Changing light frequency without altering its direction is not feasible; any attempt to do so typically results in a loss of directionality. Light can experience a change in wavelength when transitioning through different media, but the original wavelength is restored upon exiting. Non-linear processes, such as frequency doubling, can generate higher harmonics, but they require specific conditions and do not maintain direction. Fluorescence is a related process where a photon of shorter wavelength is absorbed and a longer wavelength photon is emitted, but this also results in a loss of direction. Ultimately, achieving frequency change while preserving direction remains unattainable with current methods.
