Perhaps the most well-known, everyday scientific phenomenon is the rainbow, a source of wonder - and even folklore - for young and old alike. Children learn the word 'rainbow' long before the words 'chemistry' or 'physics', yet many reach adulthood without ever questioning just how these colourful light patterns come about.
Characteristics Of Light
To understand how light interacts with matter, we must first come to terms with the nature of light. Rainbows are the result of a number of different tendencies of light coming into play: refraction, reflection and dispersion. For full explanations of refraction and reflection, both of which I have covered in the past, see the articles Images In A Spoon - The Physics and Why We Need Glasses - The Physics.
Dispersion
Light produced by most illuminating objects, such as the Sun, is considered white light. White light contains a spectrum of light wavelengths, from violet's 400 nanometers to red's 700 nanometers, and each wavelength of light behaves differently within a given material.
In most materials, the refractive index (the degree to which light is 'bent' through a material) decreases with increasing wavelength and increases with decreasing wavelength.
In other words, violet light is generally deviated the most and red light the least. Materials of greater refractive index produce greater degrees of dispersion since the fixed percentage difference between wavelengths represents a greater numerical index disparity.
Prisms of glass or perspex are used in classrooms to demonstrate the difference in refractive behaviours between colours of light.
Rainbows
Sunlight entering a droplet of water in the air is refracted, then partially reflected from the back surface of the droplet, and refracted again upon exit. This means that, while a light ray entering head-on through the middle of a raindrop will exit along the same line, all other rays exit at an angle between 0° and a maximum angle. A visual aid helps:
You may notice that the density of the rays leaving the droplet increases as the angle of exit increases (they 'pile up').
This pile-up effect explains why the bright area of the red disk is slightly larger than the bright area of the orange disk, the orange larger than the yellow, and so on.
Secondary Rainbows
You may also have seen what is termed a 'secondary rainbow': a larger, dimmer rainbow of reversed colour order alongside the primary rainbow.
A secondary rainbow is formed by rays that undergo two internal reflections in addition to the two refractions. Remember, each reflection is partial, meaning that some light is transmitted (considered 'lost' with respect to internal reflection) each time, which is why the secondary rainbow is dimmer than the original.
The second reflection results in a larger violet ray exit-angle than red ray angle - hence the secondary rainbow appears 'reversed' or 'reflected'.
'Roy G Biv'
Mnemonics and monikers are often used to teach children things which would prove difficult to remember otherwise.
ROYGBIV (red, orange, yellow, green, blue, indigo, violet) is commonly taught in classrooms as the order of light wavelength in the electromagnetic spectrum, and thus the order of colours in a primary rainbow. Secondary rainbows are not usually discussed at school - 'Vib G Yor' just doesn't have the same ring to it...
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References:
University Physics with Modern Physics by Young and Freedman
Hmm, looks like I might make a lesson plan out of your post soon, if that's ok with you! :D
That's absolutely fine. Please let me know how it goes!