Apparent Position of a Rainbow

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:
  • Are caused by sunlight reflecting, refracting and dispersing inside raindrops before being seen by an observer.
  • Appear in the section of the sky directly opposite the Sun from the point of view of an observer.
  • Become visible when millions of raindrops reproduce the same optical effects.
• 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.

Visual processing

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.

Note that:

• 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.