http://www.physicshelpforum.com/phys...pilcation.html. Question 1 has had several replies.
Therefore the distance between successive intensity minima is constant and is equal to .
For your problem, you need to find the value of d, given the following data:
Constant distance between successive intensity minima = distance between n = 1 and n = 2 minima = 19.5 metre ....... no, wait a minute ..... it = distance between n = 2 and n = 3 minima = 24 metre ..... doh!
Now you see what the trouble is ....... Perhaps there's error in the measured distances and you should use an average value ......
Note that the calculated value for the gap (slit width) is not very small compared to the distance between car and buildings (distance between detector and slit) so the above formulae probably only give a rough estimate (which would explain why the distance between successive minima is not constant).
I don't really have the time or the inclination to do an exact calculation where no approximations regarding relative size of slit widths, distances and wavelengths are made. I doubt the question requires you to do this either.