Finally, another complication is that there is no effective mirror for X-ray frequency light. In a common laser, the lasing medium is normally placed between two partial mirrors that reflect some of the output back into the media. This greatly increases the number of photons in the media and increases the chance that any given atom will be stimulated. More importantly, as the mirrors reflect only those photons traveling in a particular direction, and the stimulated photons will be released in the same direction, this causes the output to be highly focused.