Sound waves (wavelength ~0.5 m) and visible light (wavelength ~500 nm) both pass through the same 1-meter-wide doorway. Which statement correctly describes the diffraction behavior of each?
ABoth diffract equally — they pass through the same opening
BLight diffracts more because shorter wavelengths bend more sharply around edges
CSound diffracts noticeably; light travels essentially straight through — the doorway is millions of wavelengths wide for light but only ~2 wavelengths wide for sound
DNeither diffracts significantly because 1 m is larger than both wavelengths
Diffraction is significant when λ/d ≈ 1. For sound: λ/d ≈ 0.5/1 = 0.5 — substantial diffraction. For light: λ/d ≈ 500×10⁻⁹/1 ≈ 5×10⁻⁷ — essentially zero diffraction. The doorway is millions of wavelengths across for light, so edge wavelets are negligible compared to the bulk of the wavefront. Option D ignores the critical ratio — what matters is not the absolute size of the opening but its size relative to the wavelength.
Question 2 Multiple Choice
According to Huygens' principle, why does a wave bend into the geometric shadow region when it passes through a narrow opening?
AThe opening material reflects part of the wave sideways into the shadow
BThere are no secondary wavelets from the blocked region to cancel the sideways components of edge wavelets, so those components propagate into the shadow
CDestructive interference between the incident and reflected waves creates apparent bending
DResonance between the wave frequency and the opening geometry amplifies sideways propagation
Huygens' principle says every wavefront point generates secondary wavelets in all directions. In open space, sideways wavelets from adjacent points cancel each other via destructive interference, leaving only the forward-propagating wavefront. At the edge of an opening, the blocked region provides no wavelets to cancel the edge wavelets' sideways components — so those components propagate freely into the geometric shadow. This is diffraction: not a new phenomenon but the direct consequence of incomplete cancellation at boundaries.
Question 3 True / False
Diffraction becomes more pronounced when the size of an opening is much larger than the wavelength of the incident wave.
TTrue
FFalse
Answer: False
The opposite is true. When d >> λ (opening much larger than wavelength), edge wavelets are negligible relative to the vast central wavefront — the wave travels essentially straight through. Diffraction becomes significant when d ≈ λ, because then edge wavelets influence the entire aperture and the wave fans out broadly. The governing ratio is λ/d: when this approaches 1, diffraction dominates.
Question 4 True / False
According to Huygens' principle, each point on an existing wavefront can be treated as an independent source of secondary spherical (or circular in 2D) wavelets, and the next wavefront is the surface tangent to all those wavelets.
TTrue
FFalse
Answer: True
This is exactly Huygens' principle. It works because the forward-propagating components of all secondary wavelets reinforce (constructive interference in the forward direction), while sideways components cancel in open space. The principle provides a geometric recipe for wavefront propagation that naturally explains diffraction: at boundaries or openings, the cancellation is incomplete, and the resulting wavefront deviates from a plane.
Question 5 Short Answer
Why does sound diffract around the corners of buildings but visible light does not, even though both are waves that obey Huygens' principle?
Think about your answer, then reveal below.
Model answer: The key is the ratio λ/d, where λ is wavelength and d is the obstacle or opening size. Sound waves have wavelengths of roughly 0.01–10 m, comparable to everyday objects like buildings and doorways, so λ/d ≈ 1 and diffraction is significant. Visible light has wavelengths of ~400–700 nm — roughly a million times smaller — so for any everyday object, d >> λ and λ/d ≈ 0. Edge wavelets exist but are negligible compared to the bulk wavefront. Both waves obey Huygens' principle equally; the difference is purely in the ratio of wavelength to obstacle size.
This ratio λ/d is the central organizing principle of diffraction. It explains why AM radio (λ ~ 300 m) diffracts over hills, FM radio (λ ~ 3 m) is blocked by hills, and light travels in straight lines through everyday environments but diffracts dramatically through diffraction gratings with spacings of hundreds of nanometers.