Post on 19-Dec-2015
The Electromagnetic Spectrum
• Optics and Photonics is an enabling technology and science• It enables advances in many different disciplines of science,
engineering, etc.
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 – Big Bang DiscoveryCosmic background Radiation – Residual ‘Big Bang’ radiation from thermal ‘black body’ spectrum corresponding to radiation at T=2.7K
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
Concealed Threats in Packaging
Plastic knife and metal razor blade identified through packaging
TRANSMISSION
Interference (Brief)
1 2totalE E E 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 scatteringCenters….
New Wavefront
In forward direction, scattered light forms a wavefront in the same direction
(Secondary Wave)
Primary wave
New WavefrontEven 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.
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
Light refracts because speed of light and radius of Huygen’s circle is different in the two mediums
1v
2v
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=16
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