what happens to interference pattern in young doble slit experiment ,if the main source is not on central line

 

In Young's double slit experiment, the source is on the centre line between the slits and thus the central bright fringe appears at O. This is because the distance travelled by the rays SCO and SDO are equal as shown in figure (a).

 

 

If the source is moved, say if we move it upwards at point S' as shown in figure (b), the central bright fringe will appear at point O'. Here the distance traveled by rays SCO and SDO are not equal. Therefore, when the source is moved upwards from the central line, the central bright fringe will move downwards and vice-versa.