How to Identify Constructive Interference in Young's Double Slit Experiment?

[nophun6]

My question deals with Young's double slit experiment.

This figure is a graphical representation of Young’s double slit experiment in which laser light passing through a pair of closely spaced slits results in a 2-source interference pattern. The laser light, incident from the top, is diffracted as it passes through each slit (indicated by A) resulting in an interference pattern which is projected onto a viewing screen (B) which reflects the incident light. On the figure below, carefully mark the locations of
total constructive interference on the viewing screen and describe how it
would look to your eye. Label the order number,
m = 0,±1,±2,K, for each total constructive interference location on the screen.

Ok, so constructive interference occurse when the two waves are in phase at a point. I also know that when the light from the two slits arrives at a point on the screen and constructive interference occurs at that point, a bright fringe appears. How do I know where to mark the points of constructive interference? and how do i know which order number it is?
thanks so much!

 

[andrevdh]

The grey "rays" are regions where the two waves cancel each other out. One do not observe any wave motion in these areas. These rays are thus the regions were total deconstructive interference are taking place. In the regions in between these one observes wave motion - the two wavefronts interference constructively in these regions.

 

[dadi]

by 1st finding the distance between the two slits ,distance between the slits and the mirror,and wavelenght you now can find the fringe distance hence measuring from the the centrl max you can now note the 1st bright fringe to appear 'bright fringe mens a constructive zone'