15.1 Parallel light of wavelength 6563 A is incident normally on a slit 0.3850 mm wide. A lens with a focal length of 50.0 cm is located just behind the slit bringing the diffrac- tion pattern to focus on a white screen. Find the distance from the center of the principle maximum to (a) the first minimum and (b) the fifth minimum. Ans. (a) 0.852 mm, (b) 4.261 mm15.2 Plane waves of blue light, .3. = 4340 A, fall on a single slit, then pass through a lens with a focal length of 85.0 cm. If the central band of the diffraction pattern on the screen has a width of 2.450 mm. nd the width of the single slit. Ans. 0.3011 mm