Can there be a single ray of light? My physics teacher told me that a beam of light is a collection of rays of light and there cannot be a single absolute ray of light. Is this true?
 A: This is how an optical ray is defined:

In optics a ray is an idealized geometrical model of light, obtained by choosing a curve that is perpendicular to the wavefronts of the actual light, and that points in the direction of energy flow

The mathematical function that describes the classical propagation of light depends on the wave equations of Maxwell.
Here is what a wavefront starting from a point source looks like.

So the ray is the line perpendicular  to the front,that gives the direction of the energy from this single point.
Light is built up from many wavefronts next to each other so there are many optical rays, as many as the wavefronts.
To answer the title

Can there be a single ray of light?

To have a single ray of light you would have to have a single point source for the light . In nature, in order to create a classical beam the point source will not be a point, so as to have one ray. There will be many atoms radiating photons that will build up the wavefront, so there cannot be a single ray of light really because there will be many point sources building it up. (photons and atoms are another level of complexity in how classical light beams are made and needs a background  in quantum mechanics)
