phase difference=( 2pi/ lamda) X path difference
phase difference = pi [ for destructive interference]
lambda(wavelength) = 2pi/ pi X path difference.
path difference is S2M - S1M
find S2M using Pythagoras = root of 802 + 1002
=128
path difference= 128-100=28
wavelength= 2pi/pi X 28
=56cm
At f= 1KHz wavelength= 0.33m/ 33cm
At f=4kHz wavelength= 0..0825/8.25cm
wavelength changes from 33cm to 8.25cm
for minima...phase difference should be either one of them pi, 3pi,5pi,7pi [sub them in the eqn]
minima is observed when lambda= 56cm, 18.7cm, 11.2cm, 8cm
which are the 2 values between 33 and 8.25?? 18.7 and 11.2...so only 2 minimas