Young’s Two-Slit Experiment Without a Laser

Abstract

The Young’s two-slits experiment is arguably one of the most famous and beautiful experiments in physics. In introductory physics labs the experiment is almost exclusively carried out using a laser, and it is introduced this way in some introductory physics textbooks. In the case of laser illumination, light passes directly through the slits and spreads out by diffraction so that interference fringes can be viewed on a distant screen. Most students, in our experience, are unaware that the experiment can be carried out in any other way. But the laser approach depends on the remarkable light emitted by the laser: it is spatially coherent, meaning that the phase relationship between the light emitted from the slits is constant, and it is extremely bright, meaning that real interference fringes can be seen on the screen. When carried out with classical, incoherent light, a quasi-monochromatic source such a sodium lamp must first be rendered spatially coherent by focusing the light through a pinhole and then using this light to illuminate the two slits. Fringe patterns will certainly be formed, but they need to be observed by either viewing with an eyepiece or allowing them to fall directly on the photosensor of a camera. It is implied in other physics texts that the fringe pattern can be cast onto a screen and viewed, but this is not feasible because the fringes are just not bright enough. Here we report an arrangement where an extended incoherent source can be used and fringes made bright enough to be observed on a screen.