|Glove Hair Corona
Images from Hungary by Monika Landy-Gyebnar.
All is not what it seems. The early morning sun is ringed by a beautiful corona. But it is not from clouds or mist.
Monika tells the story – “..I heard a song thrush at a treetop and wanted to take a picture of it. However, I could not see it because the sunshine was too strong. I shaded the Sun from the camera lens with my gloved hand - a knitted glove with lots of thin hairs on the yarn. These hairs produced the corona! At first I thought it was made by my breath because it was a chilly morning.
The camera focus was far beyond my hand and that is when the corona showed itself best. I took other pictures close focused on the thin hairs and no corona was visible that way."
Monika then experimented indoors in the warm with the hairy gloves and a halogen lamp. The corona appeared again.
All images ©Monika Landy-Gyebnar
|Diffraction from two points only on a sphere.|
The glove hairs imaged at 200X by Monika.
|Most coronae we see are produced by light diffracted around the edges of small water drops. At left is a much simplified instance of light diffracted by just two points on a droplet's rim. The two outgoing diffracted spherical waves overlap and interfere. There is brightness where the overlapping wave crests are in phase. Waves scattered from the whole of the drop periphery produce the familiar coronal pattern.
Periphery is the key. The scattering particles need not be transparent or even spherical. Pollen grains or spores produce coronae - even telescope lenses. Here, the diffraction is by fine hairs. Had the hairs been all pointing in the same direction the corona (if it formed) would have been much elongated.
Experiment with hairy objects. But focus the eyes or camera on infinity.
|The corona is strongest when the lens is focussed at infinity. That is the effective distance of any corona produced by near parallel light.
When the lens is focussed on the hairs themselves we see them glowing bright as they diffract light around them - but no corona.