Optical fibres can be used for measurement of temperature.This is because the temperature induces changes in refractive index of the fibre.In fibre the refractive index is a function of  both composition and temperature and it changes at different rates in response to a temperature changes.A higher temperature change cause the critical angle of the fibre to change slightly,which in turn changes the amount of light lost through leakage in the cladding as shown in fig.,total internal reflection occurs at angle of incidence greater than the critical angle.At angles of incidence less than the critical angle,some light is lost due to refraction.