$x_1 = 180 \textrm{ nm}$ $x_2 = 360 \textrm{ nm, } x_3 = 540 \textrm{ nm}$ there would be only a phase difference of one half wavelength, which is destructive. Giancoli 7th Global Edition, Chapter 24, Problem 45(3:20) Chapter 24, Problem 45 is solved. Start My Free Week View sample solution