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# The Propagation of Light in Straight Lines The propagation of light in straight lines is the technical term that describes the principle that light travels in straight lines. The fact that objects can cast sharp shadows is a good illustration of this principle. Another illustration is found in a...

# The Propagation of Light in Straight Lines The propagation of light in straight lines is the technical term that describes the principle that light travels in straight lines. The fact that objects can cast sharp shadows is a good illustration of this principle. Another illustration is found in a camera obscura. In this simple and inexpensive device, a stationary object's image is formed on a photographic film or plate by light passing through a small hole, as shown in Figure (1). In this figure, the object is an electric decorative lamp emitting white light. To see how the image is formed, consider the light rays emanating from a single point 'a' near the top of the lamp. Among the many rays emitted from this point in different directions, one ray travels directly towards the hole, passing through it to a point near the base of the image screen. Similarly, the ray emitted from point 'b' near the base of the lamp, which passes through the hole, will reach point 'b' near the top of the image screen. This shows how an inverted image of the entire lamp is formed on the screen. **Figure (1):** A demonstration experiment to illustrate the principle that light rays travel in straight lines; this is the propagation of light in straight lines. # The Speed of Light In the past, astronomers believed light traveled at an infinite speed. It was also thought that any major event occurring between distant stars would be observed instantaneously at all other points in the universe.