Scratched Perspex, light rings and diffraction

Scratched Perspex ~ Graham Stephen ( GeoTopoi LustreBox ipernity ) inadvertently caught a reflection of the sun when photographing a perspex covered panel about the village of Malltraeth, Anglesey, Wales. The sun's reflection is surrounded by light rings plus several orders of circular diffraction grating colours. Image ©Graham Stephen, shown with permission

Light Rings

We see light rings from wet or frozen branches in a wood, from spider webs in grass, from the hairs of seed heads, from a scratched saucepan.

In all these, cylindrical objects or linear scratches reflect light. The scratches are in all directions but only those perpendicular to a line pointing to the light source reflect to the eye. As a result, the 'lit' scratches appear to form circles around the light. The brain delights in making patterns!

Below: A computer simulation. Only scratches perpendicular to the light reflect to the eye.

Diffraction Colours

In the light rings we see large discrete scratches illuminated. But evidently this perspex sheet also has much finer closely spaced scratches. Like the larger ones they are in all directions but those in a 'light ring alignment' form a circular diffraction grating.

A more familiar circular grating is a DVD or CD that flashes with colours as it is tilted.

At left, regularly spaced scratches scatter light into outgoing waves. The individual wave crests line up in particular directions to give 1st, 2nd and higher order diffraction fringes. The directions are wavelength dependent and so with white light illumination we actually see a set of spectra.

The perspex scratches must be quite evenly spaced because 3-4 spectrum orders exist.