[单项选择]Learn about Light 1 Ancient civilizations were amazed by the existence of light for thousands of years. The Greek philosophers believed that light was made up of countless, tiny particles that enter the human eye and create what we call vision. However, Empedocles and a Dutch scientist named Christian Huygens believed that light was like a wave. According to them, light spread out and travelled like a straight line. This theory was accepted during the 19th century. 2 ln 1905, Albert Einstein published a research paper in which he explained what is referred to as the photoelectric effect. This theory explains that particles make up light. The particles Einstein was referring to are weightless bundles（束）of electromagnetic（电磁）energy called photons（光子）. Today, scientists agree that light has a dual（二重） nature — it is part particle and part wave. It is a form of energy that allows us to see things around us. 3 Things that give off light are known as sources of light. During the