A beam of light propagates through a medium 1 and falls onto another medium 2, at an angle ${\alpha }_1$ as shown in Fig. After that, it propagates in medium 2 at an angle ${\alpha }_2$ as shown. The light's wavelength in medium 1 is ${\lambda }_1$. What is the wavelength of light in medium2? 

• A

  $\frac{\sin {\alpha }_1}{\sin {\alpha }_2}{\lambda }_1$

• B

  $\frac{\sin {\alpha }_2}{\sin {\alpha }_1}{\lambda }_1$

• C

  $\frac{\cos {\alpha }_1}{\cos {\alpha }_2}{\lambda }_1$

• D

  $\frac{\cos {\alpha }_2}{\cos {\alpha }_1}{\lambda }_1$