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Homework Statement:
 In two separate double slit experiments, two different wavelengths are used. First wavelength is 708 nm and it is observed the second order bright fringe occurs at same position as third order dark fringe of second wavelength. Determine the second wavelength
Relevant Equations:

Bright fringe: d sin theta = m.lambda
Dark fringe: d sin theta = (m  1/2).lambda
m × lambda for bright = (m  1/2) × lambda for dark so:
2 × 708 = 2.5 × second lambda
Second lambda = 566.4 nm
But the answer is 495 nm. Where is my mistake? Thanks
2 × 708 = 2.5 × second lambda
Second lambda = 566.4 nm
But the answer is 495 nm. Where is my mistake? Thanks