Angular Distance & Raindrop Colour

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED

Overview of diagram

• Rainbows form when sunlight encounters a curtain of rain.
• The sunlight enters raindrops at one angle and then emerges at another.
• The water droplets have to be in just the right place to reflect coloured rays into an observer’s eyes.
• Each raindrop is made of liquid water and acts as a tiny prism.
• Raindrops breaks down sunlight into spectral colours, so into red, orange, yellow, green, blue and violet.
• The visible spectrum is composed of wavelengths between approximately 380 and 740 nanometres and each corresponds with a different colour.
• Although we recognise the rainbow colours ROYGBV there is a colour corresponding with each and every wavelength.
• Each droplet of rain can only direct one colour towards an observer’s eyes. All the other colours exit at the wrong angle and go off in other directions.
• Rainbows are described as being both atmospheric and optical phenomena.

About the diagram

• This diagram shows an observer looking towards the anti-solar point at the centre of a rainbow.
• The rainbow forms a complete circle as if seen from a plane.
• A rainbow only forms a complete circle when the ground around an observer doesn’t get in the way.
• Normally, a rainbow produced by sunlight is reduced from a circle to a semi-circle or an arc.

• The diagram shows two raindrops, one is above (red) and one is below (violet) the rainbow’s axis.
• Both the raindrops are of a similar size and shape and are within a curtain of rain falling across the observer’s field of view.
• It is the difference in angular distance from the axis that determines their colour.
• As raindrops that are in the right position at the right moment pass an elevation of 42.20 from the axis they appear red. As their angular distance decreases, they appear orange then yellow, green, blue and finally at 40⁰, violet.
• Once the angular distance drops below 40⁰ raindrops don’t contribute colour to a rainbow.
• 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.

The angle between incident and refracted rays

• The angle between incident and refracted rays is often called the angular distance. Angular distance is usually measured between the axis and the elevation of coloured raindrops as seen by an observer.
• Angular distance can also be measured using the angle between the path of an incident ray of light before it strikes a raindrop and its path after it leaves the raindrop and is approaching the observer. See our diagram The Path of a Red Ray Through a Raindrop for more details.
• For convenience and consistency angular distance is often shown in rainbow diagrams measured between the axis and the top of the rainbow as seen by an observer. In reality the angular distance for any colour is the same at every position on the arc or entire circumference of a rainbow.