NEET Physics Electromagnetic Waves MCQs

Two waves on a string are described by $y_1(x,t) = A\cos(kx - \omega t)$ and $y_2(x,t) = A\cos(kx + \omega t + \phi)$. For what value of $\phi$ will the resultant wave have an amplitude of $A$?

The idea of the quantum nature of light has emerged in an attempt to explain