speckle telescope focometry study set

• Accurately and reproducibly measuring the focal length of convergent and divergent lenses,
• An ingenious and simple kit, based on the use of the Laser Torch and the exploitation of the speckle phenomenon

• A Laser torch (204 080)
• An opaque white screen
• A Speckle glass (204 708)
• A fine mounted focusing screen
• A set of convergent lenses
• A set of divergent lenses

The speckle is related to the diffusion of a coherent source on a rough object at the wavelength-scale or a lit specular object.

This phenomenon is explained by the Huygens-Fresnel principle because it results from random interferences between different Source point s generating waves of the same frequency but of different phase and amplitude.

The size of the speckle grains depends on the laser wavelength, the beam size and the distance between this surface and that on which the speckle is formed. Thus, the speckle grains size evolves with the laser beam part in the plane of the focusing screen:

• The pattern with the largest speckle grains is observed upon the exact positioning of the focusing screen in the focal plane of a convergent lens,
• Above and below the housing, the size of the speckle grains is reduced.