As shown in fig. for reflection from the ceiling Path SCP=SC+CP=2SC [as ∠i=∠r,SC=CP] path SCP=2√602+252=130m So path difference between interfering waves along paths SCP and SP ∆x=130−120=10m
Now for constructive interference at P ∆x=nλ, i.e. 10=nλ or λ=
10
n
with n=1,2,3...... i.e. λ=10m,5m,(10∕3)m and so on