Hi. I am a bit confused with this question: A plane wave of 400-nm light is incident on a 25-µ(mu) m slit in a screen, as shown in the figure below. At what incident angle will the first null of the diffraction pattern be on a line perpendicular to the screen? (picture is attached). Do I use the formula wavelength=(dSinA)/n ? I am a bit unsure what the "first null" refers to and how this should be taken into account into my formula. Is the first null refering to "n"? Help would be appreciated. Thank you.