How to exactly overlap two light rays? I know this is a tough thing to accomplish,but it is a possibility.how to exactly overlap two light rays.thank you,this is for my project.
 A: It depends on the kind of light you are working with.  If the light comes from two polarized lasers, it is possible to align the beams perfectly by the following method:


*

*rotate the polarization of the beams so that one is vertical and one is horizontal.

*Direct the beams through the faces of a polarization beamsplitter, so that the vertically polarized beam is reflected out one side of the beamsplitter, and the horizontally polarized beam is transmitted out that same side of the beamsplitter.  


The two beams are now aligned, but their polarizations are not the same.
If the two beams come from a single laser, or if they come from two phase-locked lasers, the combined beams will add up to a single beam with elliptical polarization.  By very slightly adjusting the path length of one of the beams upstream from the beamsplitter, the resulting elliptical polarization can be changed to linear polarization oriented at 45 degrees from vertical. A polarization rotator can then be used to turn that linear polarization any way you want it.
If the two beams are, instead, from unpolarized lasers of different color, the beams can still be combined by using a dichroic beamsplitter.  In that case, the beamsplitter has multilayer optical coatings that will reflect light of one color and transmit light of the other color.  Alternatively, a prism can be used: inject the two beams at slightly different angles so that they exit the prism at the same angle.
