Chapter 7, Problem 7.92QP

Interpretation Introduction

Interpretation:

The role of lasers in reading a compact disc has to be explained.

Concept introduction:

When light of certain wavelength falls on ruby crystal (aluminum oxide contains small amount of chromium(III) ions), the chromium ions undergo transition from level one to level three . Most of ${\text{Cr}}^{\text{3+}}$ ions in level three undergo radiation less transition to level two by losing energy in form of heat than emitting photons. Level two has much greater lifetime which leads to accumulation of ${\text{Cr}}^{\text{3+}}$ ions in level three.  When these accumulated ions emit all at same time an intense emission will occur. The wavelength of photon from emitted ion and original photon are same.  Now there are two photons one original photon and another by stimulated emission. Finally there is increase in intensity of light at this wavelength.