Harry: Invisible Man
Is invisibility possible?
?Invisibility?
How it works
A digital video
camera captures the
scene.
A computer processes
the captured image.
A projector receives
the enhanced image
and shines an
projected image
toward the person
wearing the cloak.
The cloak acts like a
movie screen,
reflecting light
directly back.
Is this any
use?
Camera
Tripod?
Bad points
Good points
•Only works in 2D
•Results that work
•Shows some reflection
•Possible improvements
•Not very dynamic!
•Inexpensive
•Bit of a cheat really!!
What is Light?
An electromagnetic wave, made up of
different wavelengths.
See for yourselves!!
How does light interact with objects?
Natural form of invisibility is an object which neither reflects
nor absorbs light (it allows light to pass through it).
Absorption - Occurs when electrons
absorb light energy
Reflection - Occurs strongly in
smooth, conducting metals with free
electrons
Transmission - Occurs strongly
when there are large enough gaps
between atoms for the light to pass.
Does invisibility exist in nature?
Secret of the sky practical
•Pour water into tank
•Shine the flashlight beam through the tank as
shown in the diagram.
•Hold white paper in the two places shown on the
diagram and record what light you see.
•Add a pinch of powdered milk and then stir. Repeat
the previous step.
So why is the sky red/orange at
sunset?
Are there any invisible materials?
Crystals
Highly organised
“crystaline” structure
Exist in crystal
planes which light
can travel between
Man made transparent solids
Glass
Plastics
Boiled
Sweets!
How do we make glass transparent?
Glass is made from sand!
Heat to a liquid
Made from sand (silica)
Glass has strong/rigid bonds like a solid but…
An irregular particle structure like a liquid thus
allows light to pass
Cool to glass
Can we use this to make Harry
invisible?
Not without killing him!!
A different approach
Can we bend light?
Yes - Refraction!!
Air
Glass
When light passes through a material it slows as it has
to pass trillions of atoms.
The denser the material the “slower” it goes.
How does this work?
Disappearing bottle
Why do the stars twinkle?
Everyday examples of Refraction!!
Do we need any special materials?
Yes! Metamaterials!
Metamaterials have properties not found in nature
including NEGATIVE refractive index.
Metamaterials contain tiny implants which can control the
refractive index at every point in a material. This would
enable light to meander and bend in a path around an object
Cylindrical Cloak
1 mm
2006 a metamaterial
made at Durham,
North Carolina and
Imperial College
made an object
invisible to
microwave radiation.
Fiberglass rings patterned with copper elements.
The internal structures embedded in the metamaterial must
be smaller than the wavelength of the radiation.
This device works in the microwave spectrum.
http://Duke University cloak
Funded by the Pentagon’s Defence Advanced Research
Projects Agency [DARPA].
The material properties vary from point to point and in a
very specific way. This gives a subtle gradient in the
electromagnetic properties of the cloak.
Problem
The internal structures embedded in the
metamaterial must be smaller than the wavelength
of the radiation.
Wavelength of visible light 380nm (purple) to 780nm (red)
We need structures 10s of nms in size
How big is a nanometer?
1 m = 1 meter
0.01 m = 1 centimeter
0.001 m = 1 millimeter
0.000001 m = 1 micrometer .
0.000000001 m = 1 nanometer
.
There are approximately five atoms to 1 nanometer!
50 nanometers
20,000 per 1 mm!!!
Is this possible?
American news
YES!!
2007 California Institute of Technology
Is this any use?
Claims to work for
red light (780 nm)
Good points
Bad points
•The practical data
agrees with the theory
•Only works in 2D
•Large step towards an
invisible cloak
•Only works for one
wavelength (the easiest
one too!)
David Smith
Duke University
“A true invisibility cloak IS possible within the laws of physics.
With good research it should be a matter of time”
Steps to success
Invisibility for all wavelengths
in 3D
(Unclear solution based upon
layers)
Invisibility for all wavelengths in 2D
(Requires complex stacking of wafers)
Invisibility for one wavelength in 2D
(Requires sophisticated patterns to bend light smoothly)
Differences compared with Harry’s cloak
This would be a rigid, most
likely cylindrical, device.
Need flexible metamaterials.
Seeing out of any such device
would be a problem.
A spy hole would mean
floating eyes.
What are the uses for an invisibility cloak?
Hiding
Eyesores
Police
Army
Spy on
people
Stealing
Fashion
Philosophical View on Invisibility
Plato
Greek philosopher
Born 424 BC, Athens
Ring of Gyges
Morality is a social construct .
The use of invisibility removes this construct and the
temptation to steal and kill at will becomes irresistible.