In Young's experiment, slit separation d is doubled while screen distance L and wavelength λ remain unchanged. What happens to the fringe spacing?
AIt doubles — wider slits spread light further apart
BIt halves — the path-difference geometry changes more rapidly with angle
CIt stays the same — fringe spacing depends only on wavelength
DIt quadruples — the effect of d is squared in the formula
Fringe spacing Δy ≈ λL/d. Doubling d while holding λ and L constant halves Δy. The intuition: wider slit separation means the path difference dsinθ reaches one full wavelength at a smaller angle, squeezing the bright fringes closer together. Option A reverses the relationship, and options C and D misread the formula.
Question 2 Multiple Choice
Newton's corpuscular theory predicted that passing light through two slits would produce two bright bands on the screen. Young instead observed alternating bright and dark bands. Why is this a decisive argument for the wave nature of light?
AParticles travel in straight lines and would miss the screen at the dark regions
BWaves can cancel — two waves arriving out of phase produce zero amplitude, but two particles cannot cancel each other
CThe bright bands are brighter than a single slit would produce, proving energy is being added
DThe dark bands occur exactly where no light hits, proving light bends around corners
The dark fringes are the decisive evidence. If light were corpuscular, two streams of particles can only add — you would see two bright bands or a uniform glow, never a region darker than either slit alone. The existence of dark bands (destructive interference) is only possible if light has a wave nature: waves arriving exactly out of phase (half-integer path difference) cancel via superposition. Particles simply cannot cancel each other.
Question 3 True / False
Bright fringes in Young's double-slit experiment occur where the path difference from the two slits equals an integer multiple of the wavelength.
TTrue
FFalse
Answer: True
This is the constructive interference condition: d sinθ = mλ (m = 0, ±1, ±2, …). When both waves travel the same total distance (or differ by exactly one, two, … wavelengths), they arrive perfectly in phase — crest meets crest, trough meets trough — and their amplitudes add to produce a bright fringe.
Question 4 True / False
Dark fringes in Young's experiment appear because light from one slit is physically blocked from reaching those regions of the screen by the other slit.
TTrue
FFalse
Answer: False
Dark fringes are not caused by blocking — both slits are open, and light from each slit reaches every part of the screen. Dark fringes arise from destructive interference: when the path difference is a half-integer number of wavelengths (m + ½)λ, the two waves arrive exactly out of phase and their amplitudes cancel. This is wave superposition, not obstruction. The misconception misidentifies a diffraction effect as a geometric shadow effect.
Question 5 Short Answer
Why do dark fringes appear in Young's experiment, and what does their existence prove about the nature of light?
Think about your answer, then reveal below.
Model answer: Dark fringes appear because light from the two slits travels different distances to reach points off-center on the screen. Where this path difference is a half-integer number of wavelengths, the waves arrive 180° out of phase and cancel. This cancellation — two sources of light combining to produce darkness — is only possible if light is a wave. Particles cannot cancel; they can only add. The dark fringes are therefore direct physical evidence that light undergoes wave superposition.
The key distinction is between additive and cancellation behavior. Particle models always predict at least as much intensity with two sources as with one. The fact that adding a second slit can make certain regions darker is the fingerprint of wave interference and cannot be explained by any particle model.