Problem involving slits, diffraction, and incident angle.

[nobb]

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 referring to "n"?

Help would be appreciated. Thank you.

 

[andrevdh]

I would think that the first null will refer to the position next to the central maximum where destructive interference occurs "for the first time". The formula you are referring to gives the angles at which minima will be located behind the screen. So the first minima will occur at n = 1.