A source emits electromagnetic waves of wavelength 3 m. One beam reaches the observer directly and other after reflection from a water surface, travelling 1.5 m extra distance and with intensity reduced to 1/4 as compared to intensity due to the direct beam alone. The resultant intensity will be

We know that a phase change of π  occurs when the reflection take place  at the boundary of denser medium.   Due to geometry,  ∆ϕ1 = 2πλ∆x        =2π31.5=π   radian. Due to phase change,   ∆ϕ2 = π   radian  ∴Total phase difference of second wave  when it reached the observer   ∆ϕ=∆ϕ1+∆ϕ2=2π   radian  Resultant intensity =  I1+I2+2I1I2cosϕ                                     =I+I4+2.I2cos2π=5I4+I=9I4