I asked my physics teacher whether refractive index can be negative and he said that it can't be. But a group of scientist have made a meta material which has a negative R.I and it has made possible to make perfect lens which can focus at an area less than wavelenght of light. Do the laws in physics change often for example General relativity says it is imposible to travel faster than the speed of light but according to quantum mechanics it is possible for a photon to travel faster than the speed of light.

 

LINK:-

http://www.physicstoday.org/vol-57/iss-6/p37.html

A meta-material isn't a 'material'...its more a well ordered and shaped collection of materials (metals and dielectrics) such that the total contribution forces a negative refractive index. There is no natural material with negative refractive index.

Also negative refractive index materials do not break any physical laws, nor do they need any changed. They are found by application of Maxwell's equations.

i think ri can be negative