A red light-emitting diode (LED) used as a bench light source emits at a central…

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Coherence and statistical optics · Coherence and statistical optics

Optics and Photonics, Ch Two-beam interference and interferometry

Problem

A red light-emitting diode (LED) used as a bench light source emits at a central vacuum wavelength of

660

nm, with a spectral width (full width) of

\Delta\lambda = 30

nm. ((a)) Estimate its coherence length

\ell_c

using

\ell_c \approx \lambda^2/\Delta\lambda

. ((b)) This LED illuminates a Michelson interferometer whose two arms are set so that the optical path difference

\Lambda

between the beams is

100\ \mu\mathrm{m}

. State, with a one-line reason, whether fringes will be visible.