led - IR emitter in reverse bias?

What will happen if light is shown on IR LED (emitter) when it's in reverse bias condition?

Answer

It works like a photo diode. I.e. if photon energy is high enough (wavelength is short enough) to create electron-hole-pairs you get a photo current proportional to the light flux.

It might be not very efficient though for that purpose because it was not designed (optimized) for that purpose.
E.g. the sensitive area is probably much smaller than that of an ordinary photodiode.