18. The phases of the light wave at $c , d , e$ and $f$ are $\phi _ { c } , \phi _ { d } , \phi _ { e }$ and $\phi _ { f }$ respectively. It is given that $\phi _ { c } \neq \phi _ { f }$. (A) $\phi _ { c }$ cannot be equal to $\phi _ { d }$ (B) $\phi _ { d }$ can be equal to $\phi _ { e }$ (C) $\left( \phi _ { d } - \phi _ { f } \right)$ is equal to $\left( \phi _ { c } - \phi _ { e } \right)$ (D) $\left( \phi _ { d } - \phi _ { c } \right)$ is not equal to $\left( \phi _ { f } - \phi _ { e } \right)$ Answer [Figure][Figure][Figure][Figure] (A) (B) (C) (D)
18. The phases of the light wave at $c , d , e$ and $f$ are $\phi _ { c } , \phi _ { d } , \phi _ { e }$ and $\phi _ { f }$ respectively. It is given that $\phi _ { c } \neq \phi _ { f }$.\\
(A) $\phi _ { c }$ cannot be equal to $\phi _ { d }$\\
(B) $\phi _ { d }$ can be equal to $\phi _ { e }$\\
(C) $\left( \phi _ { d } - \phi _ { f } \right)$ is equal to $\left( \phi _ { c } - \phi _ { e } \right)$\\
(D) $\left( \phi _ { d } - \phi _ { c } \right)$ is not equal to $\left( \phi _ { f } - \phi _ { e } \right)$\\
Answer\\
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