- #1

christianpoved

- 15

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## Homework Statement

To solve the wave equations in vacuum for ##\vec{E}## and ##\vec{B}## we made the ansatz:

\begin{array}{cc}

\vec{E}\left(\vec{r},t\right)=\vec{E}_{0}\cos\left(\vec{k}\cdot\vec{r}-\omega t+\delta\right)

\vec{B}\left(\vec{r},t\right)=\vec{B}_{0}\cos\left(\vec{k}\cdot\vec{r}-\omega t+\delta\right)

\end{array}

Make a new ansatz of the form:

\begin{array}{cc}

\vec{E}\left(\vec{r},t\right)=\vec{E}_{0}\cos\left(\vec{k}\cdot \vec{r}-\omega t+\delta\right)

\vec{B}\left(\vec{r},t\right)=\vec{B}_{0}\cos\left(\vec{k}^{\prime} \cdot \vec{r}-omega^{\prime}t+\delta^{\prime}\right)

\end{array}

And show either that

\begin{array}{cc}

\vec{k}=\vec{k}^{\prime}

\omega=\omega^{\prime}

\end{array}

Or that, in general:

\begin{array}{cc}

\vec{k}^{\prime}\left(\omega^{\prime}\right)=\vec{k}\left(\omega\right)

\end{array}

## Homework Equations

I used Maxwell equations and some cross/dot product identities.

## The Attempt at a Solution

I put these new fields into Maxwell equations and i get this

\begin{array}{cc}

\vec{E}\cdot\vec{k}=0 & \mbox{(1)}\\

\vec{B}\cdot\vec{k}^{\prime}=0 & \mbox{(2)}\\

\vec{k}\times\vec{E}=\omega^{\prime}\vec{B} & \mbox{(3)}\\

\vec{k}^{\prime}\times\vec{B}=-\frac{\vec{k}\cdot\vec{k}}{\omega}\vec{E} & \mbox{(4)}

\end{array}

With these equations I can show that both wavenumbers are parallel, and that the factor between them is the ratio of the frequencies, shouldn't I be able to show that the frequencies are the same and then conclude that the wavenumbers are equal too?

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