Step 1: Use Compton shift formula.
\[
\Delta \lambda = \lambda_C (1 - \cos\theta)
\]
The longest wavelength occurs at \(\theta = 180^\circ\):
\[
\Delta \lambda_{\max} = 2\lambda_C = 2(0.024) = 0.048\, \text{Å}
\]
\[
\lambda'_{\max} = \lambda + \Delta\lambda = 0.500 + 0.048 = 0.548\, \text{Å}
\]
Step 2: Conclusion.
Hence, the longest possible scattered wavelength = 0.548 Å.
