Light from a He-Ne laser ( = 632.8 ) strikes a pair of slits at normal incidence, forming a double-slit interference pattern on a screen located 1.90...

Light from a He-Ne laser ( = 632.8 ) strikes a pair of slits at normal incidence, forming a double-slit interference pattern on a screen located 1.90 from the slits. The figure shows the interference pattern observed on the screen. (figure shows distance between dark spots 4 apart to be 23.0 cm, which is located here: http://session.masteringphysics.com/problemAsset/1122881/1/Walker.28.17.jpg) What is slit separation?