Define the unit of length m in terms of size of the earth, meter bar and wavelength of light.

What are the advantages of defining the unit of length in terms of wavelength of light? 

Unit of length in terms of size of earth: One meter is one ten millionth of the distance from equator to the north pole along the meridian line through Paris.

Unit of length in terms of meter bar: One meter is the distance between two fine lines engraved on gold plugs near the platinum-iridium rod kept at temperature of 273.16K, preserved at International Bureau of weights and measures near Paris.

Unit of length in terms of wavelength: It is the distance occupied by 1650763.73 wave (in vacuum) of wave emitted due to electronic transition from 5d₅ to 2p₁₀ by individual atom of Kr-86 isotope. 

The advantage of defining the unit of length in terms of wavelength of light is that this standard does not change with time, physical conditions and is easily reproducible.