Find the thickness of a plate which will produce a change in optical path equal to half the wavelength $\lambda$ of the light passing through it normally. The refractive index of the plate is $\mathrm{\mu }$

• A

  $\frac{\lambda }{4(\mu -1)}$

• B

  $\frac{2\lambda }{4(\mu -1)}$

• C

  $\frac{\lambda }{(\mu -1)}$

• D

  $\frac{\lambda }{2(\mu -1)}$