The Electromagnetic Spectrum
Optics and Photonics is an enabling technology and science It enables advances in many different disciplines of science, engineering, etc.
Radio Waves Power Transmission Radio Broadcasts
Radio Astronomy
Microwaves
Microwave source (Magnetron) melted a candy bar in Spencer’s pocket. The next day, he put an egg near the microwave source. It cooked and exploded….. The rest is history
Microwaves
Microwaves – Big Bang Discovery Cosmic background Radiation – Residual ‘Big Bang’ radiation from thermal ‘black body’ spectrum corresponding to radiation at T=2.7K
Terahertz (THz) Non-destructive Evaluation Security Screening
Picometrix NDE of Space Shuttle SOFI (Sprayed On Foam Insulation) –Insulates external tank –2 to 10 inches thick –Can break off during launch Non-destructive evaluation (NDE) desired to validate tank New NDE technique required: –<2 mm lateral resolution –Sensitive to voids and disbonds
High Resolution Imaging: 5x 400 x 400 mm
T-Ray Image of “Natural Voids” Space between stringers clear of voids or defects
Concealed Threats in Packaging Plastic knife and metal razor blade identified through packaging TRANSMISSION
Concealed Threats in Clothing Knife in shirt pocketExplosive simulant REFLECTION Unmodified
Infrared – Thermal Imaging
Vegetation Mapping
Visible - Oximeter
UV-Sun/ Crab Nebula – Skin Cancer
X-rays
X-ray
Other things appear blue due to scattering…..
Three general classes of Scattering Depending on Particle size
Interference (Brief) Waves which are IN PHASE will add together. Waves which are OUT of phase, when added together will cancel each other
Consider a column of MANY scattering Centers…. In forward direction, scattered light forms a wavefront in the same direction (Secondary Wave) Primary wave
Even if scattering centers NOT in an organized row, forward going scattered light forms a new wavefront which transmits through medium
What about lateral (side) scattering? For a particular spacing between scattering centers, the scattered waves will DESTRUCTIVELY interfere and cancel each other Destructive interference Since there are MANY scattering centers (eg. number of atoms in a solid), we can ALWAYS add the scattering centers in pairs which individually destructively interfere.
Reflection from a boundary
Specular Reflection (from flat surface) Diffusion Reflection (from rough surface) Stealth fighter designed to avoid Diffuse Reflection from radar waves. Reflected waves directed so that reflected light does not retrace path back to be detected.
Huygen’s Principle Break up wavefront into individual points which each acting as a point source. Allow each spherical wavefront to propagate according to the local speed of light in the medium. Combination of spheres defines new wavefront. New wavefront is the line or curve of the crests/ troughs of the point sources of waves Direction of travel (ray) is perpendicular to wavefront
Refraction Pencil appears to be Bent at surface of water Object is DEEPER in water than it appears.
Light refracts because speed of light and radius of Huygen’s circle is different in the two mediums
Generally, as light interacts at an interface between two medium, there is a reflected wave a refracted wave Animation of reflection/ refraction of light at a boundary
Example of Fermat’s Principle - Mirages Hot Air less dense so air travels FASTER Cold Air more dense so air travels SLOWER