(a(i))[1]
Explain what the term diffracted waves means.
(a(ii))[1]
Explain what the term coherent waves means.
(b(i))[2]
Light that contains only two wavelengths $\lambda_1$ and $\lambda_2$ is incident normally on a diffraction grating. The third order diffraction maximum for light of wavelength $\lambda_1$ and the fourth order diffraction maximum for light of wavelength $\lambda_2$ are at the same angle $\theta$ to the direction of the incident light. Show that the ratio $\dfrac{\lambda_2}{\lambda_1}$ is $0.75$. Explain your working.
(b(ii))[1]
The wavelengths differ by $170\,\text{nm}$. Determine wavelength $\lambda_1$.