Light travels as a wave. But unlike sound waves or water waves, it does not need any matter or material to carry its energy along. This means that light can travel through a vacuum—a completely airless space. ... It speeds through the vacuum of space at 186,400 miles (300,000 km) per second.
Primary sources means that it is original article or book created by an individual or sometimes a group of people. What types of primary sources are available? It might be surprising to know that a novel is a primary source. Other types of primary sources are paintings created by the artist. If it were a photocopy of the painting, then it would be a secondary source. Some other primary sources are letters, films, short stories, plays, poems, photographs, court cases, journal articles, newspaper events, and speeches. For instance, a speech by President Bush would be a primary source.
Secondary sources are sources that are written about primary sources. Secondary sources analyze, interpret, and discuss information about the primary source. If a magazine writer wrote about the speech President Bush gave on September 11th, it would be a secondary source. The information is not original, but it is an analysis of the speech.
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Keep the airways clear of mucus and dirt, allowing us to breathe easily and without irritation.