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VICPhysicsQuick questions
Unit 4: How have new ideas and ways of thinking developed our understanding of the physical world?
Quick questions on Wave model of light and interference: VCE Physics Unit 4
14short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is example 1. Calculate fringe spacing?Show answer
Wavelength 600 nm, slit separation 0.20 mm, screen distance 1.5 m.
What is example 2. Wavelength from fringe spacing?Show answer
Fringe spacing 2.0 mm, slit separation 0.50 mm, screen distance 1.2 m. Find $\lambda$.
What is example 3. Path difference and fringe identification?Show answer
In a setup with $\lambda = 500$ nm, the path difference at a point on the screen is 1500 nm.
What is setup?Show answer
Monochromatic (single-wavelength), coherent light passes through two narrow slits separated by distance $d$. The light reaching the screen at distance $L$ from the slits is the superposition of two waves, one from each slit.
What is coherence requirement?Show answer
The two sources must have a fixed phase relationship. In practice, both slits are illuminated by the same monochromatic source (e.g., a laser, or a single slit illuminated first), ensuring coherence.
What is observation?Show answer
A regular pattern of bright and dark fringes appears on the screen. Bright fringes correspond to constructive interference (the waves arrive in phase); dark fringes correspond to destructive interference (the waves arrive out of phase by $\pi$).
What is pattern?Show answer
A central bright band, flanked by progressively dimmer side bands separated by dark fringes. The central band is twice as wide as the side bands.
What is dark fringe condition?Show answer
Dark minima occur at angles where:
What is width of the central maximum?Show answer
From the first dark fringes on either side: $\sin \theta_1 \approx \lambda / w$, so the angular half-width is $\theta_1 \approx \lambda / w$. The angular full-width is $2 \lambda / w$.
What is unit conversion forgotten?Show answer
Wavelengths in nm, slit separation in mm, screen distance in m. Convert all to metres before substituting.
What is confusing $d$ and $w$?Show answer
$d$ is the slit separation in Young's double-slit. $w$ is the slit width in single-slit diffraction. Different quantities, different formulas.
What is applying small-angle formula at large angles?Show answer
$\Delta x = \lambda L / d$ assumes small $\theta$. For large $\theta$ (close to the slits, or first-order maxima with very short $L$), use $d \sin \theta = m \lambda$ directly.
What is forgetting coherence?Show answer
Two independent light sources are not coherent, so they do not produce a stable interference pattern. Young used one source illuminating both slits to ensure coherence.
What is confusing path difference with phase difference?Show answer
Path difference (in metres) becomes phase difference (in radians) via $\phi = 2 \pi \Delta / \lambda$. Constructive: $\Delta = m \lambda$ (so $\phi = 2 \pi m$). Destructive: $\Delta = (m + 0.5) \lambda$ (so $\phi = (2m + 1) \pi$).