A monochromatic beam of light falls on Young's double slit experiment apparatus as shown in figure. A thin sheet of glass is inserted in front of lower slit ${\mathrm{S}}_2(\mathrm{\lambda }=600\mathrm{ }\mathrm{n}\mathrm{m}$ is wavelength of source)




If the central bright fringe is obtained on screen at 'O' then

• Correct answer
  A

  $\left(\mathrm{\mu }-1\right)\mathrm{t}=\mathrm{d}\sin \mathrm{\theta }$
• B

  $\left(\mathrm{\mu }-1\right)\mathrm{t}=\mathrm{d}\cos \mathrm{\theta }$
• C

  $\left(\mathrm{\mu }\mathrm{t}\right)=\mathrm{d}\mathrm{\theta }$
• D

  Not possible

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