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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?
Good points
•Results that work
•Possible improvements
•Inexpensive
Bad points
•Only works in 2D
•Shows some reflection
•Not very dynamic! •Bit of a cheat really!!
Camera
Tripod?
What is Light?An electromagnetic wave, made up of
different wavelengths.
See for yourselves!!
Natural form of invisibility is an object which neither reflects nor absorbs light (it allows light to pass through it).
Reflection - Occurs strongly in smooth, conducting metals with free electrons
Absorption - Occurs when electrons absorb light energy
Transmission - Occurs strongly when there are large enough gaps between atoms for the light to pass.
How does light interact with objects?
Does invisibility exist in nature?
Secret of the sky practical
•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.
•Pour water into tank
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
PlasticsGlass Boiled Sweets!
Man made transparent solids
How do we make glass transparent?
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
Heat to a liquid
Cool to glass
Glass is made from sand!
Can we use this to make Harry invisible?
Not without killing him!!
A different approach
Can we bend light?
When light passes through a material it slows as it has to pass trillions of atoms.
The denser the material the “slower” it goes.
Yes - Refraction!!
Air
Glass
Disappearing bottle
How does this work?
Why do the stars twinkle?
Everyday examples of Refraction!!
Do we need any special materials?
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
Metamaterials have properties not found in nature including NEGATIVE refractive index.
Yes! Metamaterials!
Cylindrical Cloak1 mm
http://Duke University cloak
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.
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.
Funded by the Pentagon’s Defence Advanced Research Projects Agency [DARPA].
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
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!
How big is a nanometer?
20,000 per 1 mm!!!50 nanometers
Is this possible?YES!!
2007 California Institute of Technology American news
Is this any use?
Good points
•The practical data agrees with the theory
•Large step towards an invisible cloak
Bad points
•Only works in 2D
•Only works for one wavelength (the easiest one too!)
Claims to work for red light (780 nm)
Steps to success
Invisibility for all wavelengths in 2D(Requires complex stacking of wafers)
Invisibility for all wavelengths in 3D
(Unclear solution based upon layers)
Invisibility for one wavelength in 2D(Requires sophisticated patterns to bend light smoothly)
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”
Differences compared with Harry’s cloak
Need flexible metamaterials.
This would be a rigid, most likely cylindrical, device.
A spy hole would mean floating eyes.
Seeing out of any such device would be a problem.
What are the uses for an invisibility cloak?
Hiding Eyesores
Stealing
FashionPolice
Army
Spy on people
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.
Philosophical View on Invisibility
