A Simple Apparatus for Demonstrating How Light Rays Emanate From a Point Source

A point source of light sends out light rays in all directions and in straight lines. This is an important concept for all students who study elementary optics at the high school level. Most physics textbooks discuss this topic only briefly.^1–6 They usually explain the concept by using two-dimensional diagrams even though a three-dimensional discussion may be more instructive. Much of the commercial demonstration equipment that can be used to explain this concept is very expensive (e.g., the optics kit from Suksapan Panit Company⁷ in Thailand costs approximately $70 U.S.). This and similar optics kits show that light propagates in straight lines but do not represent where the light beams come from. Because it is an abstract idea, most students still have considerable misunderstanding about this concept.^8–14 This paper describes an effective but inexpensive (approximately $20 U.S.) apparatus to illustrate the behavior of light rays emitted isotropically from a point source of light. The apparatus is simple and is intelligible to students.