Rainbows Appear as Arcs of Colour
This is one of a set of almost 40 diagrams exploring Rainbows.
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Rainbows Appear as Arcs of Colour
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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:
- 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.
- Light is a form of radiation, a type of energy that travels in the form of electromagnetic waves and can also be described as a flow of particle-like ‘wave-packets’, called photons.
- Radiation, electromagnetic waves and photons are all concepts that are interchangeable with the more general concept 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 energy, photons. 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:
- Photoelectric effect
Some facts about electromagnetic waves
- An electromagnetic wave carries electromagnetic radiation.
- Electromagnetic radiation is measured in terms of the amount of electromagnetic energy carried by an electromagnetic wave.
- Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields propagating at the speed of light in a vacuum.
- The kinetic energy carried by electromagnetic waves is often simply called radiant energy or light.
- Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
- Other terms for the amplitude of light are intensity and brightness.
- Another term for the speed at which light travels is its velocity.
- We can feel electromagnetic waves release energy when sunlight warms our skin.
- The position of an electromagnetic wave within the electromagnetic spectrum can be identified by its frequency, wavelength or energy.
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
Bands of colour
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