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Two slits act as coherent sources of equal amplitude A and wavelength lambda in a Young's experiment. In a second experiment with the same arrangement the two slits emit equal amplitude A and wavelength lambda but are now incoherent. What is the ratio of the average intensity at the centre of the screen for the coherent case to that for the incoherent case?

1. 2: 1
2. 1: 1
3. 4: 1
4. none of these

Correct answer: 2: 1

Solution

At the centre, coherent sources interfere constructively: amplitude = 2A so intensity = 4I0 (I0 = intensity of one slit). Incoherent sources simply add intensities: 2I0. Ratio = 4I0: 2I0 = 2: 1.

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