Rainbow over the town

Double rainbow over Cirencester Market Place

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Image of the day – 204

What’s in an image? Sometimes quite a lot, more than meets the eye. I’m posting an image whenever I can.

On 27th October I walked to town for some shopping, coming out of Boots in Cricklade Street I found it had started raining, so went into Bishop’s Walk to find shelter. As the rain eased I moved toward the entrance and looking out saw a bright rainbow so made my way to the Market Place for a better look.

Rainbows are interesting apparitions. Sunlight makes two internal reflections in a raindrop in mid air and these reflections send the light back at a fixed angle from the sun, but as a ray of sunlight enters a raindrop it is not only bent at an angle, each wavelength (or colour) is bent by a slightly different amount. The same happens again as the light leaves the raindrop. So these rays of separated, coloured light now come back at similar but not identical angles from the sun so raindrops in exactly the right place send back a single colour to the viewer eyes. The rainbow is part of a circle, cut off where it meets the ground as there are no raindrops below that point.

Furthermore, a small amount of the light is reflected three times internally in each raindrop and come out at a wider angle. If there are enough raindrops in the air, you can then see another rainbow outside the circle of the main bow. This outer rainbow is faint and not always visible though you can just make it out in the photo. The colours in the outer rainbow are reversed because of the additional reflection inside the raindrops. In the second photo (click it to enlarge it), I’ve increased the intensity of the colours for you. You can just see the outer rainbow in both images (look above the chimneys over the yellow shop on the right (Joules). You may notice the colours are reversed in the outer bow, red on the inside and blue outside.

Finally, took a look at the final photo where I’ve pushed things to the limit. I’ve enlarged a small part of the photo and enhanced the colours even more. Again, click the image for a closer look. Traditionally a rainbow was said to have seven colours – red, orange, yellow, green, blue (a pale blue), indigo (a deeper blue) and violet. In actual fact it’s a steady slide through the range of colours.

Do you notice anything else about this rainbow? If so, leave a comment below.

See also:

• Cirencester – Wikipedia
• Prism – Wikipedia
• Rainbow – Wikipedia
• Spectrum – Wikipedia

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