9702 · 8.3
Interference flashcards
Revision flashcards for Cambridge 9702 Interference (syllabus 8.3). Flip, recall, then mark a real past-paper question.
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What is the fundamental principle behind wave interference?
The Principle of Superposition.
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Define 'coherent' waves for interference.
Waves with the same frequency, wavelength, and a constant phase difference.
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When does constructive interference occur?
When waves meet precisely in phase, leading to maximum amplitude.
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What path difference leads to destructive interference?
An odd multiple of half a wavelength: $(n + \frac{1}{2})\lambda$ (where $n = 0, 1, 2, ...$).
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How does diffraction contribute to observing interference patterns?
It spreads waves from a single source, creating multiple coherent secondary sources.
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State the formula for fringe spacing in Young's double-slit experiment.
$x = \frac{\lambda D}{a}$ (where $x$ is fringe spacing, $\lambda$ wavelength, $D$ screen distance, $a$ slit separation).
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What is a diffraction grating?
A device with a large number of equally spaced parallel slits, producing sharp interference maxima.
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What does 'n' represent in the diffraction grating formula ($d \sin \theta = n\lambda$)?
The order of the maximum (e.g., $n=0$ for central, $n=1$ for first order).
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How does increasing the wavelength affect the angular separation of maxima from a grating?
A longer wavelength leads to larger diffraction angles, causing the pattern to spread out more.
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Name an application of diffraction gratings.
Spectroscopy (analysing light spectra) or X-ray crystallography.
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How is the grating spacing 'd' calculated if you know the number of lines per mm (N)?
First convert N to lines per metre (N × 1000). Then, d = 1 / (N × 1000).
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What is the condition that limits the maximum observable order (n_max) in a diffraction grating experiment?
The sine of the diffraction angle, sin θ, cannot exceed 1. Therefore, n_max is the largest integer satisfying n ≤ d/λ.
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In Young's double-slit experiment, what happens to the fringe spacing 'x' if the slit separation 'a' is decreased?
The fringe spacing 'x' increases, as they are inversely proportional (x = λD/a). The fringes spread further apart.
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What is the 'small angle approximation' and when is it used in interference?
It's the approximation that for small angles, sin θ ≈ tan θ ≈ θ (in radians). It's used to derive the formula λ = ax/D for Young's double-slit experiment.