We all admire the beautiful blue of a clear sky or the superb contrast between the brilliant white of the clouds and the skyblue between them. This colour, however, is not natural in the atmosphere; it is an optical effect(光学效应). When light encounters a cloud of small particles, it is diffused. This may be illustrated by a familiar example. When sunlight enters a darkened room through a gap in the blinds, it will appear as a brilliant ray of light. This is because there are tiny particles of dust suspended in the air. These, although normally invisible, catch and diffuse any strong light shining on them. The same effect can be seen in the sky when broken clouds hide the sun. Thus we can see that diffusion occurs owing to all sorts of impurities in the atmosphere. This explains all the delightfully varied shades of colour seen at different times of the day. White light is a mixture of all the colours of the rainbow or spectrum (光谱) , which, as Isaac Newton proved, can be separated by a prism. In order, they are red, orange, yellow, green, blue, violet, and they are all to be found in the solar spectrum. Red light has the longest wavelength and violet the shortest but diffused approximately six times more strongly than red. The size of the particles is important. Impurities such as dust and water droplets are much larger than the molecules of gas. In the lower parts of the atmosphere where most of these impurities are concentrated, the sky is different from what it is at heights where no impurities exist. A observation will demonstrate this principle. Examine the smoke from a cigarette before and after it has been inhaled. At first the smoke will be plainly bluish since it is made up of very minute particles; after it has been inhaled and then puffed out through the mouth, it will be yellowish because it now contains water droplets which have condensed round the particles and which are much more efficient diffuse of light. This is why the color of the sky is much more intense and dark blue when seen from the top of a mountain than from sealevel. In the first place there is an almost complete absence of water vapour and, more important, dust. Secondly, we are above the thickest portions of the atmosphere, and this naturally has the most powerful diffusing effect. The color of the sky varies from horizon to zenith (天顶) , since there is a different thickness of air in these two directions. The horizon always appears lighter, more whitish or even reddish, according to the state of the atmosphere. All this, incidentally, also intensifies the illusion of being under a great dome—the celestial roof. The sky has a darker blue color when we look at it______.