Rainbow Rays

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This is one of a set of almost 40 diagrams exploring Rainbows.


Each diagram appears on a separate page and is supported by a full explanation.

  • Follow the links embedded in the text for definitions of all the key terms.
  • For quick reference don’t miss the summaries of key terms further down each page.

Description

Rainbow Rays

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
The term rainbow rays refers to the deflected rays of light that produce the most intense colour experience for any particular wavelength of light passing through a raindrop.
Yes! Every observer has a unique view of the world because: Each one of us sees the world from a different physical location and so has a unique point of view Every one of us has different life experiences including educational, social and cultural factors that affect how we see the world.
Yes! 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.drops.
A human observer is a person who watches something from their own unique point of view.

About the diagram

An overview of  rainbow rays

  • Rainbows are composed of rainbow rays.
  • Rainbow rays are responsible for an observer’s perception of a rainbow.
  • Rainbow rays are rays of light of a single wavelength that have their origin in individual raindrops. They can be explained in terms of their angular distance from the rainbow axis at the moment they contribute to an observer’s view of a rainbow.
  • Rainbow rays are ephemeral. They are not individually observable but more a way of conceptualizing the fact that at a specific moment and in a specific position a raindrop will transmit one spectral colour towards an observer before falling further, perhaps to reappear in a different position and another colour.
  • Individual rainbow rays produce the intense appearance of each of the different spectral colours that together constitute the phenomenon of rainbows.
  • Rainbows are composed of millions of rainbow rays and each one has its origin within a single raindrop.
  • A rainbow ray is a ray of a single wavelength that for a second is responsible for a bright flash of its corresponding colour as a result of being in exactly the right place at the right time.
  • Rainbow rays are always located amongst the rays that deviate the least as they pass through a raindrop and bunch together around the minimum angle of deviation.
  • The millions of microscopic images of the Sun that produce the impression of a rainbow function in a similar way to the pixels that produce the images we see on digital displays.
  • Rainbow rays tend to out-shine all other sources of light in the sky (other than the Sun) and account for the brilliance and imposing appearance of rainbows.
  • Because raindrops polarize light at a tangent to the circumference of a rainbow, the path of rainbow rays dissects raindrops exactly in half.
  • So:
    • Individual rainbow rays account for the appearance of spectral colours of a single wavelength within the arcs of a rainbow.
    • Bands of colour within a rainbow are composed of rainbow rays that together transmit narrow spreads of wavelengths towards an observer.
    • The overall appearance of a rainbow as a singular phenomenon can be accounted for by optical and geometric rules that determine the passage of light through raindrops and in the process account for rainbow rays.
  • Remember: the notion of light rays and rainbow rays are useful when considering the path of light through different media in a simple and easily understandable way. But in the real world, light is not really made up of rays. More accurate descriptions use terms such as photons or electromagnetic waves.
A bit more about rays

A ray of light (light ray or just ray) is a common term when talking about how and why rainbows appear.

  • The idea that light is made up of rays is so commonplace when describing and explaining rainbows that it is easily taken for granted.
  • The idea of light rays is useful when trying to model how light and raindrops produce the rainbow effects seen by an observer.
  • Light rays don’t exist in the sense that the term accurately describes a physical property of light. More accurate descriptions use terms like photons or waves.
  • Modelling light as rays is a way to discuss and represent the path of light through different media in a simple and easily understandable way.
  • When light rays are drawn in a ray-tracing diagram they are represented as straight lines connected at angles to illustrate how light moves and what happens when it encounters different situations and conditions.
  • More accurate descriptions of light use terms such as photons or electromagnetic waves.
  • Don’t forget that:
    • The incident rays of light that contribute to a rainbow seen by an observer are those that approach raindrops parallel with the rainbow axis.
    • To understand why incident rays are always parallel with the rainbow axis we need to think in terms of what the observer sees. See: 2.7 Observer’s point of view.
About the diagram
  • The main drawing on the left shows the paths taken by parallel rays of yellow light (589.29nm) after initially striking the surface of a raindrop at different points.
  • The detail on the right shows the path of the deflected rays after they exit the raindrop.
  • The incident rays correspond with 0.7 to 0.925 on the impact parameter scale.
  • All the rays are of the same wavelength but are coded with coloured buttons so that their paths can be compared.
  • The dotted yellow path at 8.50 on the impact parameter scale corresponds with the rainbow ray.
The important thing to notice
  • The rainbow ray shown in the diagram is located amongst the rays that deviate the least as they pass through a raindrop and bunch together around the minimum angle of deviation.
  • It is difficult to show clearly just how tightly rays bunch together around the minimum angle of deviation.
  • It is also difficult to show that at some considerable distance from a raindrop the path of a rainbow ray and a line corresponding with the minimum angle of deviation will always intersect one another.
  • This Angles of Deviation Table explains why rays bunch together around the minimum angle of deviation.
Other things to notice
  • The paths of the rays follow a predictable pattern because the only variable is their point of impact, as shown on the impact parameter scale.
  • The detail on the right shows that after leaving the raindrop that the rays scatter. This scattering is not random but can be shown to follow a predictable pattern determined by their initial angle of refraction
  • To understand the geometry that determines their different angles of deflection check out our Angle of Deviation diagram and the carousel of images that appears on that page.
  • Don’t forget that the angle of deflection and the minimum angle of deviation always add up to 1800.

Some key terms

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

  • The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
  • Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
  • Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
  • Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.

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