L.ocatlng the Fringes
Light waves produce fringes in a Young’s double-slit interference experiment, as it is called. but what exactly determines the locations of the fringes’? To answer. we shall use the arrangement in . There. a plane wave of monochromatic light is incident on two slits SI and S2 in screen B; the light diffracts through the slits and produces an interference pattern on screen C. We draw a central axis from the point halfway between the slits to screen C as a reference. We then pick. for discussion. an arbitrary point P on the screen. at angle ()to the central axis: This point intercepts the wave of ray rl from the bottom slit and the wave of ray r2 from the top slit.These waves are in phase when they pass through the two slits because there they are just portions of the same incident wave. However. once they have passed the slits. the two waves must travel different distances to reach P. We saw a similar situation in Section 18-4 with sound waves and concluded that.
The phase difference between two waves can change if the waves travel paths of different