A first order spectrum is obtained using an optical grating with 500 lines/m. What is the angular separation of red , with wavelength 700nm and violet, with wavelength 400nm?

cosA = Lambda / 2*10-3

cos A1 = 700*10^-6 / 2^10^-3 =.7/2 = .35
cos A2= 400*10^-6 / 2*10^-3 = .4/2 = .2
A1 = 69.5
A2 = 78.5
difference = 9 degrees