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In a Young's double-slit experiment the intensity ratio of bright to dark fringes is found to be 9. From this it follows that:

1. the intensities from the two slits are 5 units and 4 units
2. the intensities from the two slits are 4 units and 1 unit
3. the amplitude ratio is 6
4. the amplitude ratio is 2

Correct answer: the intensities from the two slits are 4 units and 1 unit

Solution

Imax/Imin = ((a1+a2)/(a1-a2))² = 9, so (a1+a2)/(a1-a2) = 3. Solving gives a1/a2 = 2. Then the intensity ratio I1/I2 = (a1/a2)² = 4, i.e. the two slit intensities are in ratio 4:1, for example 4 units and 1 unit. (The amplitude ratio is 2, not 6.)

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