Rainbows Appear as Arcs of Colour

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


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Description

Rainbows Appear as Arcs of Colour

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! Each colour in a rainbow between red and violet is a spectral colour.
Red is always on the outside edge of a primary rainbow.
No! The outermost colour on a secondary rainbow is violet because the colours of a secondary rainbow are reversed.

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
  • This diagram shows an observer looking up towards a curtain of rain as parallel rays of incident white light from the Sun are reflected back towards them.
  • The observer sees the rainbow because of the combined effects of refraction, reflection and dispersion of light within each raindrop.
  • In this primary rainbow, the observer sees bands of colour stacked one above the other with red at the top and violet at the bottom.
  • The raindrops are all of a similar size and shape and are falling across the observer’s field of view.
  • As raindrops pass a point that is at 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.
  • Bands of colour result from the fact that the human eye perceives some colours more strongly than others.
Rainbows and light

Rainbows result from light encountering raindrops in the presence of an observer. The phenomenon of rainbows offers many clues as to the nature of light.

Theories of light

There are four principal theories that underpin our understanding of the physical properties of light as it relates to rainbows:

  • Wave theory – the idea that light is transmitted from luminous bodies in an undulatory wave-like motion.
  • Particle theory – the idea that the constitution and properties of light can be described in terms of the interactions of elementary particles.
  • Electromagnetic theory – the classical theory of electromagnetism that describes light as coupled electric and magnetic fields, transporting energy as it propagates through space as a wave. The energy is stored in its electric and magnetic fields and can be measured in terms of its intensity.
  • Quantum theory – explains the interactions of light with matter (atoms, molecules etc.) and describes light as consisting of discrete packets of energyphotons. Quantum physics suggests that electromagnetic radiation behaves more like a classical wave at lower frequencies and more like a classical particle at higher frequencies, but never completely loses all the qualities of one or the other.
These theories tell us things about the properties of light
  • Light is electromagnetic radiation, the force carrier of radiant energy.
  • Whilst it carries energy and has momentum, light has no mass and so is not matter.
  • Light is the result of the interaction and oscillation of electric and magnetic fields.
  • Light is a microscopic phenomenon that needs macroscopic metaphors such as waves and particles to describe it.
  • Once emitted at its source, light can propagate indefinitely through a vacuum in a straight line at the speed of light (299,792,458 metres a second) but can be deflected by gravity.
  • In any specific instance, light can be described in terms of the inter-relationship of its wavelength, frequency and energy.
  • Light slows down and is deflected as it propagates through air, water, glass and other transparent media as photons interact with matter.
Phenomena associated with light include:
Some facts about electromagnetic waves
Some facts about photons
  • Photons are the elementary building blocks and so the smallest unit used to describe light.
  • Photons are the carriers of electromagnetic force and travel in harmonic waves.
  • Photons are zero mass bosons.
  • Photons have no electric charge.
  • The amount of energy a photon carries can make it behave like a wave or a particle. This is called the “wave-particle duality” of light.
Facts about the electromagnetic spectrum
  • Visible light is just one tiny part of the electromagnetic spectrum.
  • Our eyes only respond to the visible light which we see as colours between red and violet.
  • The electromagnetic spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
  • The size of the longest wavelengths is unknown but the shortest is believed to be in the vicinity of the Planck length (approximately 1.6 x 1035 meters).

Some key terms

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

  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • As light travels through the air it is invisible to our eyes.
  • Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
  • The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
  • Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.

White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
  • As light travels through the air it is invisible to our eyes.
  • When we look around we see through the air because it is very transparent and light passes through it.
  • The term white light doesn’t mean light is white as it travels through the air.
  • One situation in which light becomes visible is when it reflects off the surface of an object.
  • When white light strikes a neutral coloured object and all wavelengths are reflected then it appears white to an observer.

On a sunny day, stand with the Sun on your back and look at the ground, the shadow of your head coincides with the antisolar point.

  • The anti-solar point is the position on the rainbow axis around which the arcs of a rainbow appear.
  • 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.
  • The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
  • As seen in side elevation, the centre-point of a rainbow is called the anti-solar point.
  • ‘Anti’, because it is opposite the Sun with respect to the location of an observer.
  • Unless seen from the air, the anti-solar point is always below the horizon.
  • The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
  • First, second, fifth and sixth-order bows all share the same anti-solar point.

The Sun is the star at the centre of our solar system.

A human observer is a person who engages in observation by watching things.

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