Question

Of the two slits producing interference in Young’s experiment, one is covered with glass so that light intensity passing is reduced to 50%. Which of the following is correct?

• A. Intensity of fringes remains unaltered
• B. Intensity of bright fringe decreases and that of dark fringe increases
• C. Intensity of bright fringe increases and that of dark fringe decreases
• D. Intensity of both bright and dark fringes decreases

Answer 1

The correct option is B Intensity of bright fringe decreases and that of dark fringe increases

Initial intensity of bright fringes = $(\sqrt{I}+\sqrt{I}{)}^2$
Final intensity of bright fringes after covering one slit with glass sheet = ${(\sqrt{I}+\sqrt{\frac{I}{2}})}^2$
Clearly the intensity of bright fringes has decreased.
For dark fringes, initial intensity = $(\sqrt{I}-\sqrt{I}{)}^2=0$ but new intensity ${(\sqrt{I}-\sqrt{\frac{I}{2}})}^2\ne 0$
Clearly the intensity of dark fringes has increased.