In an experiment conducted by Young using sodium light with λ=589 nm, the slits are 1 μm wide. You notice that every fourth fringes missing.

a) What is the slit separation?
d = na = 4(1 μm) = 4 μm.

b) How many fringes appear on the screen?
I'm not sure how to do this part.

1 answer

so for part b:
d*sin(theta)=m*λ
when you plug in the d from part a and λ and 90 degree for theta(maximum theta). you get m=6.8. This is for one side of the slit. You must subtract the missing fringes, 4 th one. you get m=5. m=10 for both side of the screen and add one for the center fringe you get m=11.