Whether its source is the Sun, a flashlight, or a fluorescent bulb overhead, light is a form of energy that is common in our everyday lives. One of light's characteristic properties is that, in a transparent medium like air, glass, or still water, it travels in a straight line.
Light exhibits characteristics of both waves and particles, the latter of which are described as packets of energy called photons. These waves, or photons, travel in narrow beams called rays. Only when light rays move from one medium to another, such as from air to water, are their linear paths altered.
So how do we know that light travels in a straight line? Because of the nature of light—for example, it moves in a vacuum at a speed of more than 186,000 miles per second, or 300,000 kilometers per second—it is, for all practical purposes, impossible to observe individual particles. But evidence of its linear pathway can be seen in a number of demonstrations.
While the flashlight tag and pinhole demonstrations in this video segment provide compelling, if not irrefutable, evidence that light rays travel in a straight line, shadows offer another demonstration of the phenomenon. On a clear day, shadows are more pronounced. When light rays reach an object, provided the object is opaque, the rays do not pass through it. Instead, the object absorbs or reflects the rays. A shadow forms because the object has blocked the light's path. Just outside the edge of the object, however, light continues along its path unobstructed. If you were to trace a line from the source of light to the edge of the object and then on to the edge of the object's shadow, it would be straight.
Similarly, if you stand in an object's shadow looking back toward the object casting it, you will not see a light source behind it. But if you shift so that you can just see the light, you will be able to draw a straight line from your eye, past the edge of the object in front of you, and on to the light. [Don't try this with the Sun, or another bright light, because it can damage your eyes!]
To learn how scientists determined the speed of light, check out Speed of Light.
To learn more about why light travels at different speeds through different materials, check out Observing Refraction of Light.
To learn about what happens to light when it passes through a lens, check out Geometric Optics.