everything you need to know about invisibility...
the invisibility machineThe first, 3D-working invisibility cloak has been built by researchers at the University of Rochester. Made out of four simple lenses it is arranged in a special manner so that the lenses create a region of invisibility. The two outer lenses focus the light from a wide area onto two smaller lenses in the middle. This creates a region where incident light can’t reach the object and reflected light can’t reach the observer. The region of invisibility is in the form of a hollow cylinder. However, with larger lenses, the region could be big enough to hide most things. These lenses have the potential not only to make things disappear, but also to be put into practical use. Surgeons could use this setup to see through their hands and truck drivers could use it to see through the front and rear pillars of their vehicle, helping to avoid accidents.
In order for the invisibility cloak to work most effectively, achromatic lenses are used. This is a lense which is designed to limit the effects of chromatic and spherical abberation (imperfection in the image) by bringing two wavelengths (usually red and blue) into focus in the same plane. The most common type of these lenses is the achromatic doublet which places a concave and a convex lense (usually made of flint glass), next to eachother. In this way the chromatic abberation of one is counter balanced by that of the other. |
try your own1) Buy two sets of two lenses with different focal lengths. (The lens provider should provide the focal length information).
2) Use an optics bench to select one lens with the first focal length and a second lens with a second focal length. Use the sum of their focal lengths to find the distance between the lenses e.g. if the first lens has a focal length of 5 inches and the second lens has a focal length of 3 inches then these lenses are separated by a distance of 5+3= 8 inches. 3) Do the same with the two remaining lenses. 4) Then find the distance which separates the two sets using some simple maths. Using the figures from step 2 - distance = [2 (3) (5+3)]/(5-3)= 12 inches |
a thing of the future?Although it may be a long time until we are able to create an invisibility cloak large enough to make a human disappear, scientists have recently developed metamaterials- artificially structured materials used to control and manipulate light, sound and many other physical phenomena.
The behaviour of natural materials is based on their molecular structure i.e. the interaction between the particles which make up the material and the electromagnetic waves of light. However, with metamaterials, it is the sum of the parts which characterise their behaviour by combining the properties of the materials which make it up e.g. by arranging gold and copper in certain patterns and shapes. How do they make things invisible? Metamaterials arrange materials in such a way that light is only allowed to flow over it. If a hole is punched into it light will continue around the hole and continue its original flow, so the hole doesn’t exist to lightwaves. Therefore, if an object is placed in the hole it will effectively disappear. What metamaterials essentially do is guide light around an object instead of reflecting or refracting it so the human eye perceives it to not be there. This method can also be used to manipulate microwaves (used in radar). To manipulate any electromagnetic wavelength the metamaterial must be smaller than the wavelength. As the wavelength of microwaves is measured in cm, scientists can manipulate them to move waves around an object and in this way objects such as planes can become invisible to radar. However, making a whole plane invisible to the human eye poses a much greater challenge as currently technology does not allow us to manufacture materials on the small scale required for light wave manipulation, which are measured in nanometers. Another problem is that colours exist on different wavelengths and so the metamaterial would have to cover the entire visible spectrum. Apart from this, anyone within this plane covered in the metamaterial would be in complete darkness as the light usually reaching them would have been diverted around the cloaking device. However, if these challenges were overcome this device could be very useful for military organisations. |