What is the basic physical principle responsible for the presence of energy bands rather than specific energy levels in a solid?

In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energies that an electron within the solid may have (called energy bands, allowed bands, or simply bands) and ranges of energy that it may not have (called band gaps or forbidden bands).Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, periodic lattice of atoms or molecules. Band theory has been successfully used to explain many physical properties of solids, such as electrical resistivity and optical absorption, and forms the foundation of the understanding of all solid-state devices (transistors, solar cells, etc.).

 How does Laue’s approach differ from Bragg’s approach?

According to Bragg's law,the crystal is considered as a set of equal spaced parallel planes of ions on atoms.X-ray are reflected by the ions on atoms on one plane.Reflected rays from two successive planes interfere constructively.

But in Laue’s approach ,there is no concept of parallel planes.X-rays are scattered from the lattice points(atoms or ions) in all directions and maximum will occur when scattered rays interfere constructively.

 WHAT DO YOU MEAN BY METASTABLE ENERGY?

A metastable state may thus be considered a kind of temporary energy trap or a somewhat stable intermediate stage of a system the energy of which may be lost in discrete amounts. In quantum mechanical terms, transitions from metastable states are “forbidden” and are much less probable than the “allowed” transitions from other excited states.

Why is optical pumping not used in gas?

Optical pumping methods are very ineffective for gas lasers because gas atoms show sharp energy levels as compared to solids, which are characterized by energy bands rather than energy levels. Thus for gas laser the electrical discharge is used for pumping.

Why is hologram called wave front reconstruction?

Because the very complex waves that are reflected from an object when it was sitting there are reproduced(reconstructed) the very same way in the hologram. The hologram appears to be a window behind which the object is sitting, but the object is not there.

All of the light that was reflected off the object was Reconstructed, rebuilt, duplicated by the hologram.

 Why laser action cannot occur without population inversion between atomic levels?

• Population inversion is the state in which the number of atoms in higher energy state is more than those in lower energy state.
• It is required in laser so that stimulated emission is more probable than induced absorption.

Population inversion that results in beam amplification.

 Why is the optical resonator required in lasers?

An optical resonator is needed to build up the light energy in the beam.The resonator is formed by placing a pair of mirrors facing each other so that light emitted along the line between the mirrors is reflected back and forth When a population inversion is created in the medium, light reflected back and forth increases in intensity with each pass through the laser medium.