In several fiber-optic-based probes in medical imaging fields, the light travels towards an object through an optical fiber (or even free space), interacts with the object and then travels back through the same fiber (or the same path in free space) and is captured by a camera or photodetector, etc. How come the light that is returning from the object does not interact with the light that is traveling toward the object? In case of fluorescence imaging, the excitation beam reflects off of a dichroic mirror, passes through set of focusing lenses, falls on a sample and induces fluorescence in the sample. The fluorescence (higher wavelength) - captured by the lenses - travels 'backwards', passes through the dichroic mirror and is then captured by a camera. The two wavelengths (excitation and emission) travel the same path in opposite directions. Why do they not interact with each other? Thanks.