6C20.40 - Fresnel Zone Plate Lens
To demonstrate how to focus light using diffraction instead of refraction or reflection with visible light.
A laser with beam expander (Camera Lens Computar) is aligned with a rather tight collamated beam. The expanded beam is then passed through the Fresnel Zone Plate. Using an image plate and camera, move them along the axis to locate the image position.
Turn off the room lights and make sure the camera output is visible to the audience. Turn on the laser and move the zone plate in front of the beam. Move the image plate with camera setup until the image is located.
Traditional lens material works great for refracting visible light, but the refractive index is not the same for the entire electromagnetic spectrum. For example, X-rays are only weakly refracted by glass and other transparent materials. Zone plates get around this issue, since the plate does not need to be transparent to focus the rays. The same zone plate will focus light of many wavelengths to different foci, which allows this to be a type of filter.