A system and method of photon trapping spectroscopy to vary the path length of light for use in spectroscopy. The systems and method include a rotating reflector with slits for selectively permitting light to enter and exit into a reflection cavity containing a sample to be analyzed. After entering the cavity, but before exiting, the light is trapped and repeatedly reflects back and forth through a sample, effectively increasing the path length of light through a sample. The effective path length is quickly adjustable by altering the rotation speed of the rotating reflector to alter the time in which the light is trapped within the cavity. The systems and methods provide a spectroscope with a wide dynamic range, low detection limits, and usable with broadband and monochromatic light sources throughout the optical region (ultraviolet to infrared).