Aspherical wave with a wavelength of 4.0 m is emitted from the origin. at one instant of time, the phase at r = 8.0 m is πrad. at that instant, what is the phase at r = 7.5 m ?

phase of the wave at r = 7.5 m is given as

Explanation:

As we know the relation between phase difference and path difference is given by

now we know that phase of wave is 180 degree at a distance r = 8 m

so here we have

now we have to find the phase at r = 7.5 m

so again by above formula

what is the actual question though?