A critical wavelength refractometer is provided. A broadband light source (413) is optically coupled to a sensor (401), the sensor having at least one sensing surface (407). As the light from the broadband light source passes through the sensor, it undergoes multiple internal reflections against the sensing surface. Due to the index of refraction of the material in contact with the sensing surface, a portion of the light passing through the sensor is reflected while a second portion of the light is transmitted through the sensing surface and into the material. A detector (421) coupled to the sensor measures the spectral intensity of the light that passes completely through the sensor after having undergone the multiple internal reflections against the sensing surface. A microprocessor (423) coupled to the detector determines the critical wavelength based on the spectral intensity measurement, thereby allowing the index of refraction of the material to be determined.