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4 Electrons, waves and photons

Quick questions on Superposition and interference: stationary waves, double slits and gratings - OCR A-Level Physics A

4short 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 are stationary waves?
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On a string fixed at both ends, the fundamental (first harmonic) has a node at each end and one antinode in the middle, so the string length equals half a wavelength: L=λ2L = \frac{\lambda}{2}, giving λ=2L\lambda = 2L and frequency f1=v2Lf_1 = \frac{v}{2L}. Higher harmonics occur at integer multiples, fn=nf1f_n = nf_1.
What is q1?
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State the principle of superposition. [2 marks]
What is q2?
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A double-slit pattern has fringes 1.5 mm1.5\ \text{mm} apart with slits 0.50 mm0.50\ \text{mm} apart and a screen 1.2 m1.2\ \text{m} away. Find the wavelength. [2 marks]
What is q3?
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State the two conditions for two sources to be coherent. [2 marks]

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