Monday, October 29, 2012

Rainbows: Optical Wonders


The awe-inspiring sight of a rainbow has brought comfort and hope to people for thousands of years. Wordsworth wrote in an 1802 poem, “My heart leaps up when I behold a rainbow in the sky…” Though the rainbow can cause us to look up in wonder, it can be easily understood scientifically. 

The two basic ingredients of a rainbow are sunlight, particularly from a low angle, and water droplets. That is why rainbows appear after a rain shower. When the sun appears through the clouds, sunlight enters millions of water droplets bending at a slight angle because of the different densities of air and water (index of refraction), bounces off the back surfaces (reflection) of the droplets and then fans out into different colors in different directions upon exiting the droplets, deflected in the process (dispersion). This dispersion happens because each color has a different wavelength. White light is composed of a mixture of wavelengths each of which will refract at a different angle, thus creating the rainbow effect. 

The visible colors of the rainbow are always arrayed in the same order beginning from the outer bow: red, orange, yellow, green, blue, indigo and violet. Sometimes you can see a secondary rainbow, above the primary one. Its color order is reversed because the light within the droplet is refracted twice before it is dispersed. Rainbows can be circular. The ground cuts our rainbows in half, but if you view from above the ground, perhaps from a hot air balloon, you can see the full circular rainbow. 

After Newton explained the optical science of rainbows, the poet John Keats was dismayed. He wrote a poem in 1820 entitled Lamia, declaring that the scientific explanation had removed all the beauty and mystery from this wonder. 
“…Philosophy (science) will clip an Angel’s wings,
Conquer all mysteries by rule and line,
Empty the haunted air and the gnomed mine – 
Unweave the rainbow.”

Rather than removing the wonder, understanding the optics only adds to the awe of the rainbow. The rainbow is a sign of God’s love for the world (Genesis 9:12-16) and at the same time, demonstrates the amazing science of optics.

For more detailed analysis of the optics of rainbows, open this link in Wikipedia: Snell's Law of Refraction

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