In an isotropic and homogeneous medium, light propagates in straight lines. This is the most fundamental optical phenomenon, capable of explaining everyday occurrences such as the formation of shadows, solar eclipses, and pinhole imaging.

Physical Implication: This law is a direct manifestation of Fermat's principle in a homogeneous medium. Fermat's principle states that light travels along a path of extremal optical path length (maximum, minimum, or constant), where the optical path length equals the geometric path length multiplied by the refractive index of the medium. Since the refractive index is constant in a homogeneous medium, the optical path length is proportional to the geometric path length. In this case, the extremum corresponds only to the unique straight-line path of the “shortest optical path length” (as there is no practically realizable “longest optical path length”), and thus light propagates in a straight line.

Homogeneous Medium: Physical properties (such as density, concentration, and refractive index n) are identical at all internal points, so light does not deviate due to medium variations during propagation (e.g., pinhole imaging);
Inhomogeneous Medium: Physical properties vary with spatial position. Differences in density or composition lead to an uneven refractive index, which alters the propagation speed of light and consequently causes the light path to bend continuously (e.g., mirages).

Mirage (Inferior Mirage)
This diagram intuitively reveals the mystery behind the formation of an inferior mirage. The observer on the left sees a bizarre scene: a real camel with an inverted virtual image beneath it.
Why does this “reflection” appear? The key lies in the light passing through an inhomogeneous atmosphere. The desert surface is scorching hot, causing the air near the ground to have a high temperature and low density (low refractive index), while the upper air is cooler and denser (high refractive index). This causes the light rays from the object to propagate not in straight lines, but along a downward-curving arc (path b).

However, our brain and eyes default to the assumption that “light travels in straight lines.” Therefore, when the curved light rays (b) finally enter the human eye, the brain unthinkingly traces them backward along their final incident direction, “filling in” a straight line (path c). It is this imagined line of sight © that convinces us the light source (i.e., the virtual image) is located below the real object, allowing us to see the non-existent inverted phantom. For comparison, path (a) in the diagram shows the straight-line path that light would follow in a homogeneous medium.

Pinhole Imaging
Pinhole imaging (also known as the camera obscura effect) is one of the most classic and intuitive applications of the law of rectilinear propagation of light.
Imagine a completely enclosed dark box with a very small circular hole (pinhole) on one of its walls. When light emitted or reflected by external objects strikes the box, most of the light is blocked by the walls. Only a tiny fraction of light rays traveling in specific straight-line directions that happen to pass through the pinhole can enter the interior of the box.

Since light propagates in straight lines, rays emitted from different points converge at the pinhole and continue to travel in straight lines, ultimately forming an inverted (upside-down) and laterally reversed real image on the back wall. This phenomenon perfectly demonstrates how light maintains its straight-line trajectory. The smaller the pinhole, the narrower the light beams that pass through, and the sharper the resulting image (though dimmer); conversely, the larger the pinhole, the more the light scatters, and the blurrier the image becomes.