21. Two towers of a radio station are 0.6 km apart along an east-west line. The towers act as point sources radiating at a frequency of 1.0 x 106 Hz. Radio waves travel at a speed of 3.0 x 108 mfs. Determine the rst (non-zero) angle at which the radio signal strength is at a maximum for listeners who are on a line 20.0 km north of the station (c = 3 x 108 mfs). [4 marks] 22. A lens that has an index of refraction of 1.51 is coated with a nonreective coating that has an index of refraction of 1.75. Determine the minimum thickness for the lm if it is to be nonreecting for light of wavelength 5.60 x 102 nm in air. [2 marks]