DO YOU remember the unfortunate Lady of Shalott, doomed forever to labour at her loom, and permitted by virtue of an enigmatic curse to observe the world only through a rear-view mirror?
And moving through a mirror clear
That hangs before her all the year.
Shadows of the world appear;
There she sees the high-way near
Winding down to Camelot.
Now, for those of us whom a rear-view mirror is an optional rather than a compulsory accessory, the device can be instructive. It allows us, for example, to look in two opposing directions simultaneously, an exercise which makes it obvious that there is a remarkable difference in colour and texture to be seen when viewing a landscape, depending on whether we are looking at it towards the sun, or with the light behind us.
This in turn is helpful in trying to understand why a freshly-mown football field or cricket pitch, or even the lawn chez vous if it is of good quality, are often tastefully arrayed with stripes of differing shades of green.
Almost any object, be it a blade of grass or anything else, appears to be a certain colour because its molecular structure is such as to absorb certain wave-lengths of light and to reflect others. In the case of grass, all colours except those which make up the colour green are absorbed; the unabsorbed green light is "rejected" and registers as that colour when it hits our eyes.
When sunlight hits a blade of grass at an oblique angle, however, some of the incident light in all parts of the spectrum is reflected before the process of selective absorption can take place. This, as far as an observer is concerned, has the effect of over-laying the green with a whitish tinge; the green appears lighter than if the leaf were viewed at some different angle, or that it would if not directly illuminated by the sun at all.
Now, mowing a lawn or a football pitch gives nearly all the blades of grass mown in the one direction a common orientation; when the mower turns and cuts a parallel stripe the opposite way, the leaves acquire a different - but again nearly uniform - orientation. The sunlight, therefore, hits the blades of grass in two adjacent strips at a different angle, so they reflect and absorb the incident light in different proportions, and give a different perception to the eye. Moreover, if you view the stripes from one end of a football pitch, and then move to observe them from the other, you will find the shading on the alternating stripes reversed.