For the case of a relativistic incident electron, the "magic" angle is defined by the equality of two different functions
(denoted below by  and ) of the collection angle :
and
where  is the speed of the incoming electron divided by the speed of light (N.B., the symbol  is also often used in the older literature to denote the collection angle instead of ).
Of course, the above integrals may easily be evaluated in terms of elementary functions, but they are presented as above because in the above form it is easier to see that the former integral is due to momentum transfers which are perpendicular to the beam direction, whereas the latter is due to momentum transfers parallel to the beam direction.
Using the above definition, it is then found that