Harry: Invisible Man. Is invisibility possible? ?Invisibility?

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?

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