Within a side, the color stays the same because of the light's parallel rays.

每面内的颜色都相同，这是因为光线是平行的。

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So we can presume that the Sun's rays are parallel.

所以我们可以近似地认为太阳光是平行的。

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An analysis is made about Descartes rays of transparent round-section fibers-a special internal surface reflection rays of the fiber when parallel rays strike the fibers perpendicularly to fiber axis.

摘要分析了透明圆形截面纤维在垂直纤维轴光线照射时纱维内表面反射光的一种特殊现象—笛卡尔线现象。

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Instead of converging to a point on a focal plane behind the lens, the rays spread out so as to run in a thick stream roughly parallel to each other (see chart).

ECONOMIST: MONITOR