1. A double slit apparatus is held 1.2 m from a screen. (a) When red light ( = 600 nm) is sent through the double slit, the interference pattern on the screen shows a distance of 12.5 cm between the first and tenth dark fringes. What is the separation of the slits? (b) What will be the difference in path length for the waves travelling from each slit to the tenth nodal line?

2. A diffraction grating gives a second-order maximum at as angle of 31 for violet light ( = 4.0 10² nm). If the diffraction grating is 1.0 cm in width, how many lines are on this diffraction grating?