Grass covers three quarters of this is land. Three factors control its growth rate: temperature, the moisture supply and sunshine. For most of the year, temperature is the limiting factor.
When the air temperature is less than about 5 Celsius there is no growth; above 5, the warmer it is the faster the grass grows until about 10; at 10, growth is at a maximum and any further rise in temperature has little effect.
Sometimes in the summer, lack of moisture may diminish the amount of grass produced, and during long dry periods growth may stop altogether unless water is added artificially. Finally, sunshine affects the rate of photosynthesis, so that greater quantities of grass are produced when daytime skies are clear than when conditions are cloudy.
A consequence of all this is that the patch of green we call the lawn requires to be mown at intervals in every season except winter. Sometimes the result is interesting: the freshly mown grass may be adorned with tasteful stripes of differing shades of green. But this is odd, because if you look closely you will see that the grass is just plain green - not 40 shades of it, or even two.
A blade of grass acquires its colour from the fact that it absorbs all wavelengths of sunlight except the green; the unabsorbed green light is "rejected" and registers as that colour when it strikes 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 has the effect of overlaying the green with a whitish tinge; the green app ears lighter than if the leaf were viewed at some different angle or than it would if not directly illuminated by the sun.
Mowing a lawn, particularly when the blades are followed by a roller, gives nearly all the blades of grass mown in the one direction a common orientation; when the mower turns and cuts a parallel strip 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 differing proportions and give a different perception to the eye.
Moreover, if you view the stripes from one end of your lawn, and then move to observe them from the other, you will find the shading on the alternating stripes reversed.