This volume consists of lectures delivered at the Sixth Inter- national Nathiagali Summer College on Physics and Contemporary Needs held at Islamabad from June 15 to July 2, 1981. The College used to be held at one of the scenic hill resorts of Pakistan, Nathiagali, hence the name of the College. The College was organized by the Pakistan Atomic Energy Commission (PAEC), under the patronage of the International Centre for Theoretical Physics (ICTP), Trieste, with a view to providing an opportunity for local physicists and physicists from developing countries for learning of the latest...