Easy ones on waves

for answer 2, what is to be known is that there is a phase change of ∩ when a longitudinal/transverse wave reflects from in interface which separates two mediums, the one in which the wave velocity is smaller being the one through which the wave is transmitted and the incident ray is travelling through the medium in which the wave velocity is larger ! just take the example of a light string and a heavy string joined together and the wave travels from the lighter to the heavier string !

for answer 3, intensity at a distance (r) due to a point source = I/ 4∩r²
intensity at a distance (r) due to a point source =I/4∩r

debo, will the answer for (2) be valid even if it is a sound wave?

can u pls derive the exps for (3)?

but in HCV while calculating path difference has ignored the phase differnce of π.. see solved eg.11 sound waves (pg 348)

what derivation u need for 3 ???

4) coz the distance is too small for the fall in intensity

as i is inversely prop to r^2 and the typical distance in a ydse distance practical setup is 1m

1) u think it will be 1

as intensity is enrgy per unit surface area

and the enrgy by a group of phtons is prop to the number of the phtons strinking the surface