Light Travels in a Straight Line

Rectilinear propagation defines the path of light rays in a medium with a constant refractive index. It is the foundational concept for geometric optics.

While light behaves as a wave, the rectilinear approximation holds true for short distances. In reality, light is subject to diffraction and will spread out over time, a behavior described by Maxwell's equations.

This approximation is essential for everyday optical applications. As long as the medium is homogeneous, we can accurately predict the path of light rays.

Pierre de Fermat identified this property, providing a basis for understanding light propagation before more complex wave theories were developed.

The pinhole cardboard experiment demonstrates that light is unable to deviate from its path. If the alignment of the holes is disrupted, the light source is no longer visible, confirming the straight-line path.