A Rainbow is an Optical Phenomenon

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A Rainbow is an Optical Phenomenon

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! A rainbow can form a complete circle when seen from a plane.
No! Rainbows are not always visible at midday because their whole circumference can be below the horizon when the sun is high in the sky.
Red is always on the outside edge of a primary rainbow.
Rainbows produce spectral colours as sunlight is refracted by raindrops.

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:
    • 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.
About the diagram

A rainbow is an optical effect, a trick of the light, caused by the behaviour of light waves travelling through transparent water droplets towards an observer.

  • Sunlight and raindrops are always present when a rainbow appears but without an observer, there is nothing, because eyes are needed to produce the visual experience.
  • A rainbow isn’t an object in the sense that we understand physical things in the world around us. A rainbow is simply light caught up in raindrops.
  • A rainbow has no fixed location. Where rainbows appear depends on where the observer is standing, the position of the Sun and where rain is falling.
  • The exact paths of light through raindrops is so critical to the formation of rainbows that when two observers stand together their rainbows are produced by different sets of raindrops.
Atmospheric rainbow summary
THE EXPERIENCE OF COLOUR

Colour is something we see every moment of our lives if we are conscious and exposed to light. Some people have limited colour vision and so rely more heavily on other senses – touch, hearing, taste and smell.

Colour is always there whether we are aware and pay attention to it or not. Colour is what human beings experience in the presence of light. It is important to be clear about this. Unless light strikes something, whether it is air, a substance like water, a physical object or the retina at the back of our eyes, light, as it travels through space, is invisible and so has no colour whatsoever. As suggested in the previous section, colour is an artefact of human vision, something that only exists for living things like ourselves. Seeing is a sensation that makes us aware of light and takes the form of colour.

The experience of colour is unmediated. This means that it is simply what we see and how the world appears. In normal circumstances, we feel little or nothing of what is going on as light enters our eyes. We have no awareness whatsoever of the chemical processes going on within photosensitive neurons or of electrical signals on their journey towards the brain. We know nothing of what goes on within our visual cortex when we register a yellow ball or a red house. The reality is, we rarely even notice when we blink! In terms of immediate present perception, colour is simply something that is here and now, it is an aspect of the world we see as life unfolds before us and is augmented by our other senses, as well as by words, thoughts and feelings etc.

It takes about 0.15 seconds from the moment light enters the human eye to conscious recognition of basic objects. What happens during this time is related to the visual pathway that can be traced from the inner surface of the eyeball to the brain and then into conscious experience. The route is formed from cellular tissue including chains of neurons some of which are photosensitive, with others tuned to fulfil related functions.

So, let’s start at the beginning!

Before light enters the eye and stimulates the visual system of a human observer it is often reflected off the surfaces of objects within the field of view. When this happens, unless the surface is mirror-like, it scatters in all directions and so only a small proportion travels directly towards the eyes. Some of the scattered light may illuminate the body or face of the observer or miss them completely. Some is reflected off the iris and enables us to see the colour of a person’s eyes. A little more is reflected off the retina – think of red-eye in flash photography.

Cross-section of the human eyeball

If we think of light in terms of rays, then some rays will be in line with the eyes of our observer as they look at an object. Rays that strike the outer surface of the eyeball directly in front of the pupil encounter various transparent media including the cornea, then the lens followed by vitreous humour, the gel that fills the eyeball. Then, they arrive at the retina.

Light enters our eyes perpendicular to the curvature of the cornea along an axis corresponding with the central line of vision, and travels straight towards the retina. It strikes the fovea centralis at the centre of the macula where the sharpest image is formed. All the rays of light around this central line of vision change direction slightly because of refraction. The lens also affects their direction of travel as it adjusts in shape to ensure that as many rays as possible are focused exactly onto the retinal surface.

References: http://www.dsource.in/course/visual-design-colour-theory/colour-perception-and-human-responses

Some key terms

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between ...

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the ...

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another ...

Visible spectrum

The visible part of the electromagnetic spectrum is called the visible spectrum. The visible spectrum is the range of wavelengths ...

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