The Electromagnetic Spectrum

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

http://www.ovsa.njit.edu/

Radio Astronomy

Microwaves

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 DiscoveryCosmic 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

400 x 400 mm400 x 400 mm

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 pocket Explosive simulant

REFLECTION

Unmodified

Infrared – Thermal Imaging

Infrared – Thermal Imaging

http://speclab.cr.usgs.gov

Vegetation Mapping

Visible - Oximeter

Visible - Oximeter

UV-Sun/ Crab Nebula – Skin Cancer

X-rays

X-ray

X-ray

Other things appear blue due to scattering…..

Three general classes of Scattering Depending on Particle size

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.

1/222 2 2 2

1e oo

eo

q Ex

m

12 2

tano

i toE e

( )i tox e

Reflection from a boundary

Reflection from a boundary

Reflection from a boundary

Reflection from a boundary

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

RefractionPencil 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

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

Example of Fermat’s Principle - Mirages

Hot Air less dense so air travels FASTER

Cold Air more dense so air travels SLOWER