  A new class of generalized solutions related to the essential spectrum of
linear Maxwell's equations is presented. The essential modes are given in terms
of normalized singular Weyl's sequences, whose square represents Dirac's delta
functions in spatial and angular frequency domains. The action integral
associated with essential modes is well-defined. We claim that these modes
represent the collapsed state of the electromagnetic field and, with some
additional assumptions on the conservation of action, are suitable for
describing the double-slit experiment in accordance with the orthodox point of
view.
