Four light waves are represented by : $($i$)$ a$1$sin$\backslash$omega t $($ii$)$ a$1$sin$(\backslash$omega t $+\backslash$delta $)($iii$)$ a$2$sin$2\backslash$omega t $($iv$)$ a$2$sin$(2\backslash$omega t $+\backslash$delta $)$ Interference fringes may be observed due to superposition of