Atmospheric
Optics
Fireweed Optics
Fireweed, rosebay willowherb, seed heads pictured in Scotland by Lesley Jennings.
Each seed has a silky hair to aid wind dispersal.
Reflections from them make light rings around the sun. Wet branches or scratched saucepans also form light rings. The hairs point in all directions but only those tangent to the sun reflect its light to the camera.
The tiny hairs do more, they diffract light to give the mysterious colour bands also seen on another silk – that of spiders.
All images ©Lesley Jennings, shown with permission
Diffraction
Deliberately throw the seed hairs out of focus and multi-coloured bands appear. Similar bands are formed by spider webs, moths , dandelion seeds and bicycle wheels
Irregularities on the silk surface act as a pseudo diffraction grating. The irregularities scatter sunlight and some of the outgoing waves are sufficiently in phase to form the colour bands.
A regular diffraction grating. Outgoing scattered waves of particular colour and direction are all in phase.
Reflection
A simulation of light rings. The computer draws lines pointing in random directions. They represent fireweed seed hairs, wet branches, scratches on glass or metal, spider webs in grass.
The computer crawls along each line and calculates the angle between the line and the direction of the central ‘sun’. Only when the angle is close to 90 degrees does it light up that section of line.
Real light rings form the same way. Grooves or the shiny surfaces of hair or wet twigs act as mirrors to reflect light towards us only when they are tangent to the sun.
