General Relativity: Gravitational Redshift
While the constancy over the speed of light in a vacuum is a fundamental law that cannot be challenged (in fact it was one of the postulates of special relativity), the wavelength of light is definitely a factor that can vary. In general relativity, Einstein highlights the consequence of gravitational redshift, a phenomenon where light emitted from a huge mass changes its wavelength when escaping the mass' gravitational field .