After developing the concept of quantum mechanical angular momentum

[ I mean the relation L_z=m (h') where h' means h/2π

newbielink:http://hyperphysics.phy-astr.gsu.edu [nonactive]...um/qangm.html]

a book says:

"The quantum mechanical origin of these strange restrictions lies in the require-

ment that if either the particle or the laboratory is turned through a complete

rotation around any axis,the observed situation will be the same as before the

rotation.Because observables are related to the square of the wavefunction,the

wavefunction must turn into either plus or minus itself under a rotation by 2π

radians.Its sign remains unchanged if the angular momentum around the rotation

axis is an integer multiple of h(i.e.,forbosons)but changes if the angular momen-

tum around the rotation axis is a half-integer multiple of h (i.e.,forfermions).

Because of this difference in sign under 2π rotations,bosons and fermions each

obey a different type of quantum statistics"

I cannot exactly follow the book here.How can exp[i 2π] result in - of the same wave function?

can anyone please explain?