In Fig. 16. 57, a wave front AB moving in air is incident on a plane glass surface xy. Its position CD after refraction through a glass slab is shown also along with normals drawn at A and D. The refractive index  of glass with respect to air will be equal to :

• A

  $\frac{\sin \mathrm{\theta }}{\sin \mathrm{\theta }\text{'}}$

• B

  $\frac{\sin \mathrm{\theta }}{\sin \mathrm{\varphi }\text{'}}$

• C

  (BD/AC)

• D

  (AB/CD)