When light travels from an optically rarer medium to an optically denser medium, the velocity decreases because of change in

Four different independent waves are represented by

$$\begin{gathered} i) y_1 = a_1\sin \omega t \\ ii) y_2 = a_2\sin 2\omega t \\ iii) y_3 =a_3\cos \omega t \\ iv) y_4 = a_4\sin \left(\omega t + \pi /3\right) \end{gathered}$$

With which of the two waves interference is possible