# Radius of wavelet using Huygen's principle

I was trying problem 1.6 and couldn't solve it. Any help is appreciated.

My effort : I considered a point $$P_1$$ at distance y from the x-axis and another point $$P_2$$at a distance $$y-\delta y$$ from the x-axis, but lying on the same wavelet. Then in small time $$\tau$$, the point $$P_1$$ and $$P_2$$ produce a wavelet of radius $$R_1= \frac{c \tau}{n(y)}$$ and $$R_2= \frac{c \tau}{n(y-\delta y)}$$. But I couldn't related this radii to the radius of wavelet.