PURPOSE: To show Fresnel and Fraunhofer diffraction from pinholes and opaque discs.
DESCRIPTION: Location of the screen determines the nature of the diffraction pattern: for nearby screen Fresnel diffraction is obtained, while for a distant screen Fraunhofer diffraction is obtained.
Lens L1 (very close to the diffraction obstacle) maps the entire region to the right of the aperture to within the focal length of the lens. Lens L2 projects one plane at the focal point of the lens onto a screen. Moving L2 from about f2 away from L1 to f1+f2 away from L1 projects successively every plane, from the aperture to infinity, onto the screen. Thus as lens L2 is moved the diffraction pattern changes, as if the screen starts close and moves to infinity, showing first Fresnel diffraction, the transition region, and ending with Fraunhofer diffraction.
The Poisson bright spot can be seen with the opaque disc if alignment along the optic axis is reasonably accurate.
REFERENCES: (PIRA unknown.)
EQUIPMENT: Laser cart with diffraction source (either pinhole of opaque disc), 20cm focal length convex lens (L1), 2cm focal length convex lens (L2), clamps and holders as photographed.
SETUP TIME: 20 min.
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