(a) Identify electromagnetic waves which:
(i) are used in radar system.
(ii) affect a photographic plate.
(iii) are used in surgery. 

Write their frequency range.
OR
(b) A plane wavefront is propagating from a rarer into a denser medium. Use Huygens principle to show the refracted wavefront and verify Snell’s law.
 

(i) Microwaves are used in radar systems. Frequency range 1 GHz to 300 GHz
(ii) Infrared radiation whose wavelength is 10⁻⁵ m affect a photographic plate.
(iii)Eye surgery: Ultraviolet rays are used in eye surgery. Frequency range 8 × 10¹⁴ to 5 × 10¹⁷ Hz.
OR

$\begin{array}{l}\text{ In }\mathrm{△}\mathrm{ABC},\\ \sin \mathrm{i}=\mathrm{BC}/\mathrm{AC}={\mathrm{v}}_1\mathrm{t}/\mathrm{AC}\\ \text{ In }\mathrm{△}\mathrm{CEA},\\ \sin \mathrm{r}=\mathrm{AE}/\mathrm{AC}={\mathrm{v}}_2\mathrm{t}/\mathrm{AC}\\ \therefore \mathrm{sini}/\sin \mathrm{r}=\mathrm{BC}/\mathrm{AE}={\mathrm{v}}_1\mathrm{t}/{\mathrm{v}}_2\mathrm{t}={\mathrm{v}}_1/{\mathrm{v}}_2\\ \therefore {\mathrm{\mu }}_1=\mathrm{c}/{\mathrm{v}}_1\\ {\mathrm{\mu }}_2=\mathrm{c}/{\mathrm{v}}_2\\ \therefore {\mathrm{\mu }}_2/{\mathrm{\mu }}_1={\mathrm{v}}_1/{\mathrm{v}}_2\\ \therefore \mathrm{sini}/\sin \mathrm{r}={\mathrm{\mu }}_2/{\mathrm{\mu }}_1\end{array}$
Hence Snell's law.