A high powered laser beam with a beam diameter of 11.8 cm is aimed at the Moon, distant. The spreading of the beam is caused only by the diffraction effects. The angular location of the edge of the central diffraction disk is given by

where is the diameter of the beam aperture. We have to find the diameter of the central diffraction disk at the Moon’s surface.

Solution:             Click For PDF Version

For the problem the following data has been given:

Diameter of the beam, ,

Wavelength of the laser bean, .

Using the result that the angular location of the edge of the central diffraction disk is given by

we calculate the angle of the central diffraction disk. We get

The diameter of the central diffraction disk on the Moon’s surface

will be given by