic and opponent-processing as interrogating, interpreting and transporting visually related information. But it would be inaccurate to conceive of this as a simple linear process because of the complexity of networking between neurons within the retina and the amount of cross-referencing and feedback loops involved.
About the diagram
The important points this diagram makes are that:
- All raindrops that form part of a rainbow appear the same colour to an observer regardless of distance.
- The raindrops that form part of a rainbow at any particular moment can be anywhere within a cone centred on the eyes of an observer.
- If raindrops are close by, and in the right position to reflect light into the eyes of an observer, a rainbow may seem almost close enough to touch.
- If there are raindrops close by and others in the middle distance (and in the right position), they will all contribute to a rainbow regardless of its apparent distance from the observer.
- Rainbows don’t have qualities like size or distance because they are simply the impression produced by millions of individual droplets of rain reflecting and refracting light in every possible direction.
- Whilst raindrops scatter light everywhere, only those rays that enter and exit at exactly the right points and at the right moment direct light towards an observer.
- This diagram shows an observer looking up towards droplets of rain as parallel rays of white light from the Sun are reflected back towards them.
- The diagram is a cross-section and shows the observer looking up towards the top of the rainbow.
- The observer sees coloured droplets at different elevations with red at the top and violet at the bottom.
- The raindrops are all of a similar size and shape but are falling across the observer’s field of view.
- As raindrops pass an elevation of 42.20 from the axis they appear red. As they continue to fall each one changes colour, first to orange then yellow, green, blue and finally at 400, violet.
- Each colour of visible light corresponds with a different wavelength but instead of seeing a smooth and continuous range of colours the observer can see distinct bands of colour.
Angle between incident and refracted rays
When light strikes a raindrop, the angle between the incident and refracted rays is often called angular distance.
- Angular distance is usually measured between the axis of a rainbow and the elevation of those raindrops seen by an observer.
- Angular distance can also be measured using the angle between the path of an incident ray of light before it strikes a raindrop and its path after it leaves the raindrop and is approaching the observer. See our diagram Path of a Red Ray Through a Raindrop for more details.
- In diagrams showing the Sun, observer and rainbow, angular distance is often shown as an angle between the axis and a point at the apex of a rainbow. In reality, the angular distance for any colour is the same at every position on the arc or entire circumference of a rainbow due to polarization.
Some Key Terms
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