Laser light with a wavelength = 665 illuminates a pair of slits at normal incidence.

Question

What slit separation will produce first-order maxima (i.e. m=1) at angles of 8 from the incident (or straight through) direction? Remember, 1 nm = m.
Express your answer using two significant figures, in micrometres ().

Answer 1

all I know is that it's not simply
n(wavelength)=dsin(theta) because I plugged the numbers in and they were wrong. (n=1)