Which type of waves move in straight lines?

Plane waves are characterized by their movement in straight lines, maintaining a constant phase across parallel planes. This occurs when waves propagate uniformly through a medium with minimal variations in amplitude. In plane waves, the wavefronts—surfaces of constant phase—are flat surfaces that move outward uniformly, as opposed to diverging outward or oscillating in place.

In contrast, spherical waves spread out from a point source in all directions, creating concentric spherical wavefronts that illustrate the difference in propagation. Pulsating waves, which can refer to waves with fluctuating amplitudes, do not inherently imply a specific direction; thus, they may not consistently move in straight lines. Standing waves, which arise from the interference of two waves traveling in opposite directions, are characterized by nodes and antinodes and do not travel through space in a traditional sense but rather oscillate in place.

Understanding these distinctions highlights why plane waves are the correct choice for waves that move in straight lines.