A group of travelers driving across the Sahara Desert were startled (hoảng) to see a cool, inviting lake suddenly appear in the distance. They gazed with delight at the broad sheet of water sparkling (long lanh) in the sunlight. Busily, they made plans to go swimming as soon as they reached the lake. But their driver, an experienced desert guide, informed the passengers regretfully that when they reached the area ahead, they would find only dust, sand, and rocks. The travelers insisted that they could see something ahead. Those with cameras were even able to take pictures of the lake the driver said wasn’t there! Later they learned that what they had seen was a reflection of the sky on the hot, dry land. Theircameras, like their eyes, had seen the image the light rays had created. They had seen a mirage (ảo tưởng). Miragesappear when there are layers of air of different density. Next to the desert floor, the air was hot. Above this hot air wasanother layer of cooler, more dense air. Light rays passing down from the layer of more dense air into the layer of lessdense air were bent upward. The bent rays reflected the sky. This bending of light rays is called refraction (khúc xạ). Ona clear summer day, drivers may see this kind of mirage on a paved highway. Their eyes tell them that a patch of roadahead is wet. A blanket of hot air lies next to the road surface, which has been warmed by the sun. Light rays bendupward as they pass into this layer. What the drivers see is just a reflection of the sky.
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