Light Travel In a Straight Line?
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One of the first things we learn about light is that it travels in straight lines. While this is a useful approximation for understanding everyday phenomena like shadows and reflections, it doesn’t fully capture how light behaves—especially when its wave nature comes into play.
Take a laser pointer as an example. At first glance, its beam seems to travel in a perfectly straight line. Yet, over long distances, the beam gradually spreads. If you shine a 20 mm laser beam at the Moon, which is approximately 384,000 km away from the Earth, it will expand to over 3.5 kilometers in diameter—almost the width of a small town! This spreading occurs not due to flaws in the laser but because light, like water waves, naturally diverges as it travels.
Figure 1: A laser beam expands significantly when traveling from Earth to the Moon due to its wave nature
Why doesn’t a laser propagate straight? To understand why a laser or light beam doesn’t propagate perfectly straight, we need to examine its wavefront. Imagine a marching band with a straight front line. If everyone steps in sync, the line stays straight—this mirrors a beam with a flat, infinite wavefront.
However, real beams are finite. While the central wavefront is nearly flat, the edges behave like uneven steps in the parade, disrupting uniformity. This sharp transition leads to diffraction, a wave phenomenon causing the beam to spread and its wavefront to curve. You may already know that a beam diffracts after passing through a small aperture. Now, imagine an finitely large beam behaving as if it’s passing through an aperture formed by its own edges.
Figure 2: As the beam propagates, its wavefronts curve more, leading to spreading.
Next let’s take a closer look of how laser travel. Lasers travel follow a special behavior called Gaussian beam optics. Near the source, the beam spreads slowly in a region, can approbatory considered as straight travel; which is called the Rayleigh range. Beyond this, the spreading becomes more noticeable and predictable, forming a cone-like shape.
Figure 3: The beam radius increases as the distance (z) grows, especially beyond the Rayleigh range
Conversely, a flashlight spreads light more quickly, emitting in all directions. Its light waves are incoherent, meaning their wavefronts don’t align, causing even greater divergence. If “incoherent” sounds unclear, don’t worry—we’ll explore this key concept in a future post!
Conclusion:
While light often appears to travel in a straight line, its wave nature reveals a more intricate behavior. Diffraction and wavefront curvature cause even tightly focused beams, like lasers, to spread. Understanding these effects through Gaussian beam optics explains phenomena like beam divergence and laser expansion. Far from being a theoretical curiosity, these principles are foundational to designing optical systems for practical applications.
Understand the Phase of Light
When most people think about light, they often associate it with color (the frequency of photons or the energy each photon carries) and brightness (the beam’s energy or photon count). But there’s another, lesser-known property of light that shapes its most intriguing behaviors: phase. Phase is the hidden rhythm of light, underlying phenomena like repeated striped pattern and diffraction rings you see sometime in the daily life, and enabling us to manipulate and control light in remarkable ways.
What Is Phase?
Light behaves as both particle and wave. Similar to water ripples or sound waves, light oscillates as it propagates, carrying energy across space. The phase of light tells us where a wave is within its oscillation cycle at a given moment.
The term “phase” originates from the Greek word φάσις (phásis), meaning “appearance” or “manifestation.” In the 17th and 18th centuries, scientists like Christiaan Huygens and Augustin-Jean Fresnel used it to describe the oscillatory state of a wave. Since waves are periodic, phase is cyclical, measured in degrees or radians (with one full cycle equaling 360° or 2π). Once a wave completes its cycle, the phase resets, repeating at 2π,4π, and beyond.
Think of phase as the “position” on a sine wave moving through space. It indicates whether the wave is at its peak, trough, or somewhere in between. If light were music, phase would determine whether notes align harmoniously or clash in discord.
Figure.1 A cycle of a light wave illustrating how phase oscillates in space. At a fixed location in space, the phase also oscillates up and down over time
You might ask: “If I stand in one spot while a light wave passes by, will I “see” the wave go up and down?” The answer is yes! But why don’t we perceive the “blinking” of the wave? That’s because the oscillation frequency is extraordinarily high. For example, with a wavelength of 550 nm, the frequency is approximately 5.5×1015 Hz (or 5.5 femtoseconds per cycle). This is far too fast for the human eye to detect. Instead, our eyes perceive the average intensity of the wave over time. If we can’t directly see the phase of light, how do we know it exists? One way is through interference phenomena, which allow us to observe the effects of phase.
Phase becomes especially critical when light waves interact. While a single photon’s phase is largely irrelevant in isolation, for beams or multiple overlapping beams, phase dramatically influences their behavior. This is especially important when light waves overlap, or so-called Interference. In Blog 1, we explored how infinite-sized beams with flat wavefronts (uniform phase) propagate in straight lines. Now, let’s examine what happens when two collimated beams overlap. In their overlapping region, the interaction depends entirely on the relative phase alignment of the waves.
If you zoom into the center of the overlap region, you’ll see a striped interference pattern. These stripes, known as fringes, repeat at regular intervals determined by the relative phase of the waves. The distance between these stripes, called the period, decreases as the angle between the beams increases. This interference pattern reflects the light’s intensity distribution. In this way, phase directly shapes optical phenomena, dictating how energy spreads in space. We will explore more phase-related phenomena in future blogs, as it serves as the conductor behind light’s most extraordinary symphonies.
Figure 2: How the Interference Pattern Changes in the center of overlap area with the Overlap Angle Between Two Collimated Beams with