Path of a Red Ray Through a Raindrop

Path of a Red Ray Through a Raindrop

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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.
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Description

Path of a Red Ray Through a Raindrop

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
In the field of optics, what is meant by a ray or a ray of light?
In a ray-tracing diagram, a light ray is a way of visualising the motion of light, including its direction of travel, and what happens when it encounters different media.
What causes dispersion of sunlight as it passes through a raindrop?
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.
What does angular distance refer to when an observer sees a rainbow?
Angular distance is the angle between the rainbow axis and the direction in which an observer must look to see the coloured arcs of a rainbow.
When is the best time to see a rainbow?
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.
Does chromatic dispersion take place as white light travels from air into a raindrop?
Yes! Every wavelength of light is affected to a different degree by the refractive index of a transparent medium and as a result, changes direction by a different amount when passing from air to water or water to air.

About the diagram

Overview of raindrops

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Overview of raindrop geometry
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About the diagram
  • In this diagram, an incident ray of white light strikes the surface of a raindrop. The diagram traces the path of a red ray as it is refracted, reflected and refracted again before being seen by an observer.
  • The points at which the ray strikes the raindrop are marked as follows:
    • A = Initial point of impact of the incident ray
    • B = Point at which ray reflects off the interior of the droplet
    • C = Point at which the ray is refracted for a second time and exits the droplet towards the observer
  • The key explains the angles marked on the diagram as follows:
    • a = Angle of incidence = Angle at which the ray strikes the raindrop
    • b = Angle of refraction = Change in direction and speed
    • d = Angle of deviation = Angle to which ray is bent back from its original path towards the observer
    • z1 = Angle of deflection = z2 = Angular distance
    • y = Matching corners of isosceles Δ’s = b
Understanding the diagram
  • This page introduces a topic that is dealt with in more detail later in this series.
  • Visitors who want to develop a comprehensive understanding of the role of individual raindrops in the appearance of rainbows will find that Section 7 of Rainbows: In detail provides a thorough introduction.

Some key terms

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Diagrams are free to download

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