# 1D string reflection and transmission phase

Ok, I must be missing something very obvious here. After applying the boundary conditions, we can write:

$$A_R e^{i \delta_R} = (\frac{v_2 - v_1}{v_1 + v_2}) A_I e^{i \delta_I}$$

and

$$A_T e^{i \delta_T} = (\frac{2v_2}{v_1 + v_2}) A_I e^{i \delta_I} .$$

Then, my book says if the second string is lighter, we have $$v_2 > v_1$$, so $$\delta_T = \delta_R = \delta_I$$. I am really not seeing how we can deduce $$\delta_T = \delta_R = \delta_I$$ from $$v_2 > v_1$$.