A plane wave passes through a slit whose width is approximately equal to the wavelength of the wave. What does Huygens's principle predict will happen?
AThe wave passes straight through the slit with minimal spreading, forming a narrow beam
BThe wave is mostly absorbed by the slit edges, with only a small fraction transmitted
CThe wave spreads in all forward directions beyond the slit, not just straight ahead
DThe wave reflects back because the slit is too narrow to allow transmission
When the slit is comparable in width to the wavelength, only a few Huygens secondary wavelets are emitted from the narrow exposed strip. These wavelets have no neighboring wavelets to cancel their sideways spreading via destructive interference. So they radiate in all forward directions — dramatic diffraction. When the slit is large compared to the wavelength, the many wavelets do cancel sideways and the wave propagates as a narrow beam. This is why sound bends audibly around doorframes (long wavelength) but light forms sharp shadows (short wavelength).
Question 2 Multiple Choice
How does Huygens's principle explain why a wavefront bends (refracts) when it passes from one medium into another at an angle?
AThe frequency of the wave changes at the boundary, causing the wavefront to tilt
BOne part of the wavefront enters the new medium first and begins moving at the new wave speed, tilting the overall wavefront direction
CThe amplitude of the wave decreases at the boundary, redirecting energy at an angle
DSecondary wavelets are absorbed and re-emitted by the new medium at a different angle
When a wavefront strikes a boundary at an angle, one edge enters the new medium before the other. That edge's secondary wavelets immediately expand at the new wave speed (slower or faster). The wavelets in the new medium have different radii than those still in the old medium. The new wavefront — the envelope of all wavelets at that instant — is tilted compared to the incident wavefront. This geometric construction directly derives Snell's law without invoking the formula.
Question 3 True / False
According to Huygens's principle, diffraction (bending around obstacles) should be most noticeable when the aperture or obstacle is much larger than the wavelength.
TTrue
FFalse
Answer: False
Diffraction is most pronounced when the aperture is *comparable to* the wavelength. When the aperture is large relative to the wavelength, many Huygens wavelets from across the opening interfere destructively in the sideways directions, and the wave propagates mostly straight through (forming a sharp-edged beam or shadow). When the aperture is small (comparable to λ), too few wavelets are present for cancellation to occur, and the wave spreads in all forward directions.
Question 4 True / False
Huygens's principle is a general wave principle that applies to sound and water waves, not just to light.
TTrue
FFalse
Answer: True
Huygens stated his principle for waves in general, not specifically for light. It applies wherever a wavefront can be defined: water waves diffract around a harbor breakwater, sound waves bend around a corner, and seismic waves refract through Earth's layers — all described by the same geometric construction. The principle predates the wave theory of light and was formulated as a general description of wavefront propagation.
Question 5 Short Answer
Why does diffraction become more pronounced when the aperture size is comparable to (rather than much larger than) the wavelength? Use Huygens's principle in your explanation.
Think about your answer, then reveal below.
Model answer: When the aperture is large relative to λ, Huygens secondary wavelets from many points across the opening spread in all directions, but the sideways wavelets from neighboring points interfere destructively — they cancel each other. Only the forward wavelets reinforce, so the wave propagates mostly straight. When the aperture shrinks to roughly λ, too few sources are present across the opening for effective destructive interference in the sideways directions. The surviving wavelets radiate freely in all forward directions, producing dramatic spreading.
This is why diffraction is an everyday experience for sound (wavelengths of centimeters to meters) — sound easily diffracts around doorframes and corners — but light (wavelengths of hundreds of nanometers) only diffracts noticeably through very narrow slits or fine gratings. The ratio of aperture size to wavelength governs the degree of diffraction, not the absolute sizes.