How to See a Rainbow

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Description

How to See a Rainbow

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
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.
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.
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.
The best rainbows appear in the morning and evening when the Sun is strong but low 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.
Conditions for seeing rainbows

There are three basic conditions that have to be fulfilled before an atmospheric rainbow appears:

  • Bright sunlight shining through clear air.
  • A curtain of falling rain in the near to middle distance.
  • An observer in the right place at the right time.
About the diagram

The weather, season and time of day are all important if you hope to see an atmospheric rainbow.

  • The best rainbows appear in the morning and evening when the Sun is strong but low in the sky.
  • Northern and southern latitudes away from the equator are good for rainbows because the Sun is lower at its zenith.
  • Mountains and coastal areas can create ideal conditions because as air sweeps over them, it cools, condenses and falls as rain.
  • Rainbows are rare in areas with little or no rainfall such as dry, desert conditions with few clouds.
  • Too much cloud is not good because it blocks direct sunlight.
  • Winter is not necessarily the best season because the light is weaker and there can be excessive cloud.
  • 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 the raindrops are in the right position to form part of a rainbow they are lost in the landscape.

Some key terms

Refraction

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

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the ...

Rainbow colours

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

Spectral colour

A spectral colour is a colour evoked in normal human vision by a single wavelength of visible light, or by ...

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