Phase difference can be expressed as an angle with units rad or degrees. To calculate the phase difference you need to find the fraction . If time t , a fraction of a period. If distance x ,a fraction of a wavelength.
The fraction should be less than 1. If your fraction is 2.5 than only 0.5 is required to calculate phase angle which is pi or 180 deg. Use the formula to convert the fraction to phase angle.
If the phase angle is 45 degrees , another valid phase angle would be 315 degrees.If the phase angle is 0.5 pi , the other valid phase angle is 1.5 pi.
In Young double slit expt, the bright fringes are distance x apart . Any 2 points of distance y apart can be converted into a fraction of x. This fraction can be used to find the phase difference. Eg. half way between 2 bright fringes will be 0.5x. This will give a fraction equal to 0.5 which works out to be 1pi( 180 degrees) phase difference.
