physics
posted by Andy .
singleslit diffraction pattern is formed when light of 680.0 nm is passed through a narrow slit. The pattern is viewed on a screen placed one meter from the slit. What is the width of the slit (mm) if the width of the central maximum is 2.69 cm?

Use the singleslit diffraction formula for the angular distance between the two minima on either side of the maximum. If you need a refresher course on that, see
http://en.wikipedia.org/wiki/Diffraction
That angle is given (for small angles) by
theta = 2 (wavelength)/d
Set that angle (in radians) equal to 2.69/100 = 0.0269
Then solve for the slit width, d.