Question

If Huygens's Principle holds everywhere, why does a laser beam not spread out? a) All the light waves that spread in the perpendicular direction from the beam interfere destructively. b) It does spread out, but the spread is so small that we don't notice it. c) Huygens's Principle isn't true in general; it only applies to slits, edges, and other obstacles. d) Lasers employ additional special beams to keep the main beam from spreading.

Short answer

Answer: The laser beam does spread out, but the spread is so small that we don't notice it.

Step-by-step solution

Analyze option a

Option a states that all the light waves that spread in the perpendicular direction from the beam interfere destructively. This is unlikely for a laser beam because laser light is coherent and monochromatic (single wavelength). Therefore, we can eliminate this option as the reason for the laser beam not spreading out.

Analyze option b

Option b suggests that the laser beam does spread out, but the spread is so small that we don't notice it. This is true as a laser beam, while more focused than other light sources, does still exhibit diffraction and will spread out over long distances. This spreading out effect is known as "laser divergence." However, laser divergence is very small for most practical applications, so the spread out is negligible or not easily noticeable.

Analyze option c

Option c states that Huygens's Principle isn't true in general, and it only applies to slits, edges, and other obstacles. This statement is not accurate. Huygens's Principle can be applied to any wave that propagates through a medium, including light waves. Also, laser beams should, in theory, also adhere to Huygens's Principle. Therefore, we can eliminate this option as the reason for the laser beam not spreading out.

Analyze option d

Option d suggests that lasers employ additional special beams to keep the main beam from spreading. This statement is not true, as lasers do not typically use additional beams to prevent divergence. The focusing properties of lasers are due to their coherent and monochromatic nature, which allows them to maintain a tight beam of light. Therefore, we can eliminate this option as the reason for the laser beam not spreading out.

Choose the correct option

Based on the analysis of each option, the correct answer is option b: The laser beam does spread out, but the spread is so small that we don't notice it.

Key concepts

Laser Beam Divergence

The phenomenon of laser beam divergence refers to the gradual expansion of a laser beam's diameter as it travels through space. At its source, a laser beam is highly collimated, meaning the light waves travel nearly parallel to each other. However, due to diffraction, a basic property of waves, the beam will inevitably start to spread out over distance.

This effect is remarkably minimal for laser beams, especially when compared to other light sources, due to their coherent and monochromatic nature. Coherent light, which consists of waves that are in phase and have the same wavelength, is less prone to diverging quickly. Despite this, as per Huygens's Principle, every point on a wavefront can be considered a source of secondary spherical waves, which would result in some divergence.

While the divergence angle for laser beams is small, it is a critical factor that needs to be considered in applications such as laser targeting, communication, and cutting, where precision is key. The divergence can be quantitatively described by the beam divergence angle, a measure that indicates how quickly the beam expands with distance.

Wave Interference

Understanding Wave Interference

Wave interference is a fundamental concept that describes the phenomenon where two or more waves superimpose to form a resultant wave of greater, lower, or the same amplitude. This process can result in two main types of interference patterns: constructive and destructive interference.

Constructive interference occurs when the crest of one wave aligns with the crest of another, resulting in a wave with a higher amplitude. In contrast, destructive interference happens when the crest of one wave aligns with the trough of another, effectively reducing or cancelling out the wave amplitude.

In the context of the exercise, it's important to note that for a laser beam, the light waves are coherent and thus are more likely to exhibit constructive interference within the beam's path, maintaining the beam's intensity over a short range. On the sides of the beam, where the path lengths differ significantly, the coherent waves do not typically result in a persistent destructive interference pattern, which would prevent the spreading of the beam.

Coherent Light

The term coherent light is used to describe light waves that have a stable, consistent phase relationship. Coherence is essential for applications that require highly controlled light properties, such as holography, interferometry, and of course, laser operations.

Temporal coherence refers to the consistency of phase over time, and spatial coherence describes the uniformity of phase across the wavefront of the light beam. Lasers produce a high degree of both temporal and spatial coherence because they emit light that is not only monochromatic, meaning it consists of a single wavelength, but also in phase across the wavefront.

This coherence makes laser light starkly different from light emitted by conventional sources such as bulbs or LEDs, which emit a wide range of wavelengths and phases, resulting in much less focus and increase in the light's spread, known as divergence. For lasers, it is this coherent nature that allows for the highly focused and minimal divergent beam, facilitating its many uses in science, medicine, and technology.