Fresnel's zone plate

Principle

A zone plate is illuminated with parallel laser light. The focal points of several orders of the zone plate are projected on a ground glass screen.

Benefits

• Find out that optical lenses can be completely different from what you are used to
• Interference is used to produce effects commonly achieved with diffraction
• The use of a laser lightsource provides clear results

Tasks

1. The laser beam must be widened so that the zone plate is well illuminated. It must be assured that the laser lightbeam runs parallel over several meters.
2. The focal points of several orders of the zone plate are projected on a ground glass screen. The focal lengths to be determined are plotted against the reciprocal value of their order.
3. The radii of the zone plate are calculated.

Learning objectives

• Huygens Fresnel principle
• Fresnel and Fraunhofer diffraction
• Interference
• Coherence
• Fresnel's zone construction
• Zone plates