Answer
$1.3m$
Work Step by Step
The wavelength difference can be calculated as
$\Delta \lambda=\lambda_2-\lambda_1$
$\implies \Delta \lambda=\frac{v}{f_2}-\frac{v}{f_1}$
$\implies \Delta \lambda=v(\frac{1}{f_2}-\frac{1}{f_1})$
We plug in the known values to obtain:
$\Delta \lambda=343(\frac{1}{170}-\frac{1}{510})=1.3m$
