When an occultation of a bright star occurs, Fresnel diffraction effects may be visible. Fresnel diffraction is the light intensity pattern that occurs at the edge of an object. Approaching the edge of the object from the unobscured side, the light intensity oscillates with increasing intensity until it rapidly falls off. At the location corresponding to the edge of the object, the light intensity has reduced to 0.25 of the full illumination. The light intensity continues to fall off in an exponential manner as one moves into the fully obscured region.
Where a large magnitude drop occurs, the exponential drop in the light intensity will be observable and can be confused with the effects of stellar diameter. The following table lists indicative angular distances from the true edge of the object for a given magnitude drop for the star. The Fresnel diffraction is dependant upon the distance to the asteroid - which is computed on the basis of 4 different distances. Also, the diffraction has been computed for light of wavelength 4000 angstroms.
Distance in masec Distance in km on Earth
Mag Drop Light drop 1 AU 2 AU 3 AU 4 AU 1 AU 2 AU 3 AU 4 AU
1 .40 0.1 0.1 0.0 0.0 0.061 0.087 0.106 0.123
2 .16 0.1 0.1 0.1 0.1 0.097 0.138 0.169 0.195
3 .063 0.2 0.2 0.1 0.1 0.154 0.218 0.267 0.308
4 .025 0.3 0.2 0.2 0.2 0.244 0.346 0.423 0.489
5 .010 0.5 0.4 0.3 0.3 0.387 0.548 0.671 0.775
6 .0039 0.9 0.6 0.5 0.4 0.614 0.868 1.063 1.228
7 .0016 1.4 1.0 0.8 0.7 0.973 1.376 1.685 1.946
8 .0006 2.1 1.5 1.2 1.1 1.542 2.181 2.671 3.084
9 .0003 3.4 2.4 2.0 1.7 2.444 3.456 4.233 4.887
10 .0001 5.4 3.8 3.1 2.7 3.873 5.477 6.708 7.746
As can be seen from this table, Fresnel diffraction can become significant if the observable magnitude drop is >4 magnitudes.
For a more technical discussion on Fresnel diffraction
For sample light curves, see Fresnel sample light curves