Likewise, about variation in light inten­sities reaching at

Likewise, sun’s altitude changes due to differences in latitude, changes in the season and in the time of day. When sun remains overhead, the intensity of sunlight over the earth’s surface will be greatest. At higher latitudes the intensity of light becomes corre­spondingly reduced.

The illumination or intensity of daylight is greatly diminished by moisture, clouds, and dust in the atmosphere and also by forest vegetation. The direction and slope also affect light intensity. There will be no light on the one side of the slope.

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The layers of vegetation bring about variation in light inten­sities reaching at various heights of a mountain. For example, in forests, major proportion of light intensity is absorbed by tree vegetation and the light reaching to the lower part of the ground vegetation is considerably reduced by 90—98 per cent of that in the exposed areas.

The amount of light reaching the forest floor depends upon the height of canopy, crown development of trees, age of trees and phenological characteristics of the constituent species. Thus in a forest, the mature tallest trees receive full inso­lation, under shrubs receive subdued illumination, and herbs and especially epigenous cryptogams grow in lighter light conditions.

When leaves of a tree are fully expanded, its canopy may reduce light to less than 1% of full solar radiation. However, in nature sun-flecks and movement of leaf canopy of trees play an important role in compensating for the reduced light and in making it avai­lable to the ground flora.

The light which enters in the aquatic media, comes from sun by passing through the atmosphere existing above the water sur­face and hence, that is subjected to all kinds of atmospheric factors like that of terrestrial media. About 10 per cent of the sunlight which falls over the water surface, is reflected back and rest 90 per cent of that pass downward in the water and is modified in respect to intensity, spectral composition, angular distribution (infraction) and time distribution.

The phytoplankton, zooplankton, suspen­ded organic and inorganic particles either reflect or absorb the light rays. Further, in water there is a selective absorption of light at various depths. The longer light rays are absorbed near the surface and in general the shortest light rays penetrate deepest.

Thus, long heat and infra-red rays are absorbed in the upper layers of water (about 4 metres) ; red and orange rays are completely absorbed upto the depth of 20 metres ; yellow rays penetrate upto 50 metres and green and blue rays penetrate up to 80 to 100 metres deep. Violet and ultraviolet rays penetrate beyond 100 metres and no light; ray penetrates beyond 200 metres depth.

Depending upon the penetration of light, oceans are divided into euphotic zone (upto 50 metres depth), disphotic zone (upto 80 to 200 metres depth) and aphotic zone (below 200 metres of depth). In the ocean, algae are distributed according to length of light rays that their colours are best suited to absorb and to utilize: green algae live in the intertidal zone, brown algae decend somewhat deeper and red algae are characteristic of deep oceanic water.

In general, light intensities are weaker at dawn, sunset and in water. At equator daylight prevails for about 12 hours out of every 24 hours, in both summer and winter. Towards the poles, in summer day length becomes longer than 12 hours.