An arrangement for observing Fresnel diffraction

Abstract

Fresnel diffraction assumes a spherical wavefront which occurs when the source-obstacle distance is small. With a conventional source there is some difficulty in projecting a sufficiently intense image of the diffraction pattern for demonstration to a large class. In this experiment the beam from a 2 mW helium-neon laser was deliberately made divergent using a double concave lens placed over the exit aperture. A projector lens, placed after the diffracting system, enabled the image to be projected on a screen 80 cm beyond. The magnification could be changed easily by moving the obstacle towards or away from the laser and in this experiment ranged from *20 to *100.