In a Young's double slit experiment the source S and the two slits A and B are vertical with slit A above slit B. The fringes are observed on a vertical screen K. The optical path length from S to B is increased very slightly (by introducing a transparent material of higher refractive index) and the optical path length from S to A is not changed, as a result the fringe system on K moves [NCERT 1984]

• A.            Vertically downwards slightly
• B.            Vertically upwards slightly
• C.            Horizontally, slightly to the left
• D.            Horizontally, slightly to the right