Apparent Position of a Rainbow

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Apparent Position of a Rainbow

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Rainbows are less common around midday because the higher the Sun is in the sky the lower the rainbow. If the Sun is too high, then by the time raindrops are in the right position to form a rainbow they are lost in the landscape.
It is the small difference in the refractive index of different wavelengths of incident light that causes dispersion and separation of white light into rainbow colours.
Rainbows are at their best early morning and late afternoon when a shower has just passed over and the Sun is illuminating the curtain of raindrops formed on the trailing edge of the falling rain.
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.

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.
Overview of diagram
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

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

Refraction

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

Dispersion

Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the ...

Observer

A human observer is a person who engages in observation by watching things. In the presence of visible light, an ...

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its ...

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