A laboratory photograph shows a series of equally spaced bright vertical lines on a screen that is 1.80 m from a pair of identical narrow slits. The slits are illuminated by monochromatic light of wavelength 550 nm. A transparent metric ruler is taped to the screen so that the distance between the third bright line above the central maximum and the third bright line below it is measured to be 24.6 mm.
(1) Calculate the average spacing between adjacent bright lines on the screen.
(2) Using the spacing you found in part (a), determine the separation of the two slits.
(3) State one reasonable experimental assumption you made in your calculation and describe qualitatively how the numerical value of the slit separation would change if that assumption were not satisfied.