Physics
posted by EM .
Light from a sodium lamp ( wavelength= 589 nm) illuminates a narrow slit and is observed on a screen 75 cm behind the slit. The dstance between the first and third dark fringes is 7.5 mm. What is the width (in mm) of the slit?
Cant seem to get the answer =/

Read the "single slit diffraction" section of
http://en.wikipedia.org/wiki/Diffraction
The formula you need can be found there.
The angle theta at any distance x from the central maximum is
theta = tan^1 (x/75)
where x is in cm 
I've tried that and im still not getting the answer (which is 0.12 mm) =/ i used that formula to give me the theta and i substituted that into the d= lambda/ sin theta