(i) Explain the concepts of: traction on an element of surface; the stress tensor; the strain tensor in an elastic medium. Derive a relationship between the two tensors for a linear isotropic elastic medium, stating clearly any assumption you need to make.

(ii) State what is meant by an $\mathrm{SH}$ wave in a homogeneous isotropic elastic medium. An SH wave in a medium with shear modulus $\mu$ and density $\rho$ is incident at angle $\theta$ on an interface with a medium with shear modulus $\mu^{\prime}$ and density $\rho^{\prime}$. Evaluate the form and amplitude of the reflected wave and transmitted wave. Comment on the case $c^{\prime} \sin \theta / c>1$, where $c^{2}=\mu / \rho$ and $\left(c^{\prime}\right)^{2}=\mu^{\prime} / \rho^{\prime}$.