In the ideal double-slit experiment, when a glass-plate (refractive index 1.5) of thickness $t$ is introduced in the path of one of the interfering beams (wavelength $\lambda$), the intensity at the position where the central maximum occurred previously remains unchanged. The minimum thickness of the glass-plate is

• Correct answer
  A

  $2\lambda$
• B

  $\frac{2\lambda }{3}$
• C

  $\frac{\lambda }{3}$
• D

  $\lambda$

Hence, the correct answer is option: 'A'.