velocity of light is minimum in which medium
Speed of light
The phase velocity is important in determining how a light wave travels through a material or from one material to another . It is often represented in terms of a refractive index . The refractive index of a material is defined as the ratio of c to the phase velocity v in the material : larger indices of refraction indicate lower speeds . The refractive index of a material may depend on the light 's frequency , intensity , polarization , or direction of propagation ; in many cases , though , it can be treated as a material - dependent constant . The refractive index of air is approximately 1.0003 . Denser media , such as water , glass , and diamond , have refractive indexes of around 1.3 , 1.5 and 2.4 , respectively , for visible light . In exotic materials like Bose -- Einstein condensates near absolute zero , the effective speed of light may be only a few metres per second . However , this represents absorption and re-radiation delay between atoms , as do all slower - than - c speeds in material substances . As an extreme example of light `` slowing '' in matter , two independent teams of physicists claimed to bring light to a `` complete standstill '' by passing it through a Bose -- Einstein condensate of the element rubidium , one team at Harvard University and the Rowland Institute for Science in Cambridge , Mass. , and the other at the Harvard -- Smithsonian Center for Astrophysics , also in Cambridge . However , the popular description of light being `` stopped '' in these experiments refers only to light being stored in the excited states of atoms , then re-emitted at an arbitrarily later time , as stimulated by a second laser pulse . During the time it had `` stopped , '' it had ceased to be light . This type of behaviour is generally microscopically true of all transparent media which `` slow '' the speed of light . 
The phase velocity is important in determining how a light wave travels through a material or from one material to another . It is often represented in terms of a refractive index . The refractive index of a material is defined as the ratio of c to the phase velocity v in the material : larger indices of refraction indicate lower speeds . The refractive index of a material may depend on the light 's frequency , intensity , polarization , or direction of propagation ; in many cases , though , it can be treated as a material - dependent constant . The refractive index of air is approximately 1.0003 . Denser media , such as water , glass , and diamond , have refractive indexes of around 1.3 , 1.5 and 2.4 , respectively , for visible light . In exotic materials like Bose -- Einstein condensates near absolute zero , the effective speed of light may be only a few metres per second . However , this represents absorption and re-radiation delay between atoms , as do all slower - than - c speeds in material substances . As an extreme example of light `` slowing '' in matter , two independent teams of physicists claimed to bring light to a `` complete standstill '' by passing it through a Bose -- Einstein condensate of the element rubidium , one team at Harvard University and the Rowland Institute for Science in Cambridge , Mass. , and the other at the Harvard -- Smithsonian Center for Astrophysics , also in Cambridge . However , the popular description of light being `` stopped '' in these experiments refers only to light being stored in the excited states of atoms , then re-emitted at an arbitrarily later time , as stimulated by a second laser pulse . During the time it had `` stopped , '' it had ceased to be light . This type of behaviour is generally microscopically true of all transparent media which `` slow '' the speed of light .
In exotic materials like Bose -- Einstein condensates near absolute zero, the effective speed of light may be only a few meters per second.  This type of behaviour is generally microscopically true of all transparent media which ``slow'' the speed of light.
In exotic materials like Bose -- Einstein condensates near absolute zero , the effective speed of light may be only a few metres per second . This type of behaviour is generally microscopically true of all transparent media which `` slow '' the speed of light .
In exotic materials like Bose -- Einstein condensates near absolute zero , the effective speed of light may be only a few metres per second . Light is claimed to be brought to a `` complete standstill '' in a Bose -- Einstein condensate of the element rubidium. Stopping the light is generally microscopically true of all transparent media which `` slow '' the speed of light .
In exotic materials like Bose -- Einstein condensates near absolute zero , the effective speed of light may be only a few metres per second . As an extreme example of light `` slowing '' in matter , two independent teams of physicists claimed to bring light to a `` complete standstill '' by passing it through a Bose -- Einstein condensate of the element rubidium , one team at Harvard University and the Rowland Institute for Science in Cambridge , Mass. , and the other at the Harvard -- Smithsonian Center for Astrophysics , also in Cambridge . However , the popular description of light being `` stopped '' in these experiments refers only to light being stored in the excited states of atoms , then re-emitted at an arbitrarily later time , as stimulated by a second laser pulse . During the time it had `` stopped , '' it had ceased to be light . This type of behaviour is generally microscopically true of all transparent media which `` slow '' the speed of light .